Mechanical BE (Modern Communication System)

I ns t i t ut eofManage me nt & Te c hni c alSt udi e s MODERN COMMUNI CATI ON SYSTEM

500

BACHELORI NMECHANI CALENGI NEERI NG www. i mt s i n s t i t u t e . c o m

IMTS (ISO 9001-2008 Internationally Certified) MODERN COMMUNICATION SYSTEM

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MODERN COMMUNICATION SYSTEMS Unit – I Communication Process 01-14 Elements of a communication system – Sources of information – Transmission and Reception of a signal – Basic antennas – Analog and digital types of communication.

Unit – II

Amplitude and Frequency Modulation

15-54

Definition of Amplitude modulation – Generation and detection of AM –SSB / DSB VSB modulation–AM radio transmitter and receiver-Definition of Frequency modulation – FM generation and demodulation – Block diagram of FM radio transmitter and receiver.

Unit – III Pulse Modulation

55-70

Sampling theorem – Basic principles of pulse Amplitude modulation – Pulse width modulation – Pulse position modulation –Multiplexing- FDM-TDM-WDM. Unit – IV

PCM and Basics of data Transmission and Reception

71-86

Principle of PCM – Quantization and quantization error Delta Modulations – Adaptive delta modulation – Time division multiplexing in PCM – Coherent reception – Binary ASK, - FSK – PSKComparison of ASK, FSK, PSK. Unit – V

Microwave Devices

87-100

Introduction to microwave system – Frequency range – Waveguides (qualitative analysis only) – cavity resonators – Two cavity Klystron – reflex klystron – Magnetron – traveling wave tube – gun diode.

UNIT QUESTIONS-

101-103

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UNIT-I COMMUNICATION PROCESS 1.1.

ELEMENTS OF COMMUNICATION:

Communication involves implicitly the transmission of information from one point to another through a succession of processes. 

The generation of a message signal: voice, music, picture or computer data.



The description of that message signal with a certain measure of precision by a set of symbols: electrical or visual.



The encoding of these symbols in a form that is suitable for transmission over a physical medium of interest.



The transmission of the encoded symbols to the desired destination.



The decoding and reproduction of the original symbols.



The re-creation of the original message signal, with a definable degradation in quality; the degradation is caused by imperfections in the system.

There are many other forms of communication that do not directly involve the human mind in real time.

For example, in computer communications involving communication between two or more computers, human decisions may enter only in setting up the programs or commands for the computer or in monitoring the rules.

There are three basic elements to every communication system, namely transmitter, channel and receiveras shown in the figure.

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The transmitter is located at one point in space, the receiver is located at the point seperated from the transmitter. The channel is the physical medium that connects them. The purpose of the transmitter is to convert the message signal from the source of information into a form suitable for transmission over the channel. The transmitted signal propagetes along the channel and is added with the noise and interfering signals with the result that the received signal is a corrupted version of the transmitted signal. This received signal is then converted into a form that is suitable for the user.

There are two basic modes of communication: ďƒ˜

Broadcasting mode: Broadcasting, which involves the use of a single powerful transmitter and numerous receivers that are relatively inexpensive to build. Here information bearing signals flow only in one direction.

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Example for the broadcasting mode of communication is radio and television type of communication. 

Point-to-Point communication mode: This mode of communication process takes place over a link between a single transmitter and a receiver. In this case, there is usually a bidirectional flow of information bearing signals, which requires the use of a transmitter and a treceiver at each end of the link.

Example for the Point-toPoint mode of communication is telephone communication and a link between an Earth station and a robot navigating the surface of a distant planer.

1.2.

SOURCES OF INFORMATION: The Communications environment is dominated by four important sources of information:

Speech, Music, Pictures and Computer data. A source of information may be characterized in terms of signal that carries information. A signal is defined as asingle – valued function of time.

The signal can be one-dimentional, as in the case of speech, music or computer data; twodimentional as in the case of pictures; three-dimentional as in the case ofvideo data and four-dimentional as in the case of volume data over time.  Speech is the primary method of human communication. Specifically, the speech communication process involves the transfer of information from a speaker to a listener, which takes place in three successive stages.  Production: An intended message in the speaker’s mind is represented by a speech signal that consists of sounds generated inside the vocal tract

and whose

arrangement is governed by the rulles of language.  Propagation: The sound waves propagates through the air at a speed of 300m/s, reaching the listener’s ears.  Perception: The incoming sounds are received by the listener into a received message, and is then transferred into an information from the speaker to the listener.  The second source of information, music orginates from instruments such as the piano, violin and flute. However the musical signal differs from the speech signal in that its spectrum occupies a much wider band of frequencies that may extend up to about 15kHz. Normally musical signalsdemand a much wider channel bandwidth than speech signals for their transmission.  Third source of information, pictures. The pictures in motion are converted into electrical signals for the transmission from transmitter to receiver.

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ď ś Next is the facsimile(fax) machine, the purpose of this machine is to transmit still pictures over a communication channel.it provides a facility for the transmission of handwritten or printed text from one point to another. ď ś Finally, personal computers, which transmits data using ASCII codes. Each character in ASCII is represented by seven data bits constituting a unique binary pattern made up of 0s and 1s.

1.3.

TRANSMISSION AND RECEPTION OF A SIGNAL:

Transmission and reception of a signal invoves with the modulation and demodulation process. The signal which is going to be transmitted is intended to modulation process then its demodulated to get back as original.

1.3.1.

SIGNAL TRANSMISSION:

In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for "MOdulate-DEModulate").

A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11.

If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bit per second.

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1.4.

5

BASIC ANTENNAS

Antennas couple the current flowing in wires or waveguides into electromagnetic waves in the air. The most basic form of the antenna is the dipole antenna.

The Dipole Antenna: This is nothing more than a straight piece wire. When voltage is applied to the wire, current flows and the electrical charges pile up in either end. A balanced set of positive and negative charges separated by some distance is called a dipole. The dipole moment is equal to the charge times the distance by which it is separated.

When an alternating voltage is applied the antenna, dipole moment oscillates up and down on the antenna, corresponding to the current. The oscillating current creates oscillating electric (E) and magnetic (H) fields which in turn generate more electric and magnetic fields. Thus a outward propagating electromagnetic wave is created. The electric field is oriented along the axis of the antenna and the magnetic field is perpendicular to both the electric field and the direction of propagation. The orientation of the fields is called the polarization.

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When the electromagnetic wave passes over a conducting material, it will create an oscillating current in it. Therefore, the same design (i.e. the dipole antenna) can be used to extract signals from the air as current in a circuit. For a center-fed dipole antenna to work most effectively, it should be exactly one-half wavelength long. Receiving antennas which do not require high sensitivity need not follow this rule.

Transmitting antennas on the other generally do, except at very low frequencies. When the antenna is placed in the ground, called a ground-plane antenna, the optimum size is reduced by half again, due to signal reflection at the ground plane. This appears to make an image antenna of equal size below the ground which reduces the actual antenna requirement. So for ground-plane antennas, the optimum size is one-quarter wavelength.

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Example: Find the optimum antenna size for a ground-plane dipole used to broadcast commercial AM radio (approximately 1 MHz). The wavelength at 1 MHz is 300m , so the optimum antenna should be about 75 m tall.

Polarization In the case of a dipole antenna, the electric field in aligned with the antenna axis and remains so as it propagates. When the field remains in a particular direction the wave is considered to be linearly polarized. For practical reasons, its orientation is usually resolved into a vertical and horizontal component.

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A vertical dipole antenna will create a vertical, linearly polarized electromagnetic wave. A receiving antenna that is also aligned vertically will generate the largest current when exposed to the electromagnetic wave. Therefore it is imperative to known the polarization type and direction of the transmitting antenna when trying to receive the signal.

Example: Commercial radio broadcasts come from large vertically oriented antennas. Therefore they are linearly vertically polarized signals and are best received by a vertical antenna. So to maximize reception of a radio signal, hold the antenna upright.

Linear polarization is not the only possibility. Another type is circular polarization. The best way to visualize this is like a corkscrew. The electric field rotates as it travels along. If the rotation is clockwise as seen looking in the direction of propagation, it is called right-hand circular polarization (RHCP).

The other possibility is LHCP. Transmitting antennas for circular polarization are generally look like corkscrews. Circular polarization is often used in satellite communications because it is not required to know the orientation of the satellites antenna (called skew). Linear polarized signal from space are also subject to a rotation caused by the Earth's magnetic field called Faraday Rotation.

Circular polarization is not affected.

A wave need not be polarized. For example, sunlight is a homogeneous mixtures of waves will all orientations. It is said to be un-polarized. It can however, become polarized either by filtering and on reflection from a flat surface. Example: polarized sunglasses.

When sunlight is reflected off the road it appears as glare. In the process of reflection the light becomes horizontally polarized. Sunglasses with vertical polarization block this component and therefore

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reduce glare. These glasses can easily be checked to see if they are polarized by holding two pair at right angles. In this case, all possible orientations of linear polarization will be blocked and the lenses will appear opaque.

Antenna Beam-forming The dipole antenna we have been discussing radiates energy in all directions perpendicular to its axis. There is no signal coming from the ends. In this sense, the dipole antenna has some directionality, or preferred direction. In cases where high sensitivity is required or when it is necessary to exclude transmission or reception from unnecessary directions, antennas can be made even more directional.

The process of creating directionality is called beam-forming. It has applications in radar and sonar as well.

Beam-forming should be understood as the exploitation of interference. For example, consider two identical receiving dipole antennas, both oriented vertically in the ground. Neither one will receive signals at any significant vertical angle.

However, each one by itself, has no preferred direction in the horizontal plane. Suppose now that they are separated by exactly one-half of a wavelength of the signal they are receiving. If the signal 0

comes from a direction along the line that connects them, there will be a 180 phase shift inserted between them, which will cause complete destructive interference. Therefore they cannot receive signals along the line connecting them.

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If the signal is coming from a direction perpendicular to the line connecting them, there will be equal path lengths and therefore complete constructive interference. They will be more sensitive along the directions perpendicular to the line connecting them.

Therefore, the two-dipole antenna array is directional in the horizontal plane. As it turns out, the three-dipole antenna linear array is even more directional. There is no reception from the directions along the axis, and a more narrow region perpendicular to the array from which they receive strongly. The width of good reception is called the beamwidth.

For a many-dipole linear array, the beamwidth gets smaller proportionally. If the overall array length is L, the beamwidth can be predicted theoretically: q l/L. This looks just like the diffraction theory. In fact, it should. The model for deriving diffraction is to sum the results from many little "antennas" across the aperture. In the limit of many antennas this is exactly the linear array. These results may be immediately extended to any antenna shape. The beamwidth is simply found from the diffraction theory.

Example: Direct Satellite TV. This system uses an 18" dish to receive signals from a geo-synchronous satellites which are located at 101 W near the equator. The signal is Ku-band at about 12.5 GHz (2.4 cm 0

wavelength), and is circularly polarized. The beamwidth of the 18" (44 cm) is 2.4/44 = 0.05 radians or 3 .

This would imply that the antenna should be positioned within less than three degrees of the line of sight to the satellite. The beamwidth is made as small as possible to maximize the sensitivity of the antenna. A larger dish would have smaller beamwidth and therefore would be more sensitive but would require a more accurate aim.

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1.5.

11

ANALOG AND DIGITAL TYPES OF COMMUNICATION: In the design of a communication system the information source, communication channel and the

end user all are specified. The challenge is to design the transmitter and the receiver. There are two types f communication systems are available analog and digital communication systems.

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Digital communication system is represented by the block diagram as shown below.

In the above figure, the source encoder removes redundant information from the message signal and is responsible for the efficient use of the channel. The resulting sequence of symbols is called the source code word. The data stream is processed next by the channel code word. The channel code word is longer than the source code word.

Finally the modulator represents each symbol of the channel code word by the corresponding analog symbol, selected from a finite set of possible analog symbols. The sequence of analog symbols produced by the modulator is called waveform, which is suitable for transmission over the channel.

At the receiver, the channel output is processed in reverse order to that in the transmitter, thereby reconstructing the original message signal. The reconstructed original message signal is finally delivered to the user of information at the destination. This digital system is complex but easy to build.

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ANALOG SYSTEM: In contrast, the design of analog system is simple in conceptual terms but difficult to build because it needs linearity and system adjustments. In signal processing terms, the transmitter consists of a modulator and the receiver consists of a demodulator, the details of which are determined by the CW modulation process.

CW MODULATION: The CW modulation, on the other hand, transmits electromagnetic waves continuously towards the target and there is a continuous reflection of these waves from the targets.

It was possible to use a single antenna for transmission and reception in pulsed radars and this was achieved with the duplexer switch. In CW such, separate antennas are required for transmission and reception.

The CW radar makes use of Doppler Effect for speed measurements of targets. Figure shows the block diagram of CW Doppler radar.

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The transmission section is a low power microwave oscillator such as reflex klystron that generates sinusoidal signals in the microwave rage. This signal is transmitted by the transmitting antenna. A small fraction of the transmitter signal is fed to the transmitter mixer, to which is also fed the IF signal generated by IF oscillator. Sum of the transmitter signal frequency (f 1) and the IF signal (fc) is selected at the output of the transmitter mixer.

The receiver antenna picks up the waves reflected form the target and for moving targets, the received signal frequency equals f 1+-f. This signal is given to the receiver mixer where it mixes with the output of the transmitter mixer. At the output of the receiver mixer is obtained the difference frequency signal at (fc+-fd). This signal is amplified by the IF amplifier and given to the detector stage. The detector circuit recovers the Doppler frequency from the IF signal and passes it to the AF amplifier where it is amplified. The amplified signal is given to a frequency counter. Since the Doppler frequency shift f d is proportional to the velocity of the target, the output of the counter gives an indication of the target speed.

The frequency counter is so designed that at its output the target speed is displayed directly in kilometer/hour rather than showing the Doppler frequency. However, the display does not give indication as to whether the target is approaching or receding, because sign of the Doppler frequency shift is lost.

The CW Doppler radar is not capable of giving the range of the target.

An important advantage of CW Doppler radar is that it uses low transmitting power, low power consumption, simple circuitry and small size. This makes it mobile. It can be used to give accurate measurement of relative velocity of the target and the reading obtained is unaffected by the presence of stationary objects. It is capable of measuring a large range of target velocities from a very low value to a high value quickly.

The most common application of CW Doppler radars is in checking the speeds of vehicles and for this purpose it is widely used by Police. It is also used in aircraft navigation for speed measurement and as rate-of-climb meter.

In spite of these applications, it has certain drawbacks. Firstly, it is limited in the maximum power it transmits and this places a limit on its maximum range. Secondly, it is not capable of indicating the range of the target and can show its velocity only. Lastly, if a large number of targets are present, then it gets confused rather easily.

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UNIT – II AMPLITUDE AND FREQUENCY MODULATION 2.1 MODULATION: In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for "MOdulate-DEModulate").

A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11.

If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bit per second.

2.1.1 THE AIM OF MODULATION The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a filter limits the frequency range to between [300 and 3400 Hz) or a limited radio frequency band.

The aim of analog modulation is to transfer an analog lowpass signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel.

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2.1.2 NEED FOR MODULATION: 

To transmit information over long distances.



To reduce the height of the antenna.



To transmit the programmes of different stations within the allocated bandwidth without interference.



For wireless communication, modulation is necessary.



Also, it simplifies the power amplifier design.

2.1.3 TYPES OF MODULATION: A high frequency sinusoidal wave is given by the expression, e = E sin ( ωt + φ ) Where, E is its maximum amplitude ω is the angular frequency ( velocity ), and φ is the phase angle.

Based on the variation of these parameters, modulation is classified into three types.

They are, (i)

Amplitude modulation

(ii)

Frequency modulation

(iii)

Phase modulation

AMPLITUDE MODULATION (AM): In Amplitude modulation (AM), the information signal is mixed with the carrier signal in such a way as to cause the AMPLITUDE of the carrier to vary at the frequency of the information signal.

FREQUENCY MODULATION (FM): With frequency modulation, the modulating signal and the carrier are combined in such a way that causes the carrier FREQUENCY (fc) to vary above and below its normal(idling) frequency. The amplitude of the carrier remains constant.

PHASE MODULATION (PM OR INDIRECT FM):

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Phase modulation is a type of frequency modulation. Here, the amount of the carrier frequency shift is proportional to both the amplitude and frequency of the modulating signal. The phase of the carrier is changed by the change in amplitude of the modulating signal.

2.1.4 AMPLITUDE MODULATION: Amplitude modulation or AM as it is often called, is a form of modulation used for radio transmissions for broadcasting and two way radio communication applications. Although one of the earliest used forms of modulation it is still in widespread use today.

The first amplitude modulated signal was transmitted in 1901 by a Canadian engineer named Reginald Fessenden. He took a continuous spark transmission and placed a carbon microphone in the antenna lead.

The sound waves impacting on the microphone varied its resistance and in turn this varied the intensity of the transmission. Although very crude, signals were audible over a distance of a few hundred metres, although there was a rasping sound caused by the spark.

With the introduction of continuous sine wave signals, transmissions improved significantly, and AM soon became the standard for voice transmissions.

Nowadays, amplitude modulation, AM is used for audio broadcasting on the long medium and short wave bands, and for two way radio communication at VHF for aircraft.

DEFINITION OF AMPLITUDE MODULATION: Modulation in which the amplitude of a carrier wave is varied in accordance with some characteristic of the modulating signal.

Amplitude modulation implies the modulation of a coherent carrier wave by mixing it in a nonlinear device with the modulating signal to produce discrete upper and lower sidebands, which are the sum and difference frequencies of the carrier and signal.

The envelope of the resultant modulated wave is an analog of the modulating signal. The instantaneous value of the resultant modulated wave is the vector sum of the corresponding instantaneous values of the carrier wave, upper sideband, and lower sideband. Recovery of the modulating signal may be by direct detection or by heterodyning.

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The amplitude of the carrier wave will vary in accordance with a modulator wave. Amplitude modulation generates a pair of sidebands for every sinusoidal component in the carrier and the modulator. The amplitude of the two sidebands increases in proportion to the amount of modulation, but never exceeds half the level of the carrier.

In AM modulation, the voltage (amplitude) of the carrier is varied by the incoming signal. In this example, the modulating wave implies an analog signal.

When a carrier is modulated in any way, further signals are created that carry the actual modulation information. It is found that when a carrier is amplitude modulated, further signals are

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generated above and below the main carrier. To see how this happens, take the example of a carrier on a frequency of 1 MHz which is modulated by a steady tone of 1 kHz. The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz, and a difference frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the carrier.

If the steady state tones are replaced with audio like that encountered with speech of music, these comprise many different frequencies and an audio spectrum with frequencies over a band of frequencies is seen. When modulated onto the carrier, these spectra are seen above and below the carrier.

2.2 GENERATION OF AMPLITUDE MODULATION: The amplitude, phase, or frequency of a carrier can be varied in accordance with the intelligence to be transmitted. The process of varying one of these characteristics is called modulation. With a sinewave voltage used to amplitude-modulate the carrier, the instantaneous amplitude of the carrier changes constantly in a sinusoidal manner.

The maximum amplitude that the wave reaches in either the positive or the negative direction is termed the peak amplitude. The positive and negative peaks are equal and the full swing of the cycle from the positive to the negative peak is called the peak-to-peak amplitude.

Considering the peak-to-peak amplitude only, it can be said that the amplitude of this wave is constant. This is a general amplitude characteristic of the un-modulated carrier. In amplitude modulation, the peak-to-peak amplitude of the carrier is varied in accordance with the intelligence to be transmitted.

For example, the voice picked up by a microphone is converted into an A-F (audio-frequency) electrical signal which controls the peak-to-peak amplitude of the carrier. A single sound at the microphone modulates the carrier, with the result shown in figure.

The carrier peaks are no longer because they follow the instantaneous changes in the amplitude of the A-F signal. When the a-f signal swings in the positive direction, the carrier peaks are increased accordingly. When the A-F signal swings in the negative direction, the carrier peaks are decreased.

Therefore, the instantaneous amplitude of the a-f modulating signal determines the peak-to-peak amplitude of the modulated carrier.

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PERCENTAGE OF MODULATION: In amplitude modulation, it is common practice to express the degree to which a carrier is modulated as a percentage of modulation. When the peak-to-peak amplitude of the modulating signal is equal to the peak-to-peak amplitude of the unmodulated carrier, the carrier is said to be 100 percent modulated.

In figure, the peak-to-peak modulating voltage, EA, is equal to that of the carrier voltage, ER, and the peak-to-peak amplitude of the carrier varies from 2ER, or 2EA, to 0. In other words, the modulating signal swings far enough positive to double the peak-to-peak amplitude of the carrier, and far enough negative to reduce the peak-to-peak amplitude of the carrier to Zero.

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If EA is less than ER, percentages of modulation below 100 percent occur. If EA is one-half ER, the carrier is modulated only 50 percent shown in figure. When the modulating signal swings to its maximum value in the positive direction, the carrier amplitude is increased by 50 percent. When the modulating signal reaches its maximum negative peak value, the carrier amplitude is decreased by 50 percent.

It is possible to increase the percentage of modulation to a value greater than 100 percent by making EA greater than ER.

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In below figure, the modulated carrier is varied from 0 to some peak-to-peak amplitude greater than 2ER. Since the peak-to-peak amplitude of the carrier cannot be less than 0, the carrier is cut off completely for all negative values of EA greater than ER. This results in a distorted signal, and the intelligence is received in a distorted form. Therefore, the percentage of modulation in a-m systems of communication is limited to values from 0 to 100 percent.

The actual percentage of modulation of a carrier (M) can be calculated by using the following simple formula,

M = ((Emax - Emin) / (Emax + Emin)) * 100 Where, M = percentage of modulation Emax is the greatest peak-to-peak amplitude of the modulated carrier

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Emin the smallest peak-to-peak amplitude of the modulated carrier.

For example, assume that a modulated carrier varies in its peak-to-peak amplitude from 10 to 30 volts. Substituting in the formula, with Emax equal to 30 and Emin equal to 10,

M = ((30 - 10) / (30 + 10)) * 100 = (20 / 40) * 100 = 50 percent. This formula is accurate only for percentages between 0 and 100 percent.

SIDE BANDS: When the outputs of two oscillators beat together, or heterodyne, the two original frequencies plus their sum and difference are produced in the output. This heterodyning effect also takes place between the A-F signal and the R-F signal in the modulation process and the beat frequencies produced are known as side bands.

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Assume that an A-F signal whose frequency is 1,000 cps (cycles per second) is modulating an R-F carrier of 500 kc (kilocycles). The modulated carrier consists mainly of three frequency components: the original R-F signal at 500 kc, the sum of the A-F and R-F signals at 501 kc, and the difference between the A-F and R-F signals at 499 kc.

The component at 501 kc is known as the upper sideband, and the component at 499 kc is known as the lower side band. Since these side bands are always present in amplitude modulation, the a-m wave consists of a center frequency, an upper side-band frequency, and a lower side-band frequency. The amplitude of each of these is constant in value but the resultant wave varies in amplitude in accordance with the audio signal.

The carrier with the two sidebands, with the amplitude of each component plotted against its frequency, is represented in figure 7 for the example given above. The modulating signal, fA, beats against the carrier, fC, to produce upper side band fH and lower side band fL. The modulated carrier occupies a section of the radio-frequency spectrum extending from f L to fH, or 2 kc.To receive this signal, a receiver must have R-F stages whose bandwidth is at least 2 kc. When the receiver is tuned to 500 kc, it also must be able to receive 499 kc and 501 kc with relatively little loss in response.

The audio-frequency range extends approximately from 16 to 16,000 cps. To accommodate the highest audio frequency, the a-m frequency channel should extend from 16 kc below to 16 kc above the

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carrier frequency, with the receiver having a corresponding bandwidth. Therefore, if the carrier frequency is 500 kc, the a-m channel should extend from 484 to 516 kc. This bandwidth represents an ideal condition; in practice, however, the entire a-m bandwith for audio reproduction rarely exceeds 16 kc. For any specific set of audio-modulating frequencies, the a-m channel or bandwidth is twice the highest audio frequency present.

The R-F energy radiated from the transmitter antenna in the form of a modulated carrier is divided among the carrier and its two side bands. With a carrier componet of 1,000 watts, an audio signal of 500 watts is necessary for 100-percent modulation. Therefore, the modulated carrier should not exceed a total power of 1,500 watts. The 500 watts of audio power is divided equally between the side bands, and no audio power is associated with the carrier.

Since none of the audio power is associated with the carrier component, it contains none of the intelligence. From the standpoint of communication efficiency, the 1,000 watts of carriercomponent power is wasted. Furthermore, one side band alone is sufficient to transmit intelligence. It is possible to eliminate the carrier and one side band, but the complexity of the equipment needed cancels the gain in efficiency.

AMPLITUDE DEMODULATION: The process of detection (or) demodulation consist in recovering the original modulating voltage from the modulated carrier voltage, thus the detection is a process reverse of the modulation.

The detection process is accomplished by mixing the carrier with the sideband components carrying the intelligence, in a non – linear device. The mixing process results in sum and difference frequency terms. The detection process is similar to the modulation process.

TYPES OF DEMODULATION PROCESS: 

Square law diode detector.



Linear diode (or) envelope detector.

SQUARE LAW DIODE DETECTOR: It utilizes the non- linear portion of the dynamic current – voltage characteristics of diode. It differs from the linear diode detector; the applied carrier voltage is small magnitude and hence is restricted to the nonlinear portion of the dynamic characteristic, where as in linear diode detector. A large amplitude modulated carrier voltage is applied to the diode and most of the operations take place in the linear region

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Above figure shows the basic circuit of square law diode detector. The diode is biased positively to shift the zero signals operating point to the small current non-linear region of the dynamic current – voltage characteristics shown in figure.

The R, C combination constitutes the load. The detector o/p current waveform has its lower half compressed. The average current consists of a steady D.C component. The shunt capacitor C bypasses all the radio frequency components. The average component flow through the load resistor R producing the desired detected output.

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ENVELOP DETECTOR: Amplitude modulation, AM, is one of the most straightforward ways of modulating a radio signal or carrier. The process of demodulation, where the audio signal is removed from the radio carrier in the receiver is also quite simple as well. The easiest method of achieving amplitude demodulation is to use a simple diode detector. This consists of just a handful of components:- a diode, resistor and a capacitor.

AM Diode Detector In this circuit, the diode rectifies the signal, allowing only half of the alternating waveform through. The capacitor is used to store the charge and provide a smoothed output from the detector, and also to remove any unwanted radio frequency components. The resistor is used to enable the capacitor to discharge. If it were not there and no other load was present, then the charge on the capacitor would not leak away, and the circuit would reach a peak and remain there.

This is essentially just a half wave rectifier which charges a capacitor to a voltage equal to the peak voltage of the incoming AM waveform,. When the input wave's amplitude increases, the capacitor voltage is increased via the rectifying diode. When the input's amplitude falls, the capacitor voltage is reduced by being discharged by a ‘bleed’ resistor, R. The main advantage of this form of AM Demodulator is that it is very simple and cheap. That's why it is used so often. However, it does suffer from some practical problems.

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The circuit relies upon the behaviour of the diode — allowing current through when the input is +ve with respect to the capacitor voltage, hence ‘topping up’ the capacitor voltage to the peak level, but blocking any current from flowing back out through the diode when the input voltage is below the capacitor voltage. Unfortunately, all real diodes are non-linear. The current they pass varies with the applied voltage. As a result, the demodulated output is slightly distorted in a way which depends upon the diode's I/V characteristic.

The circuit also suffers from the problems known as Ripple and Negative Peak Clipping. These effects are illustrated in figure. The ripple effect happens because the capacitor will be discharged a small amount in between successive peaks of the input AM wave.

ADVANTAGES OF AMPLITUDE MODULATION: There are several advantages of amplitude modulation, and some of these reasons have meant that it is still in widespread use today: 

It is simple to implement

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

it can be demodulated using a circuit consisting of very few components



AM receivers are very cheap as no specialised components are needed.

DISADVANTAGES OF AMPLITUDE MODULATION: Amplitude modulation is a very basic form of modulation, and although its simplicity is one of its major advantages, other more sophisticated systems provide a number of advantages. Accordingly it is worth looking at some of the disadvantages of amplitude modulation. 

It is not efficient in terms of its power usage



It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency



It is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

2.3.SINGLE SIDEBAND MODULATION:The system to save the bandwidth, it is sufficient to transmit only one sideband, and rejecting the other side band and carrier. The system which adopts this technique is known as SINGLE SIDEBAND SUSTEM.For 100% modulation, only 1/6 of the total power is present in one of the sidebands. 2/3 of the total power is present in carrier signal which contains no information.

If the carrier and one of the sideband are eliminated from the signal before transmission, only half of the bandwidth is required for transmission for which 1/6 of the total power is required.

GENERATION OF SSB USING FILTER METHOD: The block diagram of SSB using filter method is shown in the following figur

Low frequency crystal oscillator

Balanced modulator

USB filter

LSB Audio input

filter

Balanced mixer

Linear power amplifier

Crystal oscillator Matching network

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Band pass filters are used in this method to eliminate the unwanted sideband. To obtain sharp band pass characteristics modulation takes place at a low frequency (100 KHz). So a low frequency crystal oscillator is used. Balanced modulated output contains USB and LSB, without carrier. The desired sideband is selected with a help of band pass filter and a switch. The selected sideband is then upconverted to the final transmitting frequency with the help of a balanced mixer and a crystal oscillator.

ADVANTAGES OF SSB: 

Band width reduction.



Power reduction.



Reduction in carrier interference with other stations.



Some privacy is automatically provided.



Reduction in operating cost.



Improvement in signal to noise ratio from 9 to 12 db.

DISADVANTAGES OF SSB: 

The transmitter and receiver are complicated and their performance required is of high standard.



The frequency of the re-inserted carrier in SSB receiver must be within 15Hz in case of speech and within 4Hz in case of music. So it is necessary to transmit a pilot signal for synchronizing the receiver oscillator frequency. This signal is filtered with the help of highly selective filters.

2.4. DOUBLE SIDEBAND MODULATION: In AM with a carrier scheme, there is wastage in both transmitted power and bandwidth. In order to save the power in amplitude modulation the carrier is suppressed, because it does not contain any useful information. This scheme is called double sideband modulation technique.

It contains only LSB and USB terms, resulting in a transmission bandwidth that is twice the bandwidth of the modulating signal.

Let the modulating signal, Vm (t) = Vm sin (ωm t) The carrier signal, Vc (t) = Vc sin (ωc t) When multiplying both the carrier and modulating signal, we can get the DSB –AM signal.

V (t) = Vm sin (ωm t) . Vc sin (ωc t)

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= Vm . Vc sin (ωm t) sin (ωc t) = (Vm . Vc ) / 2 [ cos (ωc – ωm) t – cos (ωc + ωm) t ] The frequency spectrum of DSB-AM is shown below. It contains only two sidebands, having the frequency of (ωc – ωm) and (ωc + ωm). Hence this is called DSB – AM system.

The graphical representation of DSB-AM system is shown below.

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GENERATION OF DSB SIGNAL: There are two ways of generating DSB-Am signal. They are, 

Balanced modulators



Ring modulators

BALANCED MODULATOR: The basic circuit of balanced modulator for generating DSB signal is shown in the figure.

The same circuit can be used to generate AM with carrier signal. The main difference between the AM with carrier generation and DSB-AM is the feeding points of the carrier and modulating signals are interchanged.

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Q1

Vm C2 C1 Modulating signal input

DSB-AM OUTPUT

Vm

Vcc Carrier input Q2

BALANCED MODULATOR

The circuit looks like a push-pull amplifier. The importance of this configuration is that it automatically eliminates the carrier or modulating frequency. It is assumed that the two transistors are identical and the circuit is symmetrical. The modulating signal is applied to the differential input and the carrier signal is applied to the common input of the push-pull amplifier.

The signal applied to Q1 is the sum of two input voltages ( Vc + Vm ), while the signal applied to Q2 is difference of input voltages ( Vc - Vm ). The common input carrier signal is cancelled as the two collector currents are subtracted in the output transformer primary.

Vbe1 = Vc + Vm Vbe2 = Vc – Vm

The collector element is given by, ic1 = a Vbe1 + b( Vbe1* Vbe1) ic2 = a Vbe2 + b( Vbe2* Vbe2) Vo = K (ic1 - ic2 )

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Vo = Vm sin ωm t + 4 Vm Vc sin ωc t. sin ωc t

This output contains the original modulating signal and the two sidebands about the carrier frequency. The modulating voltage has been suppressed. The tank circuit is tuned to the carrier frequency and it responds to a band of frequencies centered at ωc. Thus DSB-AM signal is generated.

ADVANTAGES: 

Suppression of carrier results in economy of power.



It is commonly used in current telephony systems in which one side band is filtered out to reduce the width of the channel required for transmission.



It offers secrecy.



It increases the efficiency because carrier is suppressed.

2.5. VESTEGIAL SIDEBAND AMPLITUDE MODULATION: It Is the system in which one side band is transmitted completely while just a part of vertige of the other side band. The modulating signals of very high bandwidth are having the low frequency components along with the rest of the signal. These components give rise to the sidebands, very close to the carrier frequency which are difficult to remove by using filters. Thus it is not possible to go to the extreme and fully suppress one complete sideband in the case of television signals.

The low video frequencies contain the most important information of the picture and any effort to completely suppress the lower sideband would results in phase distortion at these frequencies. Therefore a compromise had been made to suppress the part of the lower side band, and then the radiated signal consists of full upper sideband together with the carrier and vestige of the lower sideband. This pattern of modulation is known as vestigial side band modulation. A VSB – AM system is a compromise between DSB-AM and SSB-AM. It takes the advantages of both systems and avoids their disadvantages. VSB signals are very easy to generate and at the same time their bandwidth is slightly greater than SSB-AM but less than DSB-AM.

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VSB modulation is derived by DSB-AM or AM signals in such a fashion that one sideband is passed almost completely while only a trace of other sideband is added.

A typical VSB filter transfer function and its frequency responds are shown in the following figures.

VSB MODULATION

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In VSB instead of rejecting one side band completely a gradual cut of one side band is acceptable. A VSB filter can eliminate a part of one side band signals. Therefore VSB sideband filter has ----------

To obtain time domain representation , the frequency transform of VSB signal is, HVSB = HDSB (f) Where, HDSB (f) = magnitude transfer function of DSB system.

VVSB (t) = Vc/2 sin ωc t + Vm.Vc/2 [ cos (ωc – ωm) t – cos (ωc + ωm) t ]

–

Vc] /2 cos ωc t

= response of AM and response of VSB filter.

VVSB (t) = Vc/2 sin ωc t [ 1+ Vm(t) ] – γ (t) Vc/2 cos ωc t Where,

If γ(t) = 0 then the circuit will be acting as AM modulator with carrier. If γ(f) = Vm (t) then the output will be SSB with carrier.

The power calculation for the VSB system is very tedious. Therefore it can be written as, Pc + ½ Pc.Pm<= PT

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ADVANTAGES: 

It has bandwidth greater than SSb but less than DSb system.



Power transmission greater than DSB but less than SSb system.



No low frequency component lost, hence avoids the phase distortion.

2.7.AM RADIO TRANSMITTER: In this amplitude modulated radio transmitter system, the carrier is amplitude modulated by the amplitude of the modulating signal. These transmitters are used for the radio broadcast on medium and short waves, radio telephony on short waves, radio telegraphy on short waves and television picture broadcast on very short waves.

There are two types of AM transmitter based on the modulation process. They are, 

High level modulation



Low level modulation

HIGH LEVEL MODULATION: In high level AM transmitter, the carrier is modulated at a higher power level. The following block diagram shows the high level modulation.

HIGH LEVEL AM TRANSMITTER:

Transmitting Antenna

Master oscillator

Buffer amplifier

Harmonic oscillator

Modulati signal

Class-C power amplifier

Audio amplifier

Modulated amplifier

Modulating amplifier

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MASTER OSCILLATOR: Master oscillator produces sub â&#x20AC;&#x201C; harmonic of the carrier frequency. The frequency generated should be stable. It should not change due to the variation of temperature, supply voltage and ageing of components.

BUFFE AMPLIFIER: If the master oscillator directly drives the harmonic generator, it may draw input current from the master oscillator. Hence this action may load the master oscillator. To avoid the loading effect of master oscillator, buffer amplifier (or) isolating amplifier is placed between the master oscillator and the harmonic generator. It draws negligible amount of current. Thus the loading effect of master oscillator is avoided.

HARMONIC GENERATOR: Harmonic generators are class-C tuned amplifiers in which the RF voltage from master oscillator is distorted. The tuned circuit of the amplifier selects the desired harmonic frequency.

CLASS-C AMPLIFIER: The RF voltage generated by the master oscillator has very small power in the order of few watts.

This amplifier is used to increase the power level of the RF voltage. A chain of class-C amplifiers is used to provide high output circuit efficiency.

MODULATED AMPLIFIER: This amplifier modulates the carrier signal with the modulating signal. It is a class-C tuned amplifier usually of push pull type. High efficiency series plate modulation is commonly used in high power radio broadcast and radio telephone transmitters.

MODULATING AMPLIFIER: It is a class-B push pull amplifier. It feeds the audio input to the modulated amplifier. This modulated signal is transmitted at a higher power level.

ANTENNA: This converts AM electrical signal into electro magnetic waves. The electro magnetic waves are spreaded over in the air media.

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LOW LEVEL MODULATION: LOW LEVEL AM TRANSMITTER:

Transmitting Antenna

Master oscillator

Buffer amplifier

Harmonic oscillator

Modulating signal

Class-C power amplifier

Class-C Modulated amplifier

Modulating amplifier

2.8. SUPERHETERODYNE RECEIVER In electronics, the super heterodyne receiver (also known by its full name, the supersonic heterodyne receiver, or by the abbreviated form superhet) is a technique for selectively recovering information from radio waves of a particular frequency. It is used in radio and television receivers and transmitters, allowing them to be tuned to a particular frequency.

HISTORY The super heterodyne principle was originally conceived in 1918 by Edwin Armstrong. It had been noticed some time before that if a regenerative receiver was allowed to go into oscillation, other receivers nearby would suddenly start picking up stations on frequencies different from those they were actually transmitted on. These Super heterodyne circuits originally used the self-resonance of iron-cored interstage coupling transformers to filter the intermediate frequency. Even today, Intermediate Frequency tuned circuits are referred to as IF "transformers" although they would more correctly be termed "coils" like the aerial and oscillator tuned circuits.

In modern receivers electromechanical filters such as Ceramic resonators, Surface Acoustic Wave (SAW) or crystal-lattice filters are more likely used provide selectivity at the intermediate frequency.

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Early superhets used IFs as low as 20 kHz, which made them extremely susceptible to image frequency interference, but at the time the main interest was sensitivity rather than selectivity.

OVERVIEW The superhet receiver consists of three principle parts, the local oscillator that produces a highfrequency signal close to the signal being detected, a mixer that mixes the local oscillator's signal with the received signal, and a tuned amplifier.

Reception starts with an antenna signal, optionally amplified, including the frequency the user wishes to tune, fd. The local oscillator is tuned to produce a frequency close to fd, fLO. The received signal is mixed with the local oscillator's, producing four frequencies in the output; the original signal, the original fLO, and the two new frequencies fd+fLO and fd-fLO. The output signal also generally contains a number of undesirable mixtures as well.

The amplifier portion of the system is tuned to be highly selective at a single frequency, fIF. By changing fLO, the resulting fd-fID (or fd+fID) signal can be tuned to the amplifier's fIF. In typical amplitude modulation ("AM radio" in the U.S., or MW) home receivers, that frequency is usually 455 kHz; for FM receivers, it is usually 10.7 MHz; for television, 45 MHz. Although the other signals from the mixed output of the heterodyne are still present when they reach the amplifier, they are either filtered out or simply left un-amplified.

DESIGN AND ITS EVOLUTION The diagram below shows the basic elements of a single conversion superhet receiver. In practice not every design will have all these elements, nor does this convey the complexity of other designs, but the essential elements of a local oscillator and a mixer followed by a filter and IF amplifier are common to all superhet circuits. Cost-optimized designs may use one active device for both local oscillator and mixerâ&#x20AC;&#x201D;this is sometimes called a "converter" stage. One such example is the pentagrid converter.

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The advantage to this method is that most of the radio's signal path has to be sensitive to only a narrow range of frequencies. Only the front end (the part before the frequency converter stage) needs to be sensitive to a wide frequency range.

For example, the front end might need to be sensitive to 1–30 MHz, while the rest of the radio might need to be sensitive only to 455 kHz, a typical IF. Only one or two tuned stages need to be adjusted to track over the tuning range of the receiver; all the intermediate-frequency stages operate at a fixed frequency which need not be adjusted.

Sometimes, to overcome obstacles such as image response, more than one IF is used. In such a case, the front end might be sensitive to 1–30 MHz, the first half of the radio to 5 MHz, and the last half to 50 kHz. Two frequency converters would be used, and the radio would be a "Double Conversion Super Heterodyne"—a common example is a television receiver where the audio information is obtained from a second stage of intermediate frequency conversion. Occasionally special-purpose receivers will use an intermediate frequency much higher than the signal, in order to obtain very high image rejection.

ADVANTEGES Super heterodyne receivers have superior characteristics to simpler receiver types in frequency stability and selectivity. They offer much better stability than Tuned radio frequency receivers (TRF)

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because a tuneable oscillator is more easily stabilized than a tuneable filter, especially with modern frequency synthesizer technology.

IF filters can give much narrower passbands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of a crystal filter in very critical designs such as radiotelephone receivers, in which exceptionally high selectivity is necessary. Regenerative and super-regenerative receivers offer better sensitivity than a TRF receiver, but suffer from stability and selectivity problems.

In the case of modern television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigial sideband reception, first used with the original NTSC system introduced in 1941. This originally involved a complex collection of tuneable inductors which needed careful adjustment, but since the early 1980s these have been replaced with precision electromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser milling techniques, SAW filters are much cheaper to produce, can be made to extremely close tolerances, and are extremely stable in operation.

The next evolution of super heterodyne receiver design is the software defined radio architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in the latest design analog television receivers and digital set top boxes, where there are no coils or other resonant circuits used at all.

The antenna simply connects via a small capacitor to a pin on an integrated circuit and all the signal processing is carried out digitally. Similar techniques are used in the tiny FM radios incorporated into Mobile phones and MP3 players.

Radio transmitters may also use a mixer stage to produce an output frequency, working more or less as the reverse of a superheterodyne receiver.

DRAWBACKS Drawbacks to the super-heterodyne receiver include interference from signal frequencies close to the Intermediate Frequency. To prevent this, IF frequencies are generally controlled by regulatory authorities, and this is the reason most receivers use common IFs. Examples are 455 kHz for AM radio, 10.7 MHz for FM, and 45 MHz for television. Additionally, in urban environments with many strong signals, the signals from multiple transmitters may combine in the mixer stage to interfere with the desired signal.

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IMAGE FREQUENCY (FIMAGE) One major disadvantage to the superheterodyne receiver is the problem of image frequency. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency.

The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the superheterodyne receiver.

2.9. FREQUENCY MODULATION (FM) Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the instantaneous amplitude of the modulating signal without any variation in the amplitude of the carrier wave. Because the amplitude of the wave remains unchanged, the power associated with an FM wave is constant. The figure depicts an FM wave.

As can be seen from the figure, when the modulating signal is zero, the output frequency equals fc (centre frequency). When the modulating signal is maximum and equals (f c + fm). At negative peaks of the modulating signal, the frequency of the FM wave becomes minimum and equal to (f c - fm). Thus, the process of frequency modulation makes the frequency of eth FM wave to deviate from its centre frequency (fc) by an amount (±∆f) where ∆f is termed as the frequency deviation of the system. During this

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process, the total power in the wave does not change but a part of the carrier power is transferred to the side-bands.

Assume the modulating signal to be represented by em = Em cos wmt and the carrier wave being represented by ec = Ec sin (wct + Φ) (wct + Φ) represents the total phase angle Φ at a time t and Φ represents the initial phase angle.

Thus

Φ = (wct + Φ)

The angular velocity may be determined by finding the rate of change of this phase angle. i.e., angular velocity

= d Φ / dt = wc

After frequency modulation takes place, angular velocity of the carrier wave varies in proportion to the instantaneous amplitude of the modulating signal. The instantaneous angular velocity w, is given by ω1 = ωc + K. em = ωc + K. Em cos ωm t where K is a constant of proportionally.

Maximum frequency shift or derivation occurs when the cosine terms in Eq. has a value ± 1.

Under this condition, the instantaneous angular velocity is given by

ω1 = ωC ± K. Em so that the maximum frequency deviation ∆f is given by K Em ∆f = ------2∏

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This gives

45

K Em = 2∏∆f

It may be rewritten as ω1 = ωC + 2∏∆ fcos ωm t Integration of Eq, gives the instantaneous phase angle of the frequency modulated wave. Φ1 = ω1.dt = (ωc + 2∏∆ f cos ωmt). dt = ωct + (2∏∆ f / ωm ) sin ωmt + Φ1.

where Φ1 is a constant of integration representing a constant phase angle and may be neglected in the following analysis.

The instantaneous amplitude of the modulated waves is given by emod = Ec sin Φ1. = Ec . Sin (ωC +(∆ f/fm)sin ωmf)

The ratio ∆ f/fm is termed as the modulation index of the frequency modulated wave and is denoted mf. It should be noted that for a given frequency deviation ∆ f, the modulation index varies with the modulating frequency fm.

A comparison of the modulation index m a for the AM and mf for the frequency modulated wave shows that while ma is given as the ratio of the change in the carrier amplitude due to amplitude modulation to the carrier amplitude, m f is given as the ratio of the change in the carrier frequency during frequency modulation to the carrier frequency. Substituting mf in Eq. we obtain equation for the frequency modulated wave. emod = EC sin (ωct + mf sin ωmt).

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2.10. F.M. CIRCUITS The frequency of an L - C oscillator is determined by tuning capacitor and the inductor. If the system includes a reactive element whose reactance can be varied by the modulating signal, this results in production of a frequency - modulated wave. This forms the basis of varactor diode modulator and reactance tube modulator circuits.

Alternately, an FM wave may be obtained by variation of phase of the carrier in proportion to the modulating signal as in phase modulation and will be considered along with phase modulation (P.M) VARACTOR DIODE MODULATION Varactor diode is specially fabricated PN junction diode which is used as a variable capacitor in the reverse biased condition. This capacitor is dependent upon the magnitude of the reverse bias as shown in and its capacitance is given by Silicon varactor diode shows an average variation of (10 - 15) PF per volt variation of reverse bias and have capacitance, lying in the range of 150 to 200 PF for 1 volt reverse bias.

A typical varactor diode modulation circuit. Here the varactor diode is connected across the resonant circuit of an oscillator through a coupling capacitor of relatively a large value. This capacitor isolates the varactor diode from the oscillator as far as DC is concerned while providing an effective short circuit at operating frequencies.

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The DC bias to the varactor diode is regulated so that the oscillator frequency is not affected by varactor supply fluctuations. The modulating signal is fed in series with this regulated supply and at any instant, the effective bias to the varactor diode equals the algebraic sum of the DC bias voltage V and the instantaneous value of the modulating signal. As a result, the varactor, capacitance varies with the modulating signal resulting in frequency modulation of the oscillator output.

REACTANCE TUBE MODULATOR Reactance tube modulator is another circuit that was very widely employed for FM generation before the invention of varactor diodes. To understand the basic principle of reactance tube modulator, one must remember that in any tube, the anode current and voltage are in phase (neglecting plate load). o

If they could be made to have a phase difference of 90 , the tube will behave like a reactance.

Consider the circuit. The circuit consists of a pentode tube with a phase shifting network C - R connected between anode and cathode as shown and the junction of this network is connected to the control grid. The tube is coupled across the oscillator resonant circuit. Shows the equivalent circuit of the modulator in constant current generator form where gm eg is the current output o the equivalent generator.

This output must, therefore, be equal to the sum of i1 and i2.

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2.11. PHASE MODULATION As already pointed, frequency modulation and phase modulation are very closely related and may be termed as Angle modulation. However, there exist some important differences between the two that warrant this discussion.

The process of phase modulation consists in varying the phase of the carrier linearly in proportion to the modulating signal such that maximum phase shift occurs during positive and negative peaks of the modulating signal. If eC is taken to represent the instantaneous value of the carrier, then ec = EC sin (ωct + Ф) Where Ф is the initial phase of the wave.

Let the modulating signal be em = Em Sin ωmt. If the carrier phase varies sinusoidally with the modulating signal, then the phase modulated wave is represented by

ec = EC sin (ωct + Ф ± ∆Ф sin ωmt) Assuming that initially Ф = 0,

the ec = EC sin (ωCt ±∆Ф sin ωmt) Equation representing the phase modulated wave may be expanded in a way similar to FM. This expansion gives Where (m p) is termed as the modulation index for phase modulation and equals ∆ Ф. The PM waves like FM waves have identical frequency spectrum.

In PM, ∆Ф is given a fixed maximum value so that as the modulating frequency f m varies, frequency deviation ∆f also varies and ∆Ф = m p = ∆f / fm remains constant. This is different from FM where ∆f is constant and can be given a large value. As signal to noise ratio at the receiver output depends upon the frequency deviation and not upon ∆Ф, FM is preferred to PM. However, this gives an indirect method of producing frequency modulated waves.

It should be remembered that FM and PM are closely related and may be termed as angle modulation, because in both the cases, the changes in phase angle as well as frequency of the modulated carrier take place. Thus, consider a phase modulated wave given below.

Phase modulated wave ePM = EC cos [ωct + Ф0 + mpf (t)]

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wheref (t) is the modulating signal.

The term under the bracket represents the instantaneous phase ange Ф .

Where Ф0 is a constant of integration.

The instantaneous phase angle in a frequency modulated wave varies directly as the integral of the modulating signal.

An important conclusion can be derived from the preceding discussion. If the modulating signal f(t) is integrated before using it to phase modulate a carrier, the result is a FM wave. Similarly, if the modulating signal is differentiated before allowing it to frequency modulate a carrier, the result is a PM wave. An important characteristic of PM is that it does not require additional pre-emphasis and de-emphasis like FM. Hence it may be considered as an FM system with perfect pre-emphasis and de-emphasis.

PHASE MODULATION CIRCUIT : It provides an interesting and simple method of generating PM waves by shifting the carrier of an o

amplitude modulated wave by 90 with respect to the sidebands. The block schematic of phase modulator circuit based upon the principles described above.

The circuit is termed as Armstrong modulator and comprises a balanced modulator to which the carrier and the modulating signals are given as input. The output of the balanced modulator contains o

upper and lower side bands only. The carrier is given a phase shift of 90 by the phase shifting network and is fed to an adder circuit to which the balanced modulator output is also fed.

The RC network connected at the modulating signal input is an integrating circuit comprising of R and C as shown.

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The output voltage Vo is given by where K1 is the constant of proportionality and K2 = K1K Thus â&#x2C6;&#x2020;f = K2 and is constant, as in frequency modulation.

An important feature of Armstrong modulator is that it allow phase modulation with a carrier wave generated by crystal oscillator, resulting in high frequency stability. The phase shift is usually kept small in order to reduce distortion. The resulting frequency modulation produced has a small frequency shift. A number of frequency multiplier stages have to be used if Âą 75 KHz deviation is to be obtained.

2.12. FREQUENCY RADIO TRANSMITTER: Direct FM transmitter In direct FM transmitter, the carrier is generated at lower power level. A chain of class C amplifiers and harmonic generators is used to develop the required transmitter power and frequency. It shows the Direct modulated FM transmitter.

The Oscillator is connected in shunt with the reactance modulator. The frequency of the oscillator changes as the reactance of reactance modulator changes. The reactance modulator is supplkied with the modulating signal. Hence the reactance of modulator changes according to modulating voltage.

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The oscillator output consists of the deviation frequency and carrier frequency. Hence multiplier multiplies both the deviation and carrier frequencies. If the carrier frequency is multiplied, then the power requirement of the transmitter increases. To avoid this, following arrangement shown can be used.

In this circuit a down converter is used which is supplied with local frequency and output of frequency multiplier.

The oscillator output frequency is f c + â&#x2C6;&#x2020;fm (t). The local oscillator frequency is (n-1)fc. These two signals are mixed in the down converter which selects only the difference frequency component. The output of frequency multiplier is n(fc + fm(t)) The output of down converter = n(fc + fm(t)) - (n-1)fc = nfc + nfm (t) - nfc + fc = fc + nfm(t) Hence the deviation alone is multiplied and not the carrier frequency. When the main oscillator is an LC oscillator, the frequency stability of the direct modulation will be poor. Stability is improved by using AFC circuit.

A sample of the final output signal is mixed with the signal from a stable crystal oscillator. The IF produced contains the difference in frequency between the carrier and crystal oscillator. This discriminator circuit generates a voltage which is proportional to this difference frequency. The output from the discriminator contains modulation signal and varying dc voltage. Low pass filter is used to remove the audio signal. The output from the LPF contains only dc voltage. This voltage is added to the modulating signal and applied to the reactance modulator in a manner so as to correct any drift in the main oscillator.

Indirect FM Transmitter The block diagram of Armstrong indirect FM transmitter is shown. This method utilizes a balanced o

modulator with audio and carrier signals after 90 phase shift. The balanced modulator output gives the upper and lower bands only with carrier suppressed completed.

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The frequency of the side band is increased in a harmonic generator stage fed to the mixer stage, the other input of this stage being the carrier after passing through another harmonic generator stage.

The different frequency components at the mixer are the carrier frequency and side band frequencies. The output is again multiplied by number of frequency multiplier stages, raise to the desired power level and transmitted. Integrator is used to predistort the modulating signal to make its amplitude vary inversely proportional with its frequency.

Advantages : i) Deviation produced is small ii) Does not provide AFC system because crystal master oscillator is used.

FREQUENCY MODULATION RECEIVERS

A typical FM broadcast-band receiver, as pictured in Figure. The FM receiver is very similar to the AM superhet receiver, with only slight modifications. The FM receiver will usually always have an RF amplifier (usually a low-noise MOSFET transistor) and a separate mixer and local oscillator. These three subsystems, if constructed with discrete components, will usually be shielded to prevent reradiation of highfrequency energy from the receiver and to prevent oscillations resulting from unwanted feedback.

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The intermediate frequency (IF) will usually be the standard value of 10.7 MHz with a bandwidth of approximately 200 kHz to allow for a WBFM signal to be amplified and passed on to the FMdetector subsystem. Notice that because of the IF frequency chosen, image frequencies are 21.4 MHz from the desired signals, which puts them outside the 20-MHz WBFM broadcast band. As a result of this, a large part of the superhet-receiver image problem due to other FM stations has been eliminated. The detector will be quite different from the type used in the AM receiver and must reverse the process that occurred at the transmitter.Therefore, a frequency-to-voltage (F/V) converter is needed to demodulate the received signal.

FM RECEIVER:

A typical modern FM receiver uses a phase-locked loop (PLL) subsystem as the detector. The PLL is insensitive to amplitude variations and can perform the F/V function; it can therefore be used as an FM detector. The FM stereo receiver must also include further demultiplexing circuitry, which we will discuss later. A PLL detector is shown in Figure.

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The PLL detector works in the following manner: The voltagecontrolled oscillator (VCO) free runs at the 10.7-MHz intermediate frequency. The incoming signal, if unmodulated, locks up with the VCO signal, causing there to be no signal (ac) output from the LPF stage. Now let us assume that the incoming signal has been modulated by a single audio tone.

The phase detector will output an error voltage to the VCO in an attempt to drive the VCO into lock-up with the incoming signal. Because the incoming signal frequency is deviating both above and below the 10.7-MHz rest frequency at a certain number of cycles per second, the VCO will do the same, following the input-signal frequency variations. The error voltage from the LPF, which drives the VCO, will be identical to the original modulating signal, and hence is taken as the demodulated output.

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UNIT – III PULSE MODULATION 3.1. SIGNAL SAMPLING: The information [modulating] signal is sampled with a train of narrow rectangular pulses. The sampling rate must be greater than twice the highest frequency component of the information to recover the original signal with minimum distortion at the receiver. Usually a low pass filter is used before sampling to remove frequencies greater than the highest frequency of the information.

SAMPLING THEOREM A signal with highest frequency ω must be sampled at a minimum rate of 2 ω samples per second with the samples in the forms of impulses having area proportional to the amplitude of the sampled signal at the instant of sampling. If these impulses are passed through an ideal low pass filter having cut-off frequency ω, original signal can be reconstructed.

Assume that the time - switch S1 of remains in position 1 for ∆ t seconds, while rotating at the desired rate of fs =1/T seconds. The signal e1 (t) at the transmitter input is the sampled version of the signal em1 (t). fs is known as the sampling rate and T is termed as the sampling interval. If ω is the highest frequency in the signal, then fs ≥ 2ω is termed as Nyquists sampling rate.

A typical signal em (t) and its sampled output e(t) is shown. Here the amplitude of the pulses is varied in accordance with the instantaneous amplitude of the signal which the width of the pulses is kept constant.

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As a result, the area of these pulses becomes proportional to the instantaneous amplitude of the signal.

Such an arrangement is termed as Pulse Amplitude Modulation (PAM). Alternate forms of sampling the information are - pulse Duration Modulation (PWM), whereby the width of the pulses is varied in proportion to the instantaneous magnitude of the signal, Pulse Position Modulation (PPM), in which the position of a pulse is changed as a function of the signal amplitude and Pulse Code Modulation (PCM) wherein the sampled signal is first coded into a digital code and this code group is transmitted. These processes are discussed in the following section.

3.2. PULSE MODULATION: Pulse modulation is used to transmit analog information such as continous speech or data. In this system the continous waveforms are sampled at regular intervals. Information is transmitted only at the sampling times together with synchronizing pulses.

PULSE AMPLITUDE MODULATION [PAM] : PAM is a pulse modulation system. In this system, the signal is sampled at regular intervals. Each sample is made proportional to the amplitude of the signal at the instant of sampling. The pulse are used to sent by either wire or cable. These pulse are used to modulate a carrier.

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There are two types of PAM. 1. Double polarity PAM 2. Single polarity PAM

In single polarity PAM, a fixed dc level is added to the signal so that the pulses are always positive. In pulse system a method by which constant - amplitude pulses may be used in very advantageous.

When it is used the pulses are made to frequency - modulated the carrier.

3.2.1. PAM GENERATION : The block diagram of PAM generation is shown below.

The signal to be converted to PAM is fed to one input of an AND gate. Sampling pulses are fed to its other input. The gate output consists of pulses at the sampling rate. At the instant of sampling. The amplitude of the input signal voltage. The sampled pulses are passed through a pulse shapping network to give flat-tops. The pulses are then frequency modulated.

3.2.2. PAM DEMODULATION :

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In the receiver, the pulses are recovered by a FM detector. Then they are fed to an ordinary diode detector. It is followed by a low pass filter. The filter is designed so that it must pass the high signal frequency and remove the sampling frequency ripple. Thus the original waveform is obtained at the output of the filter.

3.3. PULSE WIDTH MODULATION [PWM] : Pulse width modulation is called PDM [Pulse duration modulation] and sometimes called PLM [Pulse length Modulation]

In this system the amplitude of each pulse is kept constant. But the width of each pulse is made proportional to the amplitude of the signal at the instant of sampling. The first term is a DC component while the second is the modulating signal. The third term is similar to the expression of the FM wave and may be expanded in a similar way in order to obtain the frequency spectrum of the wave.

The modulating signal may be recovered by passing the PNM pulse train through a low pass filter with cut-off frequency equal to the highest frequency in the modulating signal at the receiver.

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A negative pulse amplitude is not possible. In PWM system signal to noise ratio is better when compare to PAM system but requires a larger bandwidth to achieve this.

Method of generating PWM signals: This is one method of generating pulse width modulated signals. Three successive samples of such a PAM wave train SA(+). We generate synchronously with the samples, the linear ramp type pulse wave form R(t). These two signals are added and the sum is applied to a comparator circuit.

A comparator circuit has associated with itâ&#x20AC;&#x2122;s reference level and an input terminal pair to which it is applied, in the present case, the signal SA+R. The ramp amplitude is adjusted to be some what larger than the variation in amplitude of the OAM samples. The comparator reference may then be located. So, that it always intersects the sloping portion of the waveform SA + R. The first crossing of the reference level by the waveform SA + R generates the leading edge of a pulse output of the comparator. The trailing edge is generated by the second crossing. This is the pulse - duration modulated (or) pulse width modulated waveform.

3.3.1.GENERTION OF PWM: PWM is generated by using a monostable multivibrator. The emitter coupled monostable multivibrator acts as voltage to time converter. Trigger pulses are used to control the starting time of pulses is the multivibrator. The duration of the pulses is controlled by the input signal.

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The stable state for this multivibrator is with T 1 - OFF and T2 - ON. The applied trigger pulse switches T1- ON and T2-OFF. As soon as this happens, C beings to charge upon the collector supply potential through R. After a time, determined by the supply voltage and the RC time constant. B 2 becomes sufficiently positive to switch T 2-ON. T1 is simultaneously switched OFF by regenerative action and stays OFF until the arrival of the next trigger pulse.

Thus the applied modulation voltage controls the base voltage B2 to switch T2 - ON. Since this voltage rise is linear, the modulation voltage controls the period of time during which T 2 is OFF ie., pulse duration.

3.3.2 DEMODULATION OF PWM : PWM is fed to an integrating circuit. The output of the integrator is a signal whose amplitude at any time is proportional to the pulse width at that time.

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METHOD OF DETECTING PWM : The Low - pass filter method of base band signal recovery results in some distortion. The PWM and PPM could be generated from a PAM waveform, then by applying the inverse operations we can convert the PWM and PPM waveforms back to a PAM signal and then demodulate the PAM signal.

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WAVEFORM :

The two successive pulses in a PWM waveform, These pulse are used to generate the waveform.

The leading edge of the pulse initiate the generation of a linear ramp whose rise is proportional to the pulse duration.

A sequence of pulse generated locally at the demodulator, is added to the waveform and this waveform is applied to a clipping circuit which permits only the portion of the waveform in the above reference level.

The output of the clipping circuit constitutes the PAM waveform. The base band signal [I/P signal] recovered by passing the PAM waveform through an appropriate Low-pass filter. The output waveform is the base band signal (ie., original I/P).

ADVANTAGE: No need to provide synchronization between transmitter and receiver.

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DISADVANTAGES: o

The transmitting power changes with respect to pulse width. Hence, transmitter must be powerful.

o

It requires larger bandwidth than PAM.

3.4. PULSE POSITION MODULATION [PPM]: In this system, the amplitude and width of the pulses are kept constant. But the position of each pulse is varied in accordance with the amplitude of the signal at the instant of sampling.

Pulse position modulation waves have a better performance with respect to signal to noise ratio in comparison to PAM and PWM systems.

3.4.1. GENERATION:

PPM may be obtained from PWM. The signal to be converted to PPM is width modulated.

The resulting pulse train is differentiated. The output of the differentiator has positive going pulses corresponding to trailing edges. If the position of the trailing edge of an unmodulated pulse is counted as zero displacement, then the other trailing edges will arrive earlier or later. They will have time

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displacement other than zero. This time displacement is proportional to the instantaneous value of the signal voltage. The differentiate pulse train is then passed through a diode clipper. to remove the leading edges.

3.4.2 DEMODULATION: The PPM is first converted into PWM using a bistable multivibrator.

Trigger pulse are applied to one input of the multivibrator. The PPM pulses are fed to the other input of the multivibrator and switch the given stage ON. The period of time during which the particular stage is OFF depends on the time difference between the two triggers. The resulting pulse is PWM. The PWM pulse train is then demodulated using an integrator.

ADVANTAGE : It requires constant transmitter power.

DISADVANTAGE : It depends on transmitter - receiver synchronization.

PAM

PWM

1. PAM circuit is simpler

PWM circuit is complex

2. PAM signal has more noise

PWM signal has less noise

3. It

require

synchronization

between

transmitter and receiver circuit 4. Transmitter power will not change

In PWM synchronization circuit is not require between transmitter and receiver circuit.

Transmitter power changes with respect to pulse width.

5. In PAM, signal to noise ratio is

In PWM, signal to noise ratio is better

moderate.

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PWM 1. Transmitter

power

PPM changes

with

It requires constant transmitter power.

respect to pulse width 2. Synchronization is not require

Synchronization is require

3. In PWM, signal to noise ratio is better

In PPM, signal to noise ratio is better than PAM and PWM.

4. Transmitter and receiver circuit is complex 5. Pulse width will be modulated

Transmitter and receiver circuit is complex than PWM. Pulse position will be modulated.

3.6. MULTIPLEXING In the modulation systems AM, FM and PM discussed so far, the analogue signals are translated to a high frequency in the spectrum and are transmitted from one point to another by the continuous use of the transmission channels. If a number of signals, each with maximum frequency f m are to be transmitted simultaneously by the use of any of these modulation systems, it would require a separate transmitter for each signal with frequency separation of atleast 2f m Hz between carriers of respective transmitters in case of AM. As the demand for number of signals to be communicated simultaneously increases, it becomes impractical to use separate transmitters for each message for reasons of space, weight, power requirement and cost. In such cases, a method known as multiplexing is commonly employed.

There are two methods of multiplexing - Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). These methods are discussed briefly in this section.

FREQUENCY DIVISION MULTIPLEXING (FDM) In this method of multiplexing, each message of maximum frequency f m is translated to a different frequency spectrum by the use of base-carriers. These messages are then combined in an adder circuit and are used to modulate a common carrier using amplitude modulation.

At the receiving end, a broad-band receiver this signal and passes it on to base - band receivers which receive signals corresponding to the respective base-band frequency. At the output of these receivers, different signals are available.

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If signals to be transmitted simultaneously are em1 (t), em2 (t), em3 (t), em4 (t), and base - band carrier frequencies are f1, f2, f3 and f4, then modulated base-band carriers at the adder input are

e1 (t) = E1 [1 + ma em1 (t)] cos ω1t e2 (t) = E2 [1 + ma em2 (t)] cos ω2t e3 (t) = E3 [1 + ma em3 (t)] cos ω3t e4 (t) = E4 [1 + ma em4 (t)] cos ω4t

These signals are combined together at the adder and form a common modulating signal for the AM transmitter. em (t) = e1 (t) + e2 (t) + e3 (t) + e4 (t) and modulated wave at the transmitter output with a carrier frequency f c is e = Ec [1 + ma {e1 (t) + e2 (t) + e3 (t) + e4 (t)}] cos ωct

Thus, the transmitted wave contains all the original signals and have bandwidth 2 (f m1 + fm2 + fm3 + fm4) where fm1, fm2, fm3, fm4 are the highest modulation frequencies in the signal. At the receiver, these signals are separated and passed to respective channels.

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4.5. TIME DIVISION MULTIPLEXING (TDM) In Time Division Multiplexed system, different time-intervals rather than frequencies are allotted to different signals. During these intervals, these signals are sampled and transmitted. Thus, this system transits information intermittently rather than continuously. Continuously varying analogue signals have to be sampled at proper intervals for transmission and the receiver must recognize these samples for TDM system of operated properly. Shows the basic features of TDM system.

Each signal source is switched in for a fixed time-interval by a time switch. S1. During this time, the connected signal modulates he carrier of the transmitter. The process is repeated by the time-switch which must rotate continuously at a uniform speed for proper operation of the system. The time for which a signal is connected to the transmitter and the time gap between the instants when the first signal is disconnected and the second is connected to the transmitter is very important.

For this, not only should the speed of time-switch S1 be kept uniform, but it is also required that motion of electronic circuits are used as electronic switches and synchronizing pulses are transmitted to keep these switches synchronized. The time for which a signal remains connected to the transmitter and the frequency at which the switch rotates are very important and related to the highest frequency in the signal. The relationship between them is governed by the sampling Theorem.

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When the transmitter and receiver switches are synchronized, the signals will be fed in the proper sequence to the receiver channels. The samples from transmitter channel one will be fed to receiver channel one. In this way, many channels of audio are combined to form a single output (multiplexed) chain.

Time spacing occurs between the components of the separate channels. The chain is transmitted (via wire or radio path) to distant demultiplexing receivers. Each receiving channel functions to select and reconstruct only the information included in the originally transmitted channel.

In most present day applications, electronic switching is used as the sampling component. The main advantage to electronic sampling is the longer life of an electronic switch when compared to an electromechanical switch. We use a mechanical system in our example to make this concept easier for you to see. Now let's look at figure , view B, where channel one is shown sampled four times. (This is the output of channel one in our transmitter.)

Figure, view C, shows all six channels being sampled four times during each cycle. (This is the output of the rotating switch in our transmitter.) What you see here is a continuous, time-sharing waveform. More than six channels (perhaps 24 or more) may be used. As we increase the number of channels, the width of each sample segment must be reduced.

The problem with reducing the width of the pulse is that the bandwidth (bw) necessary for transmission is greatly increased. Decreasing the pulse width decreases the minimum required rise time of the sampling pulse and increases the required bandwidth. When you increase the number of channels, you increase the bw. The bw is also affected by the shape of the sampling pulse and the method used to vary the pulse.

WAVELENGTH-DIVISION MULTIPLEXING (WDM): Wavelength-division multiplexing (WDM) is a method of combining multiple signals on laser beams at various infared (IR) wavelengths for transmission along fiber optic media. Each laser is modulated by an independent set of signals. Wavelength-sensitive filters, the IR analog of visible-light color filters, are used at the receiving end.

WDM is similar to frequency-division multiplexing (FDM). But instead of taking place at radio frequencies (RF), WDM is done in the IR portion of the electromagnetic (EM) spectrum. Each IR channel carries several RF signals combined by means of FDM or time-division multiplexing (TDM). Each multiplexed IR channel is separated, or demultiplexed, into the original signals at the destination. Using FDM or TDM in each IR channel in combination with WDM of several IR channels, data in different formats and at different speeds can be transmitted simultaneously on a single fiber.

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In early WDM systems, there were two IR channels per fiber. At the destination, the IR channels were demultiplexed by a dichroic (two-wavelength) filter with a cutoff wavelength approximately midway between the wavelengths of the two channels. It soon became clear that more than two multiplexed IR channels could be demultiplexed using cascaded dichroic filters, giving rise to coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM).

There are three categories of wavelength division multiplexing: 

WDM (wavelength division multiplexing)

Two to four wavelengths per fiber. The original

WDM systems were dual-channel 1310/1550 nm systems. 

CWDM (coarse wavelength division multiplexing)

From four to 8 wavelengths per fiber,

sometimes more. Designed for short to medium-haul networks (regional and metropolitan area). 

DWDM (dense wavelength division multiplexing)

A typical DWDM system supports eight or

more wavelengths. Emerging systems support hundreds of wavelengths.

In CWDM, there are usually eight different IR channels, but there can be up to 18. In DWDM, there can be dozens. Because each IR channel carries its own set of multiplexed RF signals, it is theoretically possible to transmit combined data on a single fiber at a total effective speed of several hundred gigabits per second (Gbps).

The use of WDM can multiply the effective bandwidth of a fiber optic communications system by a large factor. But its cost must be weighed against the alternative of using multiple fibers bundled into a cable. A fiber optic repeater device called the erbium amplifier promises to make WDM a cost-effective long-term solution to the bandwidth exhaustion problem.

DENSE WAVELENGTH DIVISION MULTIPLEXING (DWDM): Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a lightstream transmitted on a single optical fiber.

Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM).

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Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be travelling at the same time within the optical fiber.

DWDM promises to solve the "fiber exhaust" problem and is expected to be the central technology in the all-optical networks of the future.

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UNIT IV PULSE CODE MODULATION AND BASICS OF DATA TRANSMISSION AND RECEPTION

4.1. PULSE CODE MODULATION : Abbreviated PCM. Pulse modulation in which the signal is sampled periodically and each sample is Quantized and transmitted as a digital binary code.

It is a digital technique, no distortion is introduced and no information is lost. (Analog to Binary conversion) QUANTIZATION : The process of converting a continuous analog input into a set of discrete output levels.

QUANTIZATION ERROR : The difference between the actual value of date and corresponding discrete o/p error resulting from quantization.

QUANTIZER : A device, which converts continuous analog signal into discrete levels.

Pulse Code Modulation (PCM) PCM is the digital modulation.

QUANTIZATION OF SIGNALS : A base band signal m(E) is shown in the below fig. This signal (Vi) is applied to the Quantizer input the output of the Quantizer is called (V0). The I/P and O/P characteristics of a Quantizer has a staircase form. The Quantized wave form is mq(t). the input Vi = m(t) and output (quantized) signal is V0 = mq (t).

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The dotted waveform represents the O/P waveform m(t). The quantizer is linearly related to the I/P. Vi = V0 and m(t) = m’(t).

In the receiver quantizer will generate the requantized signal the noises are removed.

In the figure the dashed lines indicate the allowable quantizer output levels, separated by amount S. The O/P of the quantizer shown in fig. The Quantizer O/P is closest to the I/P, If the noise will not appear in the O/P. Noise exceeds S/2, the error will occur in the O/P.

Quantization pulses reduced by increasing the step size S. If increasing S, results in an increased discrepancy between m’(t) & m q (t).

The difference between m’(t) - m(t) is called Quantization noise.

PCM (Pulse code modulation) An analog signal bone quantized prior to transmission is usually sampled. The quantization helps us to reduce the effect of noise. Quantizied samples are converted into code (Binary), then the code numbers are transmitted as pulses. Thus the system of transmission is called pube code modulation(pcm).

Fig : a message regularly sampled. Quantization level are indicated.

The m(t) is analog message. The voltage range is +4v to -4v. The step size ‘s’ between 1V. Eight quantization level are used -3.5V to 3.5V.

We assume code 0 for -3.5V, code -2.5V.. Code 7 for 3.5V. Each code represented in binary 000 to 111.

The sampled value considered nearest quantized values. The values 1.3, 3.6, 2.3 etc but we are transmitting representation 101,111,110 etc.

PCM Encoder : The analog signal m(t) is sampled and these sample are Quantized. The quantized samples are fed to an encoder, for each samples the encoder generates the binary pubes.

The quantizer and encoder are togethered called Analog - to - digital converter.

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Decoder : On receiving a digitally encoded signal at the receiver. first signal to be separated from noise so re-quantized. The decoder convert the digital signal to analog signal inversion operation of encoder the decoder O/P is quantized PAM. The PAM is applied to the filter then the O/P is m(t) original signal reconstructed.

Quantization noise is Pcm : The Quantization of a signal which is rise to random error variation which is called quantization noise. This noise can be reduced by choosing smaller step size S.

To calculate the rms quantization noise, equal spacing between levels. Let signal at the transmitter be initially quantized into M-levels, with S as the spacing in volts between adjacent levels.

P S = ----M

and A = (M-1) S volts. e - represents the error voltage between the actual signal and its quantized equalent, the mean - squared value of Ń&#x201D; will than be

2

S -2

e

= ---12

the power ration is S0

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---- = 12 M

74

2

N0 in decibels,

S0 ---- = 10.8 + 20 LOG10 m.

N0 M - Number of levels N - Number of pulses in the code group n - Number of code levels.

S0 N

---- = 12n 2 N0 Advantages of pcm :

1. Increase the O/P signal to noise ratio at the receiver.

2. Pcm permits repeating (or) amplifying the encoded signal.

3. Signal to noise ratio increases exponentially with bandwidth.

4. A pcm system designed for analog message transmission is readily adopted to other I/P signals.

Demerits of Pcm : 1. Requires larger band width.

2. Increases the system complexity.

Applications of Pcm : 1. Pcm can be used for multi channel telephone communication.

2. Pcm also finds use in space communication.

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4.3.DELTA MODULATION : It is a technique by which an analog signal can be encoded into binary digits. Hence delta modulation is a Pcm system.

The pulse generator gives a regular recurring train of pulses Pi(t), of fixed amplitude and polarity.

The modulator receives the input pulses Pi(t) as well as a signal s(t). The modulator output P 0(t) is the input pulse train Pi(t) multiplied by +1 or -1 depending on the polarity of ∆(t). If ∆(t) is positive when P i(t) occurs, the multiplication is by +1, if ∆(t) is negative then by -1.

The output waveform P0(t) is applied to an integrator the output of which is designated as m(t).

The signals m(t) and m(t) are compared in a difference amplifier. The amplifier output ∆(t) is given by ∆(t) = m(t)- m(t).

Hence the modulator output pulses is positive. The integrator response to this pulse is an abrupt positive step. At time t2. ∆(t) = m(t) - m(t) is still positive with the result that m(t) steps positively again. The waveform m(t) continues to rise in steps to approach m(t) through the fourth pulse, at which time m(t) overshoots its mark. Thus immediately after the fourth pulse ∆(t) is negative and the next pulse in the modulator output is of negative polarity. The step size approximation of m(t) by m(t) when there is signal variation. The pulse wave form P0(t) is the signal which is transmitted.

LIMITATIONS OF DELTA MODULATION : The fixed step in m(t), imposed on DM a limitation, not encountered in other pulse – modulation schemes and results in overloading when the signal changes too rapidly.

However, delta modulation exhibits an additional type of overload not encountered in other modems. This overload appears when the modulating signal changes between samplings, by an amount greater than the size of a step. Hence this kind of overload is not determined by the amplitude but by the slope of the modulating signal. A signal m(t) is shown which is changing about as rapidly as the modulator can follow.

We see a signal of the same peak to peak amplitude but of increased rate of rise.

In this case we see that the slope of the modulator output increases no more rapidly than it did in the previous case. Thus m(t) does not follow m’(t) in this region of increased slope. The DM system is therefore said to be slope – overloaded.

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4.4. ADAPTIVE DELTA MODULATION : Delta modulation system which adjusts its step size. It is therefore known as adaptive delta modulation system. The amplifier shown has a variable gain. ie., its gain a function of the voltage applied at its gain. Control terminal the characteristics is zero, its gain is low and that the gain rises with increasing positive gain-control voltage. The RC combination acts as an integrator.

The square law device ensures that whatever be the polarity of the voltage across the capacitance C, a positive voltage will be applied to the gain-control terminal of the amplifier.

Now assume that m(t) is making only small excursions so that the modulator does not follow it. The output p0(t) then consists of alternate polarity pulses. When integrated these pulses yeid an average output of almost zero. So, the step size is reduced.

If m(t) increases positively or negatively at a very fast rate, then m(t) cannot follow it. The output p 0(t) is then a train of all positive or negative pulses. The integrator averages and gives a large voltage output to increase the gain of the amplifier, by using square law circuit. It is necessary that there be an adaptive adjustment of the stop size at the receiver end as well.

By using this type we can avoid the problem of overloading.

4.5. TIME DIVISION MULTIPLEXING (TDM) In Time Division Multiplexed system, different time-intervals rather than frequencies are allotted to different signals. During these intervals, these signals are sampled and transmitted. Thus, this system transits information intermittently rather than continuously. Continuously varying analogue signals have to be sampled at proper intervals for transmission and the receiver must recognize these samples for TDM system of operated properly. Shows the basic features of TDM system.

Each signal source is switched in for a fixed time-interval by a time switch. S1. During this time, the connected signal modulates he carrier of the transmitter. The process is repeated by the time-switch which must rotate continuously at a uniform speed for proper operation of the system. The time for which a signal is connected to the transmitter and the time gap between the instants when the first signal is disconnected and the second is connected to the transmitter is very important.

For this, not only should the speed of time-switch S1 be kept uniform, but it is also required that motion of electronic circuits are used as electronic switches and synchronizing pulses are transmitted to keep these switches synchronized. The time for which a signal remains connected to the transmitter and the frequency at which the switch rotates are very important and related to the highest frequency in the signal. The relationship between them is governed by the sampling Theorem.

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When the transmitter and receiver switches are synchronized, the signals will be fed in the proper sequence to the receiver channels. The samples from transmitter channel one will be fed to receiver channel one. In this way, many channels of audio are combined to form a single output (multiplexed) chain.

Time spacing occurs between the components of the separate channels. The chain is transmitted (via wire or radio path) to distant demultiplexing receivers. Each receiving channel functions to select and reconstruct only the information included in the originally transmitted channel.

In most present day applications, electronic switching is used as the sampling component. The main advantage to electronic sampling is the longer life of an electronic switch when compared to an electromechanical switch. We use a mechanical system in our example to make this concept easier for you to see. Now let's look at figure , view B, where channel one is shown sampled four times. (This is the output of channel one in our transmitter.)

Figure, view C, shows all six channels being sampled four times during each cycle. (This is the output of the rotating switch in our transmitter.) What you see here is a continuous, time-sharing waveform. More than six channels (perhaps 24 or more) may be used. As we increase the number of channels, the width of each sample segment must be reduced.

The problem with reducing the width of the pulse is that the bandwidth (bw) necessary for transmission is greatly increased. Decreasing the pulse width decreases the minimum required rise time of the sampling pulse and increases the required bandwidth. When you increase the number of channels, you increase the bw. The bw is also affected by the shape of the sampling pulse and the method used to vary the pulse.

4.7 DIGITAL COMMUNICATIONS : Digital signals such as that produced by different types of pulse modulation circuits, telegraphy signals or tele printer signals are often require to be transmitted over long distance by means of radio waves. Any of the usual modulating systems AM, FM or PM may be employed. The resulting methods are termed as 1. Amplitude shift keying (ASK) 2. Frequency shift keying (FSK) 3. Phase shift keying (PSK)

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4.7.1 AMPLITUDE SHIFT KEYING : In this method, the carrier wave is switched from one level to the other by the binary signal.

During the ‘1’ state, the ‘mark’ signal increases the carrier level, while during the ‘0’ state or space signal, the carrier level is reduced. This method is also termed as ON-OFF keying [00K] The carrier frequency is generated by a crystal oscillator, which is followed by a buffer amplifier to maintain good frequency stability. Again in the interests of maintaining good frequency stability the oscillator frequency is usually lower than the required carrier frequency and one or more frequency is a power amplifier that provides the required drive for the final RF amplifier. which is a class C state Antenna.

Although the keying circuit could be used to simply interrupt the current in the final amplifier by means of “make-break” contact, this could gives rise to undesirable transients and would be avoided in high – power circuits.

The more usual method is to use the keying signal to bias the class C into cut off for the off binary bits.

For radio transmission it is undesirable to have rapid changes in amplitude because these give rise to sideband spatter, and the digital modulating waveform is filtered to remove the sharp transitions so that the modulated waveform appears.

Unmodulated carrier signal ec(t) = Ec max cos (2πfct + Φc) modulated waveform is ec(t) = Ke m(t) cos (2πfct + Φc) em(t) = Binary modulated waveform BT = 2B B  Overall system band width BT  Bandwidth for modulated wave.

The bandwidth for the modulated wave. BT is equal to the B, is the overall system bandwidth for the binary signal.

In the particular case where raised cosine filtering is used on the base band signal.

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Thus for the ideal band width P=0, 00K provides a rate of 1bps/Hz. Demodulation of the 00K waveform may take place using a simple envelope detector. A more efficient method is to use synchronous detection.

ASK DEMODULATION : Synchronous detection requires a carrier recovery circuit, which is used to generate a local carrier component exactly synchronized to the transmitted carrier.

The spectrum contains a component at the carrier frequency that can be used to phase lock the VCO in a PLL. Applying the locally generated carrier and the received signal to the multiplier results in an output.

eout(t)

= Ae(t) cos (2πfct + φc) 2

= Aem(t) [cos (2πfct + φc)] = Aem(t) [cos (2πfct + φc)]

cos2 φ = 1 + cos2 φ /2 = ½ + cos2φ/2 = Aem(t) [1/2 + ½ cos 2 (2πfct + φc)] eout(t) = Aem(t) [1/2 + ½ cos (4πfct + 2φc)].

The second harmonic carrier is easily removed by filtering, leaving as the output. eout = Aem(t) / 2

PROBABILITY ERROR FOR COHERENT : The synchronous detection just described is also referred to as coherent detection. The coherent detector is more complicated than the envelop detector, but it results in a lower probability of error for a given signal – to – noise i/p.

Pbe = ½ erfc √Eb/2No Eb  bit energy

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No  carrier error frequency PROBABILITY ERROR FOR NON-COHERENT: The optimum non coherent detector requires that Eb/No >1, and for this condition.

Pbe = ½ e

-Eb

/2N0.

4.8. FREQUENCY SHIFT KEYING [FSK] : With frequency modulation, usually referred to as frequency shift keying [FSK], the carrier frequency is shifted in steps or levels corresponding to the levels of the digital modulating signal. In the case of a binary signal, two carrier frequencies are used one corresponding to the binary 0 that is space and the another to a binary 1 that is mark.

SINGLE OSCILLATOR METHOD OR CPFSK : The two carriers may be generated from separate oscillators, independent of one another, and this is indicated by separate subscripts for the amplitudes and fixed phase angle the combined signal can therefore have discontinuities in amplitude and phase.

Where a single oscillator is frequency modulator by the digital signal the method is referred to as continuous phase frequency shift keying (CPFSK). By treating the CPFSK wave as two 00K waves. The spectrum for the 00K wave can be used for each and the resultant spectrum is as sketched. A special and important case of CPFSK known as minimum shift keying (MSK). This is the minimum separation for which correlation between the two signaling wave forms is zero.

POWER SPECTRUM FOR CPFSK : Before modulation : Binary 0, e0(t) = A0 cos (2πf0t + α0)] Binary 1, e1(t) = A1 cos (2πf1t + α1)] The modulated signal is given by Binary 0, e0(t) = A cos 2πf0t Binary 1, e1(t) = A cos 2πf1t

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For MSK, if it is assumed that the spectrum beyond the first nulls can be ignored the overall spectrum bandwidth is BT = 3/2 Rb.

COHERENT DETECTION FOR CPFSK :

The coherent receiver can be constructed by using two separate 00K coherent detectors. The outputs are combined to form a polar binary signal, which for optimum detection, is then passed to a matched filter.

Correlation between the two signaling frequencies results in general in an increased probability of bit error, but with MSK the correlation is zero and the expression for bit-error probability is the same as that for 00K, that is Pbe = ½ erfc √Eb / 2 N0 Pbe = probability bit error NON COHERRENT DETECTION FOR CPFSK : Non coherent detection can also be used with FSK signal. Again, because FSK appears as two 00K waves, the non coherent receiver need consist only of two separate paths with band pass filters tunned to the individual frequencies.

Each filter followed by a envelop detector. The outputs are combined to form a polar waveform which is then passed as input to the pulse regenerator operating at zero voltage threshold. When properly adjusted for optimum performance, the probability of bit error for the non coherent FSK detector is given by Pbe = ½ erfc √Eb / 2 N0 The non-coherrent receiver is much simpler to built than the coherent receiver, and for many applications the degradation in bit – error probability is acceptable.

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4.9. PHASE SHIFT KEYING [PSK] : With phase modulation, usually referred to as phase shift keying [PSK], the binary signal is used to switch o

o

the phase between 0 and 180 . It is also known as phase reversal keying (PRK). The modulated, carrier is e(t) = Ec max cos (2πfct + φc), binary 1 o

e(t) = Ec max cos (2πfct + φc + 180 ), binary 0

BALANCED MODULATOR PRODUCING BPSK : DETECTION OF BPSK : The modulating signal is polar NRZ, and when it is +1, the modulated output is +1 x Ec max cos (2πfct + φc) = Ecmax cos(2πfct + φc) and when it is -1, the modulated output is o

-1 x Ec max cos (2πfct + φc) = Ecmax cos(2πfct + φc + 180 ) WAVEFORM: The BPSK modulator is similar to the 00K modulator, the difference being that no dc component is present in the modulating waveform and therefore no carrier component is transmitted.

The BPSK wave has in effect a DSBSC spectrum. Coherent detection of BPSK followed by matched filter detection results in a bit error probability given by Pbe = ½ erfc √Eb / N0.

QPSK modulation : Quadrature phase shift keying (QPSK) utilizes four distinct levels of phase shift and is a widely used from a multilevel modulation.

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In this method a serial to parallel convertor is used to convert the binary signal p(t) into two separate binary signals, that is Pi(t) for in-phase and pq(t) for quadrature – phase components.

The in-phase component modulates a carrier to produce a BPSK signal, while the quadrature component o

modulates a carrier component shifted by 90 , also to produce a BPSK signal. Thus the QPSK signal is o

equivalent to two BPSK signals, but with the carriers 90 out of phase with one another.

WAVEFORM: Thus QPSK signaling requires one – half the bandwidth of BPSK signaling for the same input bit rate in both cases, and where raised cosine filtering is used.

QPSK demodulation : Detection of QPSK is similar to that for BPSK with the difference that the recovered carriers must also o

have the 90 phase difference. Assuming that the demodulated output is followed by a matched filter detector, the bit-error probably QPSK is the same as that for BPSK. By implementing of QPSK then bandwidth will be reduced and circuit will be complicated.

DIFFERENTIAL PHASE SHIFT KEYING [DPSK] : Differential encoding of base band signals is described, where a binary message sequence d k is shown differentially encoded as sequence ek. For case of comparison, this same sequence is used here, the ek sequence phase modulates a carrier cos wct. The output from the modulator is a BPSK signal, but because it is modulated by a differentially encoded signal, it will be referred to as a differentially phase shift keyed φ DPSK signal.

By assigning ak = +1V to a binary 1 and ak = -1V to binary 0, the DPSK signal can be represented by DPSK = ak cos wct. o

It will be sent that a binary 1 corresponds to no phase shift and a binary 0 to a 180 phase shift in the carrier.

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dk (logic levels)

1

1

0

1

0

0

1

dk (logic levels)

1

0

0

1

1

1

0

Dk (volts)

+1

-1

-1

+1

+1

+1

-1

ak (volts)

+1

-1

-1

+1

+1

+1

-1

ak-1 (volts)

-1*

+1

-1

-1

+1

+1

+1 * Initial state

* Initial state V0 (volts)

-1

V0

-1

+1

-1

+1

+1

(logic

levels) 0 0 1 0 1 1 0

V0 (logic

V0 0

(logic levels)

-1

1

0

1

1

0

levels) V0 (logic levels)

0

The logic levels corresponding to V0 are the message levels inverted, and an inverter is required to restore the message sequence at the output comparison of the inverted output V 0 with the binary input dk shows these to be the same.

V0 = Aak ak-1 = +A for ak = ak-1 =

-A for ak â&#x2030; ak-1

more generally, it can be shown that the multiplication followed by inversion is equivalent to the exclusive â&#x20AC;&#x201C; OR decoder operation for the differentially encoded base band signal.

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The disadvantage with DPSK is that bit errors tend to occur in pairs, because the polarity of a given bit depends on the polarity of the proceeding bit. The average bit error probability for DPSK. (-Eb/N0)

Pbe = ½ e

4.10. Comparison of ASK, FSK and PSK: Amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK) are digital modulation schemes.

ASK refers to a type of amplitude modulation that assigns bit values to discrete amplitude levels.

The carrier signal is then modulated among the members of a set of discrete values to transmit information.

FSK refers to a type of frequency modulation that assigns bit values to discrete frequency levels.

FSK is divided into noncoherent and coherent forms. In noncoherent forms of FSK, the instantaneous frequency shifts between two discrete values termed the "mark" and "space" frequencies.

In coherent forms of FSK, there is no phase discontinuity in the output signal. FSK modulation formats generate modulated waveforms that are strictly real values, and thus tend not to share common features with quadrature modulation schemes.PSK in a digital transmission refers to a type of angle modulation which the phase of the carrier is discretely varied—either in relation to a reference phase or to the phase of the immediately preceding signal element—to represent data being transmitted.

For example, when encoding bits, the phase shift could be 0 degree for encoding a "0," and 180 degrees for encoding a "1," or the phase shift could be –90 degrees for "0" and +90 degrees for a "1," thus making the representations for "0" and "1" a total of 180 degrees apart.

In PSK systems designed so that the carrier can assume only two different phase angles, each change of phase carries one bit of information, that is, the bit rate equals the modulation rate. If the number of recognizable phase angles is increased to four, then 2 bits of information can be encoded into each signal element; likewise, eight phase angles can encode 3 bits in each signal element.

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UNIT V MICROWAVE PROPAGATION AND DEVICES

5.1 MICROWAVES :

Are electromagnetic waves with wavelengths longer than those of Terahertz (TH Z) wavelengths, but relatively short for radio waves. Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). However, the boundaries between far infrared light, Terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. A credible definition comes from Pozar’s text “Microwave Engineering”, which states that the term microwave “refers to alternating current signals with frequencies between 300 8

11

MHz ( 3 x 10 Hz) and 300 GHz (3 x 10 Hz).”

The microwave range includes ultra-high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz), and extremely high frequency (EHF) (30-300 GHz) signals. Above 300 GHz, the absorption of

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electromagnetic radiation by Earth’s atmosphere is so great that it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Microwaves can be generated by a variety of means, generally divided into two categories: solid state devices and vaccum-tube based devices. Solid state microwave devices are based on semiconductors such as silicon or gallium arsenide, and include filed – effect transistors (FET’s), bipolar junction transistors (BJT’s), Gunn diodes, and IMPATT diodes. Specialized versions of standard transistors have been developed for higher speed, which are commonly used in microwave applications.

Microwave variants of BJT’s include the hetero junction bipolar transistor (HBT), and microwave variants of FET’s include the MESFET, the HEMT (also known as HFET), and LDMOS transistor. Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, traveling wave tube (TWT), and gyrotron.

Uses A microwave oven uses a magnetron microwave generator to produce microwaves at a frequency of approximately 2.45 GHz for the purpose of cooking food. Microwaves cook food by causing molecules of water and other compounds to vibrate or rotate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method.

o

Microwaves are used in broadcasting transmissions because microwaves pass easily through the earth’s atmosphere with less interference than longer wavelengths. There is also much more bandwidth in the microwave spectrum than in the rest of the radio spectrum. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van.

o

Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.

o

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and b specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses an ISM band in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5 – 4.0 GHz range.

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o

Metropolitan

Area

88

Networks

–

MAN

protocols,

such

as

WiMAX

(Worldwide

Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. o

Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some mobile phone networks, like GSM, also use the lower microwave frequencies.

o

Many semiconductor processing techniques use microwaves to generate plasma for such purpose as reactive ion etching and plasma – enhanced chemical vapor deposition (PECVD).

o

Microwaves can be used to tranmit power over long distances, and post – World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar power satellite (SPS) systems with large solar arrays that would beam power down to the earth’s surface via microwaves.

o

A maser is a device similar to a laser, except that it works at microwave frequencies.

5.2 MICROWAVE FREQUENCY BANDS

The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency Bands are defined in the table below:

Microwave frequency bands Designation

Frequency range

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L band S band C

band 1

to

2

GHz 1 to 2 GHz 2 to 4 GHz 4 to 8 GHz

X band 8 to 12 GHz

Ka band

26.5 to 40 GHz

Q band

30 to 50 GHz

U band

40 to 60 GHz

V band

50 to 75 GHz

E band

60 to 90 GHz

W band

75 to 110 GHz

F band

90 to 140 GHz

D band

1 1 0

t o

1 7 0

G H z

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The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes for UHF frequencies below L-band. For other definitions see Letter Designations of Microwave Bands.

5.3 WAVEGUIDE: A circular, elliptical or rectangular metal tube or pipe through which electromagnetic waves are propagated in microwave and RF communications. The wave passing through the medium is forced to follow the path determined by the physical structure of the guides. If the guiding pipe is perfectly conducting, the modes may be divided into transverse magnetic (TM) and transverse electric (TE) types.

For the former, the magnetic field is confined to the transverse plane, while the electric field is so confined for the latter. These classifications remain useful for the practical metallic conductors used for such guides.

Other important waveguide elements are the directional coupler and isolator. In the directional coupler, there is coupling to an auxiliary guide in such a way that the output of one of its ports is proportional to the wave traveling in the forward direction, and the output of the other is proportional to the reverse wave.

The isolator makes use of the nonreciprocal properties of ferrites with an applied magnetic field to pass the forward traveling wave of the guide but to eliminate the reflected wave.

A dielectric waveguide consists of one dielectric material, called the core, surrounded by a different dielectric, called the cladding, The permittivity (dielectric constant), or refractive index, of the core is larger than that of the cladding, and under proper conditions electromagnetic energy is confined largely to the core through the phenomenon of total reflection at the boundary between the two dielectrics. See also permittivity; Reflection of electromagnetic radiation; Refraction of waves.

INTRODUCTION : A waveguide may be connected at one end no an antenna and at the other end to a transmitter (or) receiver. The wave guide, in fact guides waves from antenna to the receiver (or) from transmitter to the antenna, or as the case may be wave guide can guide waves between any two desired points.

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The walls of a wave guide are made of conductors, and therefore waves get reflected from them.

ADVANTAGES 1. A circular wave guide has an appearance similar to a coaxial line, but with its inside conductor pulled out, This is the biggest advantage of a wave guide over coaxial.

2. Power handling capacity is ten times greater than coaxial cable.

3. Lower power loss 4. Maximum operating freq (325 GHZ)

BASIC BEHAVIOUR : A plane electromagnetic wave propagates in space in transverse. electromagnetic or TEM mode and in this mode the electric field. Magnetic field and the direction of propagation are all mutually perpendicular.

If such wave is fed into a wave guide, this wave fails to propagate, because the electric field short circuited by the walls became the walls are conductor.

So, sending the wave in a zigzag fashion and making them to reflect from the side walls, so no walls can short circuit, thus propagation is not bindered.

Two point arise for zig zag propagation, The velocity of propagation is a wave guide must be less than in free space, and the second the waves no longer be in TEM mode.

There is no component of electric field in the direction of propagation are called transverse â&#x20AC;&#x201C; electric (TE) modes, and modes with no such component of magnetic field are called transverse â&#x20AC;&#x201C; magnetic (TM).

5.3.1 CAVITY RESONATORS : o

A simplest cavity resonator consist of a wave guide closed off at both ends with metallic planes.

o

If propagation is in the longitudinal direction, standing waves will set up in the closed strecture, and the closed unit will start resonating.

BASIC OF CAVITY RESONATOR :

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92

In low frequency circuits the LC resonator circuits are used, resonant cavities are used to perform the same function.

OPERATION : o

The two ends of a waveguide may be closed, the distance between two conducting walls in n.ÎťP/2, where n is integer.

o

A little amount of microwave (signal) be fed into the closed unit, then multiple reflections occur from the two wall s and standing waves are established resulting in oscillations, the oscillations goes infinite time.

o

But some losses will occur and so these oscillations die down after some time.

o

Each cavity resonator has an infinite number of resonant frequencies, oscillations obtained at twice the frequency.

TYPES OF CAVITIES : o

Cavities can be made in various shapes, such as spheres, cylinders (or) rectangular prisms, But the drawback in the cavities are the pulse energy is required for oscillations in the cavity.

Fig a is popular in Magnetrons, These type of cavities are known as re â&#x20AC;&#x201C; entrant type, These resonators are shaped in a way that one of the walls re-enters the resonator shape.

APPLICATIONS : o

Cavity resonators are analogues to tuned LC circuits and resonant transmission lines of lower frequencies. Cavity resonators are used at high frequencies, which can be used as input or output tuned circuits or amplifier.

o

Tuned circuit of oscillators

o

resonant circuit used for filtering.

o

Cavity resonator is as a cavity wave meter, used for measuring microwave frequencies cavity wave meters are basically 2 types.

1. Transmission type 2. Reaction type

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TRANMISSION TYPE WAVE METER : o

The location of a transmission type wave meter in a microwave along with its equivalent circuit, The maximum energy reaches the load only when the cavity is tuned to its resonant frequency.

The O/P coupling loop with a detector. The detector probe will show the resonant frequency.

REACTION TYPE : It is also called absorption – type wave meter. This type of wave meter is more frequency used.

The distance d – is made either zero or approximately xp/2 so that on effective short is presented at the wave guide wall, whenever the cavity is for from resonance. Thus the magnitude depends on the Q of the cavity and distanced.

5.4. MULTICAVITY KLYSTRON : The Klystron was invented by varian brothers, just before world war II. Today it is widely used as a Microwave amplifier and oscillator.

It can provide very high powers.

OPERTION : (TWO CAVITY KLYSTRON) The following fig show two cavity klystron amplifier. A high velocity electron beam is obtained by the cathode, Focusing electrodes and collector. External Magnetic field is used.

The Electron beam passes the gap ‘A’ of buncher cavity, to which an RF signal to be amplified is coupled.

An interaction takes place between the electron beam and the RF field across the gap A. After intraction the electron beam drifts in the drift space, with out influence of RF signal, until it reaches gap B (catcher cavity). Oscillators will be excited in the catches cavity, these oscillations are much power as compared to buncher cavity oscillations, so, the amplification is achieved, the used electron beam collected is the collector electrode.

The two cavities are tunasce, the electron beam velocity is proportional to the collector current.

The electron beam velocity is contant at gap A, which will be affected by the RF voltage across this gap.

When the gap ‘A’ voltage is zero, the electron passing it will remain unaffected and travel towards the collector with the constant velocities. After some time, an input is fed to the buncher cavity (RF), when the voltage across the gap is going – ve to +ve of the sinusoidal cycle, at this instant the voltage is zero.

When the gap A (RF) voltage is zero, the passing electrons are unaffected, This electrons are called

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reference electron, which may be denoted as ‘y’. Another electron z, passes the gap slightly later than y as shown in the applegate diagram of fig.

No gap voltage, then both electron velocity unchanged. In reality, what happends is that electron Z is accelerated slightly by the now ‘+’ve voltage across the gap A, the accelerated electron catches the reference electron as shown in fig. Similarly, electron x passes gap A slightly before the reference electron. Although electron x passes the gap A before y electron, it is still not able to reach the gap B before the reference electron y. This is became the electron x is some what retarded by –ve voltage across the gap. Sine y electron was so retarded, it has an excellent change of catching electron x before gap B. The overall result is a velocity modulated electron beam by the RF voltage existing across the gap A. Velocity modulation will not provide the amplification. The intraction of electrons gives the velocity modulation beam, which passes the catcher cavity. The variation in current density is known as current – modulation. Bunching occurs only one per cycle.

Advantage : 1. Used as Amplifier 2. Used as Oscillators

5.5. REFLEX KLYSTRON : This device has only one cavity and is used as an oscillator. The electrons are made to pass twice through the same cavity.

FUNDAMENTALS : The reflex Klystron is used as a Microwave oscillator and has low O/P power with very low efficiency.

Magnetic focusing is not used because the device is very small. The cavity acts as an anode and has a high ‘+’ve voltage applied to it, which causes the electron beam to accelerate.

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Because of the large momentum obtained by the electrons, they overshoot the gap in the cavity and continue to move towards the next electrode, which infact never reach. This repeller electrode is applied a sufficient negative voltage so that all the electrons are repelled back and none is able to reach it.

Thus, electrons in the beam reach some point in the repeller space and are then turned back. By adjusting the supply the returning electrons can be adjusting the supply the returning electrons can be adjusted, and this sustained oscillators can be obtained.

OPERATIONS : The fig shows an applegate diagram, Here again we consider specific electrons, passing the gap for the first time at selected instants.

The reference electron y – is assumed to pass the gap when the gap voltage is zero and is going negative, Electron y, as was in multicavity – klystron case passes the gap without being affected and travels towards repeller electrode. It is lowever returned back to anode before even toutch the repeller.

The point from where the electron returns is governed by its velocity when it emerges from the gap, and inturn governs the time in between the two instants, i.e., when the electron left and when i.e., returned base to anode. Thus fast electron comes closer to the repeller as compared to slow ones, there fore talcs a longer time to return to the resonator gap than electrons, which do not approach the repeller so closely.

Electron x, which on its outward movement passes the resonant gap slightly before the ret. electron y. If no gap voltage, certainly the electron x returned back to the gap before the ref. electron.

The electron x gets accelerated to some extent because of the slightly ‘+’ve gap voltage, which comes closer to the repeller, and takes longer time to return the gap, than electron y.

Electron y – catches up with electrons x just as they return back to the resonator gap.

Similarly, electron 2 passes the gap a little after the reflection y. It falls –ve gap voltage, the z electron retarted. Thus electron z is not able to came closer to repeller as did x and y. The electron z takes a shorter time to return to the gap and so has a good chance of catching and bunching up with the electrons z and y. The velocity modulation converted into a current modulation then sufficient electron do bunch between the bunching limits, so the devices acts as a oscillator.

TRANSIT – TIME : Time taken by the ref electron from the instant if leaves the gap to the instant of its return, should have a correct value.

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T=n+ž T = transit time of electron is repeller spare. n = any integer.

MODES : The transit time of a reflex klystron is governed by the repeller and anode voltages, so there should be adjusted carefully. First the cavity is turned to the current freq and then repeller voltage should be adjusted.

For larger O/P power higher operating voltage is required, This result will be higher power dissipation of lower efficiency, also cause insulation problem.

When the cavity is correctly tuned and a suitable voltage is applied to the anode, any one of the available modes can produced by adjusting the repeller voltage.

5.6. MAGNETRON : A magnetron is a diode, which uses the nitration of magnetic and electric field in a cavity to provide oscillations of very high peak powers.

The first magnetion developed, used the so called cyclotron resonance principle, but suffered from erratic behaviour and low power capability .

CONSTRUCTIONAL FEATURES OF THE CAVITY MAGNETRON : Magnetron uses an axial magnetic field and a radial electric field. The cathode is surrounded by the anodes. The outer anode result in a d.c radial electric field. The magnetic field is obtained by a magnet.

The magnetic field pass through the cathode and interaction space.

In a magnetion the magnetic field is perpendicular to the plane of the electric field, it is also called a cross â&#x20AC;&#x201C; field device.

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The output can be taken from any one of the cavities. The anode construction is mostly of copper. o

o

o

If it has 8 â&#x20AC;&#x201C; cavities (anodes), the phase difference between the anodes is 30 (30 x 8 = 240 ). If the o

o

phase difference between the anodes is 45 (45 x 8 = 360 ) or 2Ď&#x20AC;. This mode is self consistent.

EFFECTS OF MAGNETIC FIELD : Any electron emitted by the cathode, which will be influenced by two different forces, namely the d.c magnetic field and the R.F electric field.

When electron moves, it constitutes an electric current. If a magnetic field is applied an this moving electron, a force is extended on this moving electron, whose magnitude is proportional to the product Bve, where V and e are the velocity and charge of the electron, respectively, and B is the magnetic field component in a plane perpendicular to the direction of electron motion.

As the magnetic field is constant, its forces depends on the velocity.

EFFECT OF ELECTRIC FIELD : The electric field acts along with the magnetic field on the electron. This results in a change in data of electron. The bellow fig shows the electron paths in the absence of oscillations and electric field is constant and radial and the axial magnetic field can have any value.

In case No Magnetic field, the electron goes straight from the cathode to the anode. The path 1 is indicated, when the magnetic field is zero. When the magnetic fields small, the electrons making to bend, the path 2 is indicated.

The electron velocity increases due to the accelerating force of electric field, the path will be more circular, the electron never reaches the anode, the path 3 is indicated.

If the anode current reduce to zero, the magnetic field is still stronger the electron takes data 4.

The electron reaches cathode much sooner. The electron path is modified by the RF field.

OPERATION : The absence of RF electric field, the electron a and b would follow the path shown by the dotted line a and b respectively the path modified RF electric field.

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EFFECTS OF COMBINED FIELDS : When the electron a is situated at point 1, the tangential component of electric field at that instant at point 1 is such that is opposes the tangential velocity of the electron, the returning electrons after being repelled back by the repeller. The electrons is returned and got retarted.

Thus, electron a also gets retarted, on the other hand, electron b is so placed as to extract an equal amount of energy from the RF field by virtue of being accelerated.

The electron a spends much more time is the RF field as compared to that spent by electron b.

Now the conditions are changed the electron ‘a’ reaches point 2 it again faces a retarding field. So that once again it has to give energy to the RF field. This is achieved by reversing the polarity of the RF electric field. Due to the magnetic field effort electron ‘a’ once again interaction takes place at point 3. The reversing polarity of RF electric field electron ‘a’ losing its energy finally ends on the anode.

The electron b is different form electron a. Electron b is immediately accelerated by the RF field. This electron returns to the cathode in a very short time. It spends less time in tae interaction space as compared to electron ‘a’. In RF field the interaction of electron like b type, resulting sustained oscillations in the magnetron. The unfavorable electron (like b) return back to the cathode, This is called back heating so, the magnetron considered as switched on.

Applications : 1. Used in pulsed microwave radars 2. Used in Microwave ovens 5.7. TRAVELLING WAVE TUBE (TWT) TWT is mostly used as a microwave wideband amplifier. The continuous intraction between the electron beam and signal takes place.

A TWT can be used as a low level. Low noise amplifier or as a high power one.

BASICS OF TWT:The RF field is fixed but the electron beam travels. There exists a great difference between the velocity of RF field and the maximum achievable electron beam velocity, the RF field is retarted with the help of a helix (or) waveguide.

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CONSTRUCTION: An electron gun is used to produce a very narrow electron beam, The collector is given a high positive potential as compared to the cathode to accelerate the electrons in the electron beam. A d.c magnetic field is used to focus the electron beam.

Helix coil is made positive with respect to the cathode.

The RF field travels around the helize, The axial speed of RF field is equal to the velocity of light multiplied by the ratio of helix pitch to helix circumferences. This, the continuous intraction can be achieved in this way.

OPERATIONS: TWT is similar to the cavity magnetion. The difference is method of intraction. In TWT electrons leave the cathode they encounter the weak input signal on the helix. Intraction, i.e., velocity modulation takes place.

The first bunch formed at the next turn of helix.

The intraction process continuous as the wave and the electron beam travel towards the output end of the helix, it becomes maximum the remaining electrons reaches the collector.

APPLICATIONS: 1. TWT is mostly used in Microwave tubes 2. Low power, low noise device.

3. Used in radars and other microwave receivers.

5.8 GUNN DIODE: A Gunn diode, also known as a transferred electron device (TED), is a form of diode used in highfrequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between.

When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Eventually, this layer starts to conduct, reducing the gradient across it, preventing further conduction. In practice, this means a Gunn diode has a region of negative differential resistance.

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The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device. The oscillation frequency is determined partly by the properties of the thin middle layer, but can be adjusted by external factors.

Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonant cavity is usually added to control frequency. The resonator can be based on a waveguide, coaxial cavity, YIG resonator, etc. Tuning is done mechanically, by adjusting the parameters of the resonator, or in case of YIG resonators by electric current.

Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.The Gunn diode is named for the physicist J.B. Gunn who, in 1963, produced the first device based upon the theoretical calculations of Cyril Hilsum.

APPLICATIONS 

Thin diodes act as amplifiers



Thick diodes show time of flight effects for the electrons and are narrow-banded



A bias tee is needed to isolate the bias current from the high frequency oscillations

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UNIT QUESTIONS UNIT â&#x20AC;&#x201C; I ASSESMENT QUESTIONS: 1. Write about the elements of communication system.

2. Describe about the soueces of information.

3. Explain the operation of signal transmission and reception.

4. Write about antennas and its types.

5. Explain about analog and digital type of communications.

UNIT II Assessment questions: 1. Define amplitude modulation.

2. Explain the generation and detection of AM modulation technique.

3. Explain the generation and detection of SSB modulation technique.

4. Write about DSB modulation and detection.

5. Explain about vestigial side band modulation.

6. Explain about frequency division multiplexing.

7. Explain the operation of radio transmitter.

8. Write about super heterodyne receiver.

9. Define frequency modulation.

10. Define phase modulation.

11. Explain the generation and detection of AM modulation technique.

12. Explain the operation of FM transmitter and receiver.

UNIT III ASSESSMENT QUESTIONS: 1. State sampling theorem.

2. Define pulse amplitude modulation.

3. Define pulse width modulation.

4. Define pulse position modulation.

5. Explain sampling theorem in detail.

6. Explain quantization process.

7. Explain about pulse modulation techniques in detail.

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UNIT IV ASSESSMENT QUESTIONS:

1. Define PCM. 2. Define quantization error.

3. Define multiplexing.

4. Explain about the principles of PCM in detail.

5. Explain about quantization process.

6. Explain about delta modulation.

7. Explain about adaptive delta modulation.

8. Write about time division multiplexing in PCM. 9. Explain about amplitude shift keying.

10. Explain about frequency shift keying.

11. Explain about phase shift keying.

12. Compare ASK, FSK and PSK in detail.

UNIT V ASSESSMENT QUESTIONS:

1. What are the types of antennas?

2. Define microwave system.

3. Define waveguides.

4. Draw the diagram of multicavity klystron.

5. Explain about antennas in detail.

6. Explain about ground wave propagation.

7. Explain about sky wave propagation.

8. Explain about space wave propagation.

9. Explain about two cavity klystron with neat sketch.

10. Explain about magnetron operation.

11. Write about TWT in detail.

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12. Explain about gun diode with neat diagram.

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