Basic Electronics
Communication Systems
Communication Systems Syllabus: Communication Systems: Introduction, Elements of Communication Systems, Modulation: Amplitude Modulation, Spectrum Power, AM Detection (Demodulation), Frequency and Phase Modulation. Amplitude and Frequency Modulation: A comparison. (6 Hours)
Introduction One of the greatest applications of electrical technology is communication systems. Communication is the process of transferring information from one point to the other. Information may be in the form of voice, text, picture or a combination of these.
Elements of Communication System Fig. 1 shows a block diagram of a communication system.
Source
Modulator and Transmitter
Channel
Demodulator and Receiver
Destination
Noise
Fig. 1 Block diagram of a communication system Source: The aim of a communication system is to convey a message and this message originates from a source. Common examples of source are analog audio, video or some digital data. Modulator and Transmitter: It processes the message signal from the source and makes it suitable for transmission over the channel. The transmitter consists of encoders, decoders, transducers, amplifiers, etc. Channel: It is the physical medium that connects transmitter and receiver. Communication channels can be a pair of conductors, optical fiber or just free space. Noise: Noise is random, unwanted energy that gets added to the message signal during transmission over the channel. Demodulator and Receiver: It performs the reverse process of modulation and transmission. The receiver processes the signal and gets back the actual message that is transmitted. It performs demodulation and extracts the message signal from the carrier wave. The receiver consists of amplifier, detector, mixer, oscillator, transducer, etc.
Modulation Baseband Communication A signal in its original frequency is called a baseband signal and transfer of these signals directly over the channel is called baseband communication. Shrishail Bhat, Dept. of ECE, AITM Bhatkal
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However, the baseband signals are not suitable for transmission, as they get attenuated and cannot travel longer distances. Hence modulation is used.
Modulation Modulation is a process in which some characteristic or property of a high frequency signal called carrier signal is varied in accordance with the instantaneous amplitude of the message signal. The message signal is called the modulating signal. The carrier signal is a sinusoidal signal that can be represented as (1)
đ?’—đ?’„ (đ?’•) = đ?‘˝đ?’„ đ??Źđ??˘đ??§(đ??Žđ?’„ đ?’• + đ?œ˝) where đ?‘Łđ?‘? (đ?‘Ą) is instantaneous voltage as a function of time đ?‘‰đ?‘? is peak amplitude đ?œ”đ?‘? is angular frequency (rad/s), đ?œ”đ?‘? = 2đ?œ‹đ?‘“đ?‘? where đ?‘“đ?‘? is carrier frequency in Hz đ?‘Ą is time in seconds đ?œƒ is phase angle in radians
The characteristic of the carrier wave that is modified may be amplitude đ?‘‰đ?‘? , frequency đ?‘“đ?‘? or phase angle đ?œƒ. Accordingly, we have three types of modulation: 1. Amplitude Modulation 2. Frequency Modulation 3. Phase Modulation The modulated signal is not a single frequency signal and it occupies a great bandwidth. The bandwidth of the modulating signal also depends on the modulating signal frequency range and the modulating scheme in use. Table 1 gives the commonly used frequency ranges and their applications. Table 1 Commonly used frequency ranges and applications Frequency Range
2
Applications
Super high frequencies (3 GHz – 30 GHz)
Radar
Ultra high frequencies (300 MHz – 3 GHz)
Communication satellites, cellular phones, personal communication systems
Very high frequencies (30 MHz – 300 MHz)
TV and FM broadcast
High frequencies (3 MHz – 30 MHz)
Short-wave broadcast commercial
Medium frequencies (300 kHz – 3 MHz)
AM broadcast
Low frequencies (30 kHz – 300 kHz)
Navigation, submarine communications
Very low frequencies (3 kHz – 30 kHz)
Submarine communications, navigation
Voice frequencies (300 Hz – 3 kHz)
Audio, submarine communications, navigation
Extremely low frequencies (30 Hz – 300 Hz)
Power transmission Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
Need for Modulation Since the baseband signals are incompatible for direct transmission over the channel, modulation technique is used. The advantages of using modulation are as listed below: 1. Reduces the height of antenna: The minimum height of antenna required is given as đ?œ†/4. đ?‘?
The wavelength đ?œ† is given as đ?œ† = đ?‘“ where đ?‘? is the velocity of light and đ?‘“ is the frequency. Modulation increases the frequency of the signal to be radiated and thus reduces the wavelength, which reduces the size of the antenna required. 2. Avoids mixing of signals: Different signals from different sources can be sent over the same channel by using different carrier frequencies for these signals. This avoids mixing of signals. 3. Increases the range of communication: Modulation increases the frequency of the signal to be radiated and thus increases the distance over which the signals can be transmitted. 4. Allows multiplexing of signals: Multiplexing means transmission of two or more signals simultaneously over the same channel. Different signals from different sources can be sent over the same channel by using different carrier frequencies for these signals. 5. Allows adjustments in the bandwidth: Bandwidth of a modulated signal can be made smaller or larger than the original signal. Signal to noise ratio (SNR), which is a function of bandwidth, can thus be improved. 6. Improves quality of reception: Modulation reduces the effect of noise to great extent and thus improves the quality of reception.
Amplitude Modulation Amplitude Modulation is a process in which the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the message signal. Fig. 2 shows a modulating signal, a higher frequency carrier and the amplitude modulated signal. The instantaneous value of the message signal (modulating signal) is đ?’—đ?’Ž (đ?’•) = đ?‘˝đ?’Ž đ??Źđ??˘đ??§ đ??Žđ?’Ž đ?’•
(2)
where đ?‘Łđ?‘š (đ?‘Ą) is instantaneous amplitude of modulating signal đ?‘‰đ?‘š is peak amplitude of modulating signal đ?œ”đ?‘š is angular frequency (rad/s), đ?œ”đ?‘š = 2đ?œ‹đ?‘“đ?‘š where đ?‘“đ?‘š is modulating frequency in Hz The instantaneous value of the carrier signal is đ?’—đ?’„ (đ?’•) = đ?‘˝đ?’„ đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’•
(3)
where đ?‘Łđ?‘? (đ?‘Ą) is instantaneous voltage of carrier signal đ?‘‰đ?‘? is peak amplitude of carrier signal đ?œ”đ?‘? is angular frequency (rad/s), đ?œ”đ?‘? = 2đ?œ‹đ?‘“đ?‘? where đ?‘“đ?‘? is carrier frequency in Hz
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
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��
đ?‘‰đ?‘?
đ?‘‰đ?‘? + đ?‘‰đ?‘š
Fig. 2 Amplitude modulation The amplitude of amplitude modulated signal is then given by
Using Eqn. (2) in (4)
đ?‘‰đ??´đ?‘€ = đ?‘‰đ?‘? + đ?‘Łđ?‘š (đ?‘Ą)
(4)
đ?‘‰đ??´đ?‘€ = đ?‘‰đ?‘? + đ?‘‰đ?‘š sin đ?œ”đ?‘š đ?‘Ą
(5)
The instantaneous value of amplitude modulated signal is then given by (6)
đ?‘Łđ??´đ?‘€ (đ?‘Ą) = đ?‘‰đ??´đ?‘€ sin đ?œ”đ?‘? đ?‘Ą Using Eqn. (4) in (6)
đ?’—đ?‘¨đ?‘´ (đ?’•) = (đ?‘˝đ?’„ + đ?‘˝đ?’Ž đ??Źđ??˘đ??§ đ??Žđ?’Ž đ?’•) đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’•
(7)
Eqn. (7) is the equation of the AM wave.
Modulation Index Modulation index is defined as the amount by which the carrier amplitude gets modified by the modulating signal. It is also called modulation factor, modulation coefficient or the degree of modulation. For amplitude modulation, the modulation index is given by đ?’Ž=
�� ��
(8)
where �� is peak amplitude of modulating signal 4
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
đ?‘‰đ?‘? is peak amplitude of carrier signal The modulation index of AM is a number between 0 and 1 and is often expressed as a percentage and called the percentage modulation.
Modulation Index in terms of đ?‘˝đ?’Žđ?’‚đ?’™ and đ?‘˝đ?’Žđ?’Šđ?’? Fig. 3 shows amplitude modulated wave in time domain.
Fig. 3 Amplitude modulated wave From Fig. 3, �� = and
đ?‘˝đ?’Žđ?’‚đ?’™ −đ?‘˝đ?’Žđ?’Šđ?’?
(9)
đ?&#x;?
đ?‘‰đ?‘? = đ?‘‰đ?‘šđ?‘Žđ?‘Ľ − đ?‘‰đ?‘š
(10)
Substituting Eqn. (9) in Eqn. (10), đ?‘‰đ?‘šđ?‘Žđ?‘Ľ −đ?‘‰đ?‘šđ?‘–đ?‘›
đ?‘‰đ?‘? = đ?‘‰đ?‘šđ?‘Žđ?‘Ľ − ( đ?‘‰đ?‘? =
2
)
2đ?‘‰đ?‘šđ?‘Žđ?‘Ľ − đ?‘‰đ?‘šđ?‘Žđ?‘Ľ + đ?‘‰đ?‘šđ?‘–đ?‘› 2
�� =
đ?‘˝đ?’Žđ?’‚đ?’™ +đ?‘˝đ?’Žđ?’Šđ?’? đ?&#x;?
(11)
Now we have modulation index đ?‘‰đ?‘šđ?‘Žđ?‘Ľ − đ?‘‰đ?‘šđ?‘–đ?‘› đ?‘‰đ?‘š 2 đ?‘š= = đ?‘‰ + đ?‘‰đ?‘šđ?‘–đ?‘› đ?‘‰đ?‘? đ?‘šđ?‘Žđ?‘Ľ 2 đ?‘˝
−đ?‘˝
đ?’Ž = đ?‘˝đ?’Žđ?’‚đ?’™ +đ?‘˝đ?’Žđ?’Šđ?’? đ?’Žđ?’‚đ?’™
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
đ?’Žđ?’Šđ?’?
(12)
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Overmodulation In AM wave, overmodulation takes place when modulation index đ?‘š > 1, i.e. when đ?‘‰đ?‘š > đ?‘‰đ?‘? . In overmodulated AM wave, loss of information takes place and hence it must be avoided. Fig. 4 shows an overmodulated wave when đ?‘š = 1.25.
Fig. 4 Overmodulated AM wave Fig. 5 shows an AM wave when đ?‘š = 1.
Fig. 5 AM wave when đ?‘š = 1
Frequency Spectrum We know that the amplitude modulated signal is đ?’—đ?‘¨đ?‘´ (đ?’•) = (đ?‘˝đ?’„ + đ?‘˝đ?’Ž đ??Źđ??˘đ??§ đ??Žđ?’Ž đ?’•) đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’• 6
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
We also know that modulation index is given by đ?’Ž=
�� �� (13)
∴ đ?‘˝đ?’Ž = đ?’Žđ?‘˝đ?’„ Using Eqn. (13) in equation for đ?‘Łđ??´đ?‘€ (đ?‘Ą), we get đ?‘Łđ??´đ?‘€ (đ?‘Ą) = (đ?‘‰đ?‘? + đ?‘šđ?‘‰đ?‘? sin đ?œ”đ?‘š đ?‘Ą) sin đ?œ”đ?‘? đ?‘Ą đ?’—đ?‘¨đ?‘´ (đ?’•) = đ?‘˝đ?’„ (đ?&#x;? + đ?’Ž đ??Źđ??˘đ??§ đ??Žđ?’Ž đ?’•) đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’•
(14)
đ?’—đ?‘¨đ?‘´ (đ?’•) = đ?‘˝đ?’„ đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’• + đ?’Žđ?‘˝đ?’„ đ??Źđ??˘đ??§ đ??Žđ?’Ž đ?’• đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’•
(15)
1
Using the trigonometric relation sin đ??´ sin đ??ľ = 2 [cos(đ??´ − đ??ľ) − cos(đ??´ + đ??ľ)], we get đ?’—đ?‘¨đ?‘´ (đ?’•) = đ?‘˝đ?’„ đ??Źđ??˘đ??§ đ??Žđ?’„ đ?’• +
đ?’Žđ?‘˝đ?’„ đ?&#x;?
đ??œđ??¨đ??Ź(đ??Žđ?’„ − đ??Žđ?’Ž )đ?’• −
đ?’Žđ?‘˝đ?’„ đ?&#x;?
Lower side band
Carrier
đ??œđ??¨đ??Ź(đ??Žđ?’„ + đ??Žđ?’Ž )đ?’•
(16)
Upper side band
Similarly, if đ?’—đ?’Ž (đ?’•) = đ?‘˝đ?’Ž đ??œđ??¨đ??Ź đ??Žđ?’Ž đ?’• and đ?’—đ?’„ (đ?’•) = đ?‘˝đ?’„ đ??œđ??¨đ??Ź đ??Žđ?’„ đ?’•, then đ?‘Łđ??´đ?‘€ (đ?‘Ą) = (đ?‘‰đ?‘? + đ?‘šđ?‘‰đ?‘? cos đ?œ”đ?‘š đ?‘Ą) cos đ?œ”đ?‘? đ?‘Ą đ?‘Łđ??´đ?‘€ (đ?‘Ą) = đ?‘‰đ?‘? cos đ?œ”đ?‘? đ?‘Ą + đ?‘šđ?‘‰đ?‘? cos đ?œ”đ?‘š đ?‘Ą cos đ?œ”đ?‘? đ?‘Ą 1
Using the trigonometric relation cos đ??´ cos đ??ľ = 2 [cos(đ??´ − đ??ľ) + cos(đ??´ + đ??ľ)], we get đ?’—đ?‘¨đ?‘´ (đ?’•) = đ?‘˝đ?’„ đ??œđ??¨đ??Ź đ??Žđ?’„ đ?’• + Carrier
đ?’Žđ?‘˝đ?’„ đ?&#x;?
đ??œđ??¨đ??Ź(đ??Žđ?’„ − đ??Žđ?’Ž )đ?’• +
���
Lower side band
đ?&#x;?
đ??œđ??¨đ??Ź(đ??Žđ?’„ + đ??Žđ?’Ž )đ?’•
(17)
Upper side band
From Eqn. (16) and (17), we can say that the first term represents unmodulated carrier and two additional terms represent two sidebands. The frequency of lower sideband is đ?‘“đ??żđ?‘†đ??ľ = đ?‘“đ?‘? − đ?‘“đ?‘š and the frequency of upper sideband is đ?‘“đ?‘ˆđ?‘†đ??ľ = đ?‘“đ?‘? + đ?‘“đ?‘š . Fig. 6 represents the frequency spectrum of AM wave.
Fig. 6 Frequency spectrum of an AM wave Shrishail Bhat, Dept. of ECE, AITM Bhatkal
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Bandwidth of AM Wave The bandwidth of an AM wave is given by đ??ľđ?‘Š = đ?‘“đ?‘ˆđ?‘†đ??ľ − đ?‘“đ??żđ?‘†đ??ľ đ??ľđ?‘Š = (đ?‘“đ?‘? + đ?‘“đ?‘š ) − (đ?‘“đ?‘? − đ?‘“đ?‘š ) đ?‘Šđ?‘ž = đ?&#x;?đ?’‡đ?’Ž
Spectrum Power The AM wave has three components: unmodulated carrier, lower sideband and upper sideband. Therefore, the power of an AM wave is the sum of carrier power đ?‘ƒđ?‘? , power in lower sideband đ?‘ƒđ??żđ?‘†đ??ľ and power in upper sideband đ?‘ƒđ?‘ˆđ?‘†đ??ľ . The total transmitted power is given as đ?‘ˇđ?‘ťđ?’?đ?’•đ?’‚đ?’? = đ?‘ˇđ?’„ + đ?‘ˇđ?‘łđ?‘şđ?‘Š + đ?‘ˇđ?‘źđ?‘şđ?‘Š
(18)
�
If average carrier voltage is ( đ?‘? ), average carrier power is given by √2
đ?‘‰ 2 ( đ?‘?) đ?‘ƒđ?‘? = √2 đ?‘… đ?‘ˇđ?’„ = Similarly, if average sideband voltage is (
đ?‘šđ?‘‰đ?‘? 2
√2
đ?‘˝đ?’„ đ?&#x;?
(19)
đ?&#x;?đ?‘š
) , average power in lower sideband and upper
sideband, đ?‘šđ?‘‰đ?‘? 2 ( 2 ) √2 đ?‘ƒđ??żđ?‘†đ??ľ = đ?‘ƒđ?‘ˆđ?‘†đ??ľ = đ?‘ˇđ?‘łđ?‘şđ?‘Š = đ?‘ˇđ?‘źđ?‘şđ?‘Š =
đ?‘… đ?’Ž đ?&#x;? đ?‘˝đ?’„ đ?&#x;?
(20)
đ?&#x;–đ?‘š
We can also write đ?‘ƒđ??żđ?‘†đ??ľ = đ?‘ƒđ?‘ˆđ?‘†đ??ľ But
đ?‘‰đ?‘? 2 2đ?‘…
đ?‘š2 đ?‘‰đ?‘? 2 đ?‘š2 đ?‘‰đ?‘? 2 = = Ă— 8đ?‘… 4 2đ?‘…
= đ?‘ƒđ?‘? . Therefore đ?‘ˇđ?‘łđ?‘şđ?‘Š = đ?‘ˇđ?‘źđ?‘şđ?‘Š =
đ?’Žđ?&#x;? đ?&#x;’
��
(21)
By using Eqn. (19) and (20) in Eqn. (18) , the average total transmitted power is then given by, đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™
8
đ?‘‰đ?‘? 2 đ?‘š2 đ?‘‰đ?‘? 2 đ?‘š2 đ?‘‰đ?‘? 2 = + + 2đ?‘… 8đ?‘… 8đ?‘…
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™ =
đ?‘‰đ?‘? 2 đ?‘š2 đ?‘š2 (1 + + ) 2đ?‘… 4 4
đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™
đ?‘‰đ?‘? 2 đ?‘š2 = (1 + ) 2đ?‘… 2
∴ đ?‘ˇđ?‘ťđ?’?đ?’•đ?’‚đ?’? = đ?‘ˇđ?’„ (đ?&#x;? +
đ?’Žđ?&#x;? đ?&#x;?
)
(22)
Modulation Index in terms of đ?‘ˇđ?‘ť and đ?‘ˇđ?’„ We know that total transmitted power đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™
đ?‘š2 = đ?‘ƒđ?‘? (1 + ) 2
đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™ đ?‘š2 = 1+ đ?‘ƒđ?‘? 2 đ?‘š2 đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™ = −1 2 đ?‘ƒđ?‘? đ?‘ƒđ?‘‡đ?‘œđ?‘Ąđ?‘Žđ?‘™ đ?‘š2 = 2 ( − 1) đ?‘ƒđ?‘? đ?‘ˇđ?‘ťđ?’?đ?’•đ?’‚đ?’?
đ?’Ž = √đ?&#x;? (
��
− đ?&#x;?)
(23)
Transmission Efficiency The transmission efficiency of AM wave is defined as the ratio of the transmitted power which contains the information to the total transmitted power. In an AM wave, the information is contained in the sidebands. The transmission efficiency is then given by,
đ?œź=
đ?‘ˇđ?‘łđ?‘şđ?‘Š +đ?‘ˇđ?‘źđ?‘şđ?‘Š đ?‘ˇđ?‘ťđ?’?đ?’•đ?’‚đ?’?
(24)
Using Eqn. (21) and (22) in Eqn. (24), we get đ?‘š2 đ?‘š2 đ?‘ƒ + đ?‘? 4 đ?‘ƒđ?‘? đ?œ‚= 4 đ?‘š2 đ?‘ƒđ?‘? (1 + 2 ) đ?‘š2 đ?‘š2 đ?‘ƒđ?‘? ( 4 + 4 ) đ?œ‚= đ?‘š2 đ?‘ƒđ?‘? (1 + 2 ) đ?‘š2 đ?‘š2 2 2 đ?œ‚= = 2 đ?‘š 2 + đ?‘š2 1+ 2 2
đ?œź= Shrishail Bhat, Dept. of ECE, AITM Bhatkal
đ?’Žđ?&#x;? đ?&#x;?+đ?’Žđ?&#x;?
(25) 9
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The percentage transmission efficiency is given as %đ?œź=
đ?’Žđ?&#x;? Ă— đ?&#x;?đ?&#x;Žđ?&#x;Ž % đ?&#x;? + đ?’Žđ?&#x;?
Improvement Techniques The modulating signal is band of frequencies with varying amplitude of ∆đ?œ” components. The spectrum of the modulating signal is shown in Fig. 7 (a). The corresponding spectrum of the modulated signal is shown in Fig. 7 (b).
đ?‘šđ?‘‰đ?‘? 2
��
(a) Modulating signal
(c) DSB-SC
(b) Modulated signal
(d) SSB
Fig. 7 AM improvement techniques We observe that the information is contained only in the sidebands and the carrier contains no information. Hence to improve the power efficiency, the carrier need not be transmitted, but only two sidebands are transmitted. This is called double sideband suppressed carrier (DSB-SC) technique as shown in Fig. 7 (c). DSB-SC requires less transmission power, but the carrier has to be generated at the receiving end by a high frequency oscillator. Furthermore, as upper and lower sidebands are mirror images of each other, it is sufficient to transmit only the upper sideband. This is called Single Side Band (SSB) as shown in Fig. 7 (d). The detector at the receiving end becomes complicated.
AM Detection (Demodulation) Detection or demodulation is the process of recovering the original modulating signal from the received signal at the receiver. The simplest demodulator for AM is the envelope detector.
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Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
Fig. 8 shows a demodulation circuit. It consists of a diode as half wave rectifier and RC circuit as a low pass filter. The received signal is passed through a diode to cut-off the lower half and the peaks detected and smoothed out by a parallel RC circuit.
Fig. 8 Demodulation circuit The time constant RC must meet the following condition: �� ≪ �� ≪ �� 1
đ?œ”
where đ?‘‡đ?‘? is carrier time period, đ?‘‡đ?‘? = đ?‘“ = 2đ?œ‹đ?‘? đ?‘?
1
đ?œ”đ?‘š
đ?‘š
2đ?œ‹
đ?‘‡đ?‘š is time period of modulating signal, đ?‘‡đ?‘š = đ?‘“ = The condition can also be written as đ??Žđ?’„ đ??Žđ?’Ž ≪ đ?‘šđ?‘Ş â‰Ş đ?&#x;?đ??… đ?&#x;?đ??… Fig. 9 shows the demodulator waveforms.
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
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Fig. 9 Demodulator waveforms
Frequency and Phase Modulation The frequency and phase of the carrier signals are closely related, since frequency is the rate of change of phase angle. If either frequency or phase is changed in a modulation system, the other will change as well. So frequency modulation and phase modulation are generally known as angle modulation.
Frequency Modulation Frequency Modulation is a process in which the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the message signal. Fig. 10 shows a sine wave modulating a higher frequency carrier signal with frequency modulation. The frequency modulated signal is represented by ∆đ?’‡
đ?’—(đ?’•) = đ?‘¨ đ?’”đ?’Šđ?’? [đ??Žđ?’„ đ?’• + đ?’‡ đ?’”đ?’Šđ?’? đ??Žđ?’Ž đ?’•] đ?’Ž
đ?’—(đ?’•) = đ?‘¨ đ?’”đ?’Šđ?’?[đ??Žđ?’„ đ?’• + đ?’Žđ?’‡ đ?’”đ?’Šđ?’? đ??Žđ?’Ž đ?’•] where đ??Žđ?’„ = 2đ?œ‹đ?‘“đ?‘? is the angular frequency of carrier signal đ??Žđ?’Ž = 2đ?œ‹đ?‘“đ?‘š is the angular frequency of modulating signal ∆đ?‘“ is the frequency deviation đ?‘šđ?‘“ is the modulation index of FM 12
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
��
đ?‘‰đ?‘?
đ?‘‰đ?‘?
Fig. 10 Frequency modulation
Frequency Deviation The amount of change in carrier frequency produced by the modulating signal is known as frequency deviation. Maximum frequency deviation occurs at the maximum amplitude of the modulating signal.
Modulation Index Modulation index of FM is the ratio of the frequency deviation to the modulating frequency. đ?’Žđ?’‡ =
∆đ?’‡ đ?’‡đ?’Ž
where ∆đ?‘“ is the frequency deviation đ?‘“đ?‘š is the modulating frequency
Bandwidth of FM Wave The bandwidth of an FM signal is given by đ?‘Šđ?‘ž = đ?&#x;?[đ?’Žđ?’‡ + đ?&#x;?]đ?’‡đ?’Ž đ??ľđ?‘Š = 2 [
∆đ?‘“ + 1] đ?‘“đ?‘š đ?‘“đ?‘š
đ?‘Šđ?‘ž = đ?&#x;?[∆đ?’‡ + đ?’‡đ?’Ž ] Shrishail Bhat, Dept. of ECE, AITM Bhatkal
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Phase Modulation Phase Modulation is a process in which the phase angle of the carrier signal is varied in accordance with the instantaneous amplitude of the message signal. The phase modulated signal is represented by đ?’—(đ?’•) = đ?‘¨ đ?’”đ?’Šđ?’?[đ??Žđ?’„ đ?’• + đ?’Žđ?’‘ đ?’”đ?’Šđ?’? đ??Žđ?’Ž đ?’•] where đ??Žđ?’„ = 2đ?œ‹đ?‘“đ?‘? is the angular frequency of carrier signal đ??Žđ?’Ž = 2đ?œ‹đ?‘“đ?‘š is the angular frequency of modulating signal đ?‘šđ?‘? is the modulation index of PM Fig. 11 shows a modulating sine wave and a phase modulated signal.
Fig. 11 Phase modulation Here the positive half cycle of modulating signal produces a lagging phase shift and negative half cycle produces a leading phase shift. As the modulating signal goes positive, amount of phase lag increases with the increase of modulating signal. This results in lower frequency of the modulated signal. As the modulating signal goes negative, amount of phase lead increases with the increase of modulating signal. This results in higher frequency of the modulated signal.
Amplitude and Frequency Modulation: A Comparison Table 2 gives a comparison of amplitude modulation and frequency modulation techniques with reference to different characteristics.
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Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
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Table 2 Comparison between AM and FM Characteristics
Amplitude Modulation
Frequency Modulation
đ?‘Ł(đ?‘Ą) = đ?‘‰đ?‘? [1 + đ?‘š sin đ?œ”đ?‘š đ?‘Ą] sin đ?œ”đ?‘? đ?‘Ą
đ?‘Ł(đ?‘Ą) = đ??´ đ?‘ đ?‘–đ?‘›[đ?œ”đ?‘? đ?‘Ą + đ?‘šđ?‘“ đ?‘ đ?‘–đ?‘› đ?œ”đ?‘š đ?‘Ą]
Principle
The amplitude of the carrier varies in accordance with the message signal. Carrier frequency remains constant.
The frequency of the carrier varies in accordance with the message signal. Carrier amplitude remains constant.
Modulation index
Modulation index can be Modulation index is always either less than one or more between zero and one than one
No. of sidebands
Only two produced
Channel bandwidth
FM has larger bandwidth AM has smaller bandwidth, because it produces a larger number of side bands. đ??ľđ?‘Š = 2đ?‘“đ?‘š đ??ľđ?‘Š = 2[∆đ?‘“ + đ?‘“đ?‘š ]
Operating carrier frequency
FM utilizes higher carrier AM utilizes lower carrier frequency (above 30 MHz) frequency because of its higher bandwidth
Transmission efficiency
AM has lesser transmission FM has better efficiency efficiency
Noise performance
AM has performance
Common Channel Interference (CCI)
Due to CCI, distortion occurs FM is better due to capture in AM effect
Externally generated noise pulses
FM receiver responds slightly to noise pulses generated by In AM, such tuning is not external sources, but if it is essential slightly mistuned, then its ability to suppress noise pulses is highly reduced
Area of reception
FM is limited to a small AM covers more distance than distance; as distance increases, FM signal quality becomes poorer
Wave equation
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
sidebands
poor
are A large number of sidebands are produced
noise FM has performance
better
noise
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Communication Systems
Basic Electronics
Questions 1. With a neat block diagram, explain the elements of communication system. (Dec ’16 – 5M, Jun ’16 – 6M, MQP ’15 – 5M) 2. What are commonly used frequency ranges in communication system? Mention the application of each range. (Dec ’14 – 5M, MQP ’14 – 4M) 3. What is modulation? Explain the need for modulation. List the different types of modulation schemes. (Jun ’16 – 5M, Dec ’15 – 5M, Jun ’15 - 6M, Dec ’14 – 4M) 4. What is amplitude modulation? Explain with neat waveforms and derive the expression for the AM wave. Also draw the frequency spectrum. (Dec ’16 – 6M, Jun ’16 – 5M, Dec ’15 – 8M, Jun ’15 – 8M, Dec ’14, MQP ’15 – 5M, MQP ‘14) 5. Define modulation index. Obtain the expression for modulation index of AM wave in terms of đ?‘‰đ?‘šđ?‘Žđ?‘Ľ and đ?‘‰đ?‘šđ?‘–đ?‘› . (Dec ‘15) 6. Derive an expression for modulation index in AM.
(Dec ’16 – 6M)
7. Derive the expression for the total power transmitted in an AM wave. (Dec ’14 – 5M, MQP ’14 – 6M) 8. With a neat diagram, explain demodulation of an AM wave.
(Jun ’16 – 5M)
9. Explain frequency modulation with neat waveforms. (Dec ’16 – 5M, Dec ’15 – 8M, MQP ’15 – 5M) 10. Mention the advantages of frequency modulation.
(Jun ’16 - 5M)
11. Differentiate between amplitude modulation and frequency modulation. (Dec ’16 – 5M, Dec ’15 – 4M, Jun ’15 – 8M, Dec ’15 – 5M, MQP ’15 – 6M, MQP ’14 – 4M) 12. A carrier of 10 V peak and frequency 100 kHz is amplitude modulated by a sine wave of 4 V and frequency 1000 Hz. Determine the modulation index for the modulated wave and draw the amplitude spectrum. (Dec ’16 – 6M) 13. An audio frequency signal 10 sin(2đ?œ‹ Ă— 500)đ?‘Ą is used to amplitude modulate a carrier of 50 sin(2đ?œ‹ Ă— 105 )đ?‘Ą. Calculate i) Modulation index ii) Sideband frequencies iii) Bandwidth iv) Amplitude of each sideband v) Total power delivered to a load of 600Ί vi) Transmission efficiency (Dec ’16 – 8M, Jun ’15 – 6M) 14. A carrier of 1 MHz, with 400 W of its power is amplitude modulated with a sinusoidal signal of 2500 Hz. The depth of modulation is 75%. Calculate the sideband frequencies, the bandwidth, the power in the sidebands and the total power in the modulated wave. (Jun ’16 – 5M)
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Shrishail Bhat, Dept. of ECE, AITM Bhatkal
Basic Electronics
Communication Systems
15. A 1 MHz carrier is amplitude modulated by a 40 kHz modulating signal with a modulation index of 0.5. The unmodulated carrier is having a power of 1 kW. Calculate the power of the amplitude modulated signal. Also find the sideband frequencies. (Jun ’16 – 5M) 16. A 500 W, 1 MHz carrier is amplitude modulated with a sinusoidal signal of 1 kHz. The depth of modulation is 60%. Calculate the bandwidth, power in the sidebands and the total power transmitted. (Dec ’15 - 7M) 17. The total power content of an AM signal is 1000 W. Determine the power being transmitted at carrier frequency and at each of the sidebands when percentage modulation is 100%. (Dec ’14 – 5M) 18. A 500 W, 100 kHz carrier is modulated to depth of 60% by modulating signal of frequency 1 kHz. Calculate the total power transmitted. What are the side band components of the AM wave? (MQP ’15 - 6M) 19. Calculate the percentage power saving when one side band and carrier is suppressed in an AM signal with modulation index equal to 1. (MQP ’14 - 5M) 20. A 15 kHz audio signal is used to frequency modulate a 100 MHz carrier, causing deviation of 75 kHz. Determine modulation index and bandwidth of the FM signal. (Dec ’16 – 4M)
References 1. D.P. Kothari, I. J. Nagrath, “Basic Electronics”, McGraw Hill Education (India) Private Limited, 2014. 2. Simon Haykins, “Communication Systems”, 5th Edition, John Willey India Pvt. Ltd., 2009. 3. Simon Haykins, “An Introduction to Analog and Digital Communication”, John Wiley India Pvt. Ltd., 2008
Shrishail Bhat, Dept. of ECE, AITM Bhatkal
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