Gordon Hunter President & CEO of Littelfuse
Electrical Engineering Community
Bringing your
concepts to reality 1.
is as easy as...
2.
Create schematics, technical diagrams, and flowcharts using your browser. • 600+ Symbol Library • Share Schematics Online • Export High Quality Images
digikey.com/schemeit
Free and easy-to-use circuit simulator that runs in your browser. • SPICE Simulator • AC/DC/Transient Sims • Waveform Viewer
partsim.com
3. Full featured online CAD application for designing and manufacturing electronics hardware. • Schematic Capture • PCB Layout • BOM Integration
pcbweb.com
Visit: digikey.com/schemeit • partsim.com • pcbweb.com Copyright ©2013 Aspen Labs LLC.
CONTENTS
PULSE
4
Gordon Hunter
PRESIDENT & CEO OF LITTELFUSE
A conversation about this circuit protection company's rich history and promising future.
Featured Products This week’s latest products from EEWeb.
Safeguarding Mission-Critical Circuitry on Earth & in Space
The ways in which NASA scientists implement circuit protection technology for long term reliability & performance.
Specifying an Electronic Enclosure
Why cutouts, tappings, and gaskets should all be considered before choosing an enclosure.
nications
n bogged
egulations
22
Fiber Optics & Consumer Telecommunications
ons often
How the emergence of fiber optics will revolutionize the way we communicate.
e, and
he stodgy
r optic
History of HF Radio Receivers: Part 2
since
rching at
wever, most
A look at some of the key characteristics of the earliest radio equipment.
16
ve remained
speed of
hich pulls
3rd in average
RTZ
speeds close
ader— Hong
Return to Zero Comic
lack of
e allowed
26
Alex Toombs
4
12 16 22 26 32 38
Electrical Engineer
Visit: eeweb.com
3
PULSE
4
Visit: eeweb.com
INTERVIEW
Visit: eeweb.com
5
PULSE
“ Rather than being only a fuse company, or a semiconductor company like some of our competitors, we’ve significantly expanded our technology base.” Could you give us an overview of Littelfuses products? Littelfuse is built on a culture of engineering– understanding the applications and compliance requirements for circuit protection around the world, and being able to develop and build products that meet these requirements. Over the last 10 years, we’ve worked to broaden the product offering well beyond fuses to several other technology platforms which are used in broader circuit protection applications. Littelfuse products protect against short circuits and voltage overloads, and include semiconductor, ceramic and polymer technologies. We are really the only company that has all of the possible technologies to meet a customer’s circuit protection needs. That’s really been the differentiating factor for Littelfuse. Rather than being only a fuse company, or a semiconductor company like some of our competitors, we’ve significantly expanded our technology base. That strategy has built a reputation for us as the leader in circuit protection. The future for us is to build off that strong base into two other areas—Power Control and Sensing. These are our newest platforms that we’re in the early stages of developing.
What directions are you planning on going in with Power Control and Sensing products? We already have some products in our portfolio for these markets. For example, in the semiconductor products category, we
6
Visit: eeweb.com
have what we call Protection Thyristors. These devices are used to protect against a voltage surge or a lightning strike on a telecomm line—they protect all of the equipment. Because thyristor technology can also be used for another product category called switching thyristors, some of our products have been used in a mode that’s not strictly circuit protection. An acquisition that we made in the automotive area a couple of years ago gave us a whole range of products for the non-passenger car market, which we wanted to get into, but didn’t have the products in our existing portfolio. This acquisition also gave us very good customer relationships with companies like John Deere and Caterpillar. We also have a range of products for power control applications—controlling the electronics inside a truck or agricultural vehicle, and battery management. We’ve been edging into power control and this past summer, we got into sensing with the acquisition of a sensor company in Sweden. This company makes products for the automotive industry. They provide technologies such as hall sensing to detect, for example, if a seat belt buckle is fastened. That’s a much better technology than a microswitch and it’s what the automotive industry is moving towards. That type of sensing is a real specialty of that company, along with using photodiodes to detect incoming sunlight to control the HVAC system in a vehicle. These applications all have a lot of growth potential and just recently we acquired Hamlin, Inc., another sensing company that has manufacturing facilities in the US, Mexico, and China. Most of
INTERVIEW that business is also in the automotive market, however this company also has some sensing applications in what is called the industrial electronics segment for sports equipment, fluid sensing, and industrial controls. Building that platform is part of the new fiveyear plan that we presented to the financial community back in December. We have a very exciting plan to increase the growth rate of our sales and earnings. Over the past 10 years, we’ve grown sales almost at a 10% compounded annual growth rate, even including the big economic downturn of 2009. We’ve grown our operating income and earnings much more than that—over 20%. We’ve managed to get a lot of leverage from that top-line growth.
What were some of the things Littelfuse had to do to get through the economic downturn? Obviously, some tough decisions had to be made. As it turns out, just before the downturn, we realized that we had a less than optimum cost structure because we had made quite a few acquisitions to get into the desired technologies. We had numerous small plants, and in the semiconductor industry, you really need critical mass and you need to reduce overhead. We closed all of the smaller plants and built a brand new semiconductor facility in China. Overall, our plan was to go from the 15 plants we had at the time to 5, in what we thought were the right locations and the right sizes. We were in the middle of that process when the downturn hit. We had to just work through it. We were in the middle of the restructuring and recruiting people for these new locations and we
What are some recent product offerings from Littelfuse? One growing area is the automotive industry. With the increasing amount of automotive electronics, there’s also a need for more circuit protection and sensors. Behind all of the electronics that goes into a vehicle is the need for automobiles to become more fuelefficient. There used to be a lot of belt drives coming off the engine of a car, and that hardly happens nowadays because that’s a drag on the efficiency of the engine. It may not be well known, but there has been a change in the electrical architecture in vehicles, so the electrical system and the battery system are now much more sophisticated. There’s a high-current architecture that drives things like compressors and fans, which require very different kinds of circuit protection. This is a completely new segment. We have several products that go into that, including our MasterFuse, which has multiple fuse elements in one module. Very often you’ll see these connected to the battery in new cars. This design has become more preferred over the past couple of years. It began in Europe, where the need to have stronger battery and electrical system performance is high. To have electric vehicle systems working efficiently, the vehicle’s computer has to realize it’s safe to turn the battery off. There’s a lot of sophistication in the electrical system to make a vehicle more fuel efficient, which is an emerging segment. We look at that MasterFuse family as a critical part of making vehicles safer and more fuel-efficient.
Protection Thyrsistors
MasterFuse Module
Visit: eeweb.com
7
PULSE obviously couldn’t slow that down. We had to just accept that we’d eventually come out on the other side. By 2010, we were in the very fortunate position of having new plants with plenty of capacity, so we were able to respond very well when the market rebounded. Our new cost structure has really turned out exactly as we had expected in terms of being much more efficient, so since 2007, our profitability has dramatically increased.
Are these acquisitions integral to meeting those growth numbers in your 5-year plan? Yes. Over the next five years, we think that we can achieve a 10% compounded annual growth rate from acquisitions. For example, this year we’re a $700 million revenue company, and we’ve already made an acquisition that will be contributing more than 10% of that in its first full year. Acquisitions of that size fit in very well. We feel if we are making slightly larger acquisitions, we’re increasing our technical capabilities in a more meaningful way and probably getting slightly higher return on investment. We’re very excited about the talent that we got with the Hamlin acquisition and all indications are that the company is going to fit in very well with the Littelfuse culture—it’s a very teambased and global company. We’re excited about getting the Hamlin products to more customers because as a smaller company, they just didn’t have the global reach or the presence or distribution channels that some of the key OEMs have. We think we can bring something to Hamlin and we certainly think they can bring something to us.
“ The good thing about circuit protection products is that they’re found in everything that uses electrical energy.”
8
Visit: eeweb.com
What are some of the other industries that Littelfuse serves? The good thing about circuit protection products is that they’re found in everything that uses electrical energy. If you look around you, whether it’s at home or at the office, it can range from the fax machine to the cell phone in your hand to your washing machine. As a result, we are in many of what we call “vertical markets.” When we say electronics, people immediately think of consumer electronics— TVs and computers. And it doesn’t really matter which OEM wins with their brand. The volume growth of electronics in general and the more and more connected devices, provide a wealth of opportunities for us. That’s because typically when there’s a connection, there’s usually a weak link in the system that needs protection. For example, there’s always a need for protection on the back side of any external port. So if someone plugs in the wrong device or the wrong thing gets inserted into that connector, that potential short circuit doesn’t get all the way to the microprocessor or microcontroller. The proliferation of circuit protection really comes with the amount of electronics and all of the connection points that are made. If you think about all of the small vertical markets like medical equipment, appliances, lighting, test and measurement—there’s a whole range of small industrial applications that don’t get all the press, but are critical places that require circuit protection. We get to that market through distributors. Making sure our distributors are well trained and we have a strong partnership helps to ensure that we are always the circuit protection provider of choice. That’s a critical part of our strategy, so there’s a lot of work in treating the channels as partners, working with the key companies and developing joint programs. Frankly, with things like Speed2Design, it’s a way of bringing something innovative to the relationship, being excited by engineers and bringing them to events where engineering is a critical component like car races, or to NASA—generally being seen by the engineering community as a company that really understands circuit protection. ■
Online Circuit Simulator PartSim includes a full SPICE simulation engine, web-based schematic capture tool, and a graphical waveform viewer.
Some Features include: • Simulate in a standard Web Browser • AC/DC and Transient Simulations • Schematic Editor • WaveForm Viewer • Easily Share Simulations
Try-it Now!
www.partsim.com
PULSE Single Modem AISG Transceiver The ASC3101HV device combines NXPs programmable DSP based AISG modem functionality with a powerful 94 MHz ARM CortexTM M3 processor with embedded flash program memory. The AISG modem functionality is identical as provided by NXPs standalone AISG transceiver ASC3011HN, with the same programmable features. This combination of functions represents a highly optimized and cost-effective solution for any tower mounted device requiring a CPU...Read More
600-V, 20-A High-Speed Switch Renesas Electronics Corporation announced the availability of three new super-junction metal-oxide-semiconductor Super Junction MOSFETs featuring figure of merit: onstate resistance x gate change in a 600 V power semiconductor device suitable for high-speed motor drives, DC-DC converters, and DC-AC inverter applications. Previously, air conditioners and other home appliances employing high-voltage, highspeed motors and inverters typically used IGBTs with discrete Fast Recovery Diodes, in one package, to enable a short reverse recovery time...Read More
Precise OFN Modules Optical Finger Navigation (OFN) products from Avago Technologies offer a compact, precise, fast speed and highly reliable navigation solution, which will help enhance the overall system performance. Our OFN solution is user friendly and can be used in electronic devices such as mobile phones, MP3 players, ultra-miniature PCs, game pads, digital cameras and keyboards. The ADBM-A350 Optical Finger Navigation Module is capable of high-speed motion detection – up to 20ips. In addition, it has an on-chip oscillator and integrated LED for optical navigation to minimize the need for additional external components...Read More
Power Management IC with PMBus The LM5056/LM5056A combines high-performance analog and digital technology with a PMBus compliant SMBus™ and I2C interface to accurately measure the electrical operating conditions of systems connected to a backplane power bus. The LM5056/LM5056A continuously supplies real-time power, voltage, current, temperature and fault data to the system management host via the SMBus interface. Accurate power averaging is accomplished by averaging the product of the input voltage and current...Read More
1-Wire Thermocouple-to-Digital Converters Maxim Integrated Products, Inc. announced that it is now sampling the MAX31850/ MAX31851 cold-junction-compensated, 1-Wire® thermocouple-to-digital converters. The devices achieve ±2.0°C accuracy (not including sensor nonlinearity) while integrating all of the functions required for a complete thermocouple-to-digital solution. Additionally, the MAX31850/MAX31851’s 1-Wire interface allows multiple sensor locations to communicate and draw power over a single data line, greatly simplifying wiring requirements...Read More
10
Visit: eeweb.com
FEATURED PRODUCTS Glitch-Free RF Digital Step Attenuator Integrated Device Technology, Inc. announced the industry’s first Glitch-Free radio frequency (RF) digital step attenuator (DSA) with integrated blocking capacitors. The new DSA is a true drop-in replacement for the most popular DSA footprint, allowing customers to reduce the bill-of-materials (BOM), optimize board area, and improve performance in base station and industrial applications. The IDTF1953 is a 6-bit RF DSA optimized for the demanding requirements of base transceiver station (BTS) receive, transmit, and digital pre-distortion (DPD) paths. The device is pin- and controlcompatible with competitors’ offerings, but integrates blocking capacitors and innovative design techniques to reduce the BOM and improve performance...Read More
AGT Platform for AC Power Control The combination of the Anode Gated Thyristor (AGT) together with a standard thyristor, gives several interesting configurations. Thyristor phase-leg configurations normally contain two separated gate driver potentials, but by replacing one thyristor with the AGT, it will reduce to only one gate driver potential. This will save one complete gate driver circuit. Also an AC switch configuration benefits from the AGT. Here the switch can be operated with only one gate drive circuit. This shows great cost saving possibilities by reducing the gate drive component count...Read More
Ultra-Low-Power 16-Bit FRAM Fujitsu Semiconductor Europe announced the addition of a new small package to the MB85RC16, the 16kbit ultra low-power Ferroelectric Random Access Memory (FRAM) device with an I2C interface. Fujitsu has been supplying MB85RC16 with standard package SOP-8. The new SON-8 plastic LCC. The combination of small footprint and ultra low power consumption makes the MB85RC16 a perfect non-volatile memory solution for portable and sensing applications in medical and industrial segments as well as for energy harvesting applications...Read More
High Voltage Dual MOSFET Driver The ISL6208C is a high frequency, dual MOSFET driver optimized to drive two NChannel power MOSFETs in a synchronous-rectified buck converter topology. It is especially suited for mobile computing applications that require high efficiency and excellent thermal performance. The driver combined with an Intersil multiphase Buck PWM controller forms a complete single-stage core-voltage regulator solution for advanced mobile microprocessors. The ISL6208C features 4A typical sinking current for the lower gate driver...Read More
Visit: eeweb.com
11
PULSE Half Bridge Resonant Switch ower Integrations released their series of integrated LLC Controllers that features High-Voltage Power MOSFETs and Drivers, the HiperLCS family. This HiperLCS series of integrated circuits combine an LLC controller, a low and a high-side driver, and two half-bridge MOSFETs in a single package. The typical application circuit of this IC family will show simplified schematic of a HiperLCS based power stage where the LLC resonant inductor is integrated into the transformer...Read More
Low Input Bias Current Op Amp The Microchip’s MCP6421 operational amplifiers (op amps) has low input bias current (1 pA, typical) and rail-to-rail input and output operation. This family is unity gain stable and has a gain bandwidth product of 90 kHz (typical). These devices operate with a single-supply voltage as low as 1.8V, while only drawing 4.4 µA/amplifier (typical) of quiescent current. These features make the family of op amps well suited for photodiode amplifier, pH electrode amplifier, low leakage amplifier, and battery powered signal conditioning applications, etc. The MCP6421 family is offered in SOT23 and SC70 packages...Read More
High Temperature Resistor Arrays Mouser Electronics, Inc. is stocking and shipping the award-winning PRAHT 100, PRAHT 135, and PRAHT 182 Series Resistor Arrays from Vishay Sfernice. Vishay Sfernice PRAHT 100, PRAHT 135, and PRAHT 182 Series Resistor Arrays are thin film wrap-around chip resistor arrays that offer an extended operating temperature range up to 215ºC and an industry high maximum storage temperature of 230ºC. The PRAHT series provides 2 ppm/ºC TCR tracking and a ratio tolerance as tight as 0.05%...Read More
LIN Slave for Relay & DC Motor Control The MLX81150 is a highly integrated motor controller for DC- and 1-/2-Phase BLDC motor control in 12V automotive applications. The IC comes in QFN32 5×5 and TQFP48 7×7 packages. It follows the Melexis high integration concept by putting a 16-bit-Flash-Microcontroller, voltage regulator, Complete LIN-Protocol, LIN-Transceiver, Relay- and Power-FET-Gate-Driver as well as several blocks for DC and BLDC motor operation in one single monolithic IC. The MLX81150 comes with an interface for Hall sensor connection in order to enable position sensing...Read More
12
Visit: eeweb.com
FEATURED PRODUCTS Integrated USB Audio Decoder ICs ROHM’s USB Audio Decoder ICs integrate a USB 2.0 host, SD memory card controller, AAC decoder, WMA decoder, MP3 decoder, Audio DAC amp, and a system controller all on a single chip, allowing easy playback of music stored in USB flash memory using an audio player. They can be easily added to virtually any audio device (i.e. CD player, minicomponent stereo, portable player, car stereo) for greater functionality. The BU94603KV outputs folder names, file names, ID3TAG (V1.0, V1.1 V2.2 V2.3 and V2.4) information and WMATAG information and AAC-TAG information via the I2C interface. This function is enabled only in MODE 2 and MODE 3. It also supports fast forward playing and fast backward playing with music...Read More
Metal Fuse Clip The 159 Series product is a metal fuse clip with preinstalled Littelfuse 461 Series TeleLinkÂŽ fuse. This fuse and clip combination can be automatically installed in PC Boards in one efficient manufacturing operation. It permits quick and easy fuse replacement without exposing the PC Board and other components to risks of rework solder heat as required with direct surface mount fuses. It meets UL 60950 power cross requirements and is designed to allow compliance with Telcordia GR-1089CORE and TIA-968-A Surge Specifications...Read More
Hall Effect IC for Pulse Encoders he AK8778 is a Hall effect latch which detects both vertical magnetic field and horizontal magnetic field (perpendicular and parallel to the marking side of the package) at the same time. The pulse output F and direction output D are switched according to the vertical and horizontal magnetic fields applied to the device. The direction is calculated internally and output D is switched at a rising or falling edge of output F. The AK8778 is for use in the incremental pulse encoders or rotational detection systems...Read More
Automotive Multiple Output Regulator Family Allegro MicroSystems, Inc. introduces three new automotive power management ICs that use an internal 2.2 MHz constant on-time buck pre-regulator to supply regulated outputs commonly used in automotive CAN and microprocessor controlled applications. These new devices are targeted at the automotive market for bias supplies within control units for functions such as electronic power steering, transmission control, antilock braking and emissions control...Read More
Portability & Power All In One... Debug digital designs on an iPad, iPhone, iPod. Can a logic analyzer be sexy? Watch the video and weigh in...
Logiscope transforms an iPhone, iPad or iPod into a 100MHz, 16 channel logic analyzer. Not only is it the most intuitive logic analyzer available, the triggering is so powerful you’ll be able to count the hair on your bug.
See why our innovation keeps getting recognized.
Technology You Can Trust
Avago Technologies Optocouplers
Safety Certified Protection... Worldwide! IEC 60747-5-5 Certified
Optocouplers are the only isolation devices that meet or exceed the IEC 60747-5-5 International Safety Standard for insulation and isolation. Stringent evaluation tests show Avago’s
optocouplers deliver outstanding performance on essential safety and deliver exceptional High Voltage protection for your equipment. Alternative isolation technologies such as ADI’s magnetic or TI’s capacitive isolators do not deliver anywhere near the high voltage insulation protection or noise isolation capabilities that optocouplers deliver. For more details on this subject, read our white paper at: www.avagoresponsecenter.com/672
PULSE
>>> SAFEGUARDING MISSION-CRITICAL CIRCUITRY >>>>>>> ON EARTH AND IN SPACE
16
Visit: eeweb.com
TECH ARTICLE
Y E
S
pace exploration has its potential hazards, but so far, little green men haven’t featured prominently among them. Instead, NASA scientists and engineers concern themselves far more with electrical hazards that can interfere with the operation of mission-critical electronics and communications equipment, whether they’re located on earth, in orbit on the International Space Station (ISS), or on the robots now roving the Martian landscape. All demand comprehensive circuit protection to ensure longterm reliability and performance. For example, ESD (electrostatic discharges or static shocks) can create major problems for critical electronics, no matter where they occur. Without the proper circuit protection, a simple touch could produce a discharge that would interfere with the operation of sensitive electrical or electronic elements and logic circuits.
Visit: eeweb.com
17
PULSE
>>>> A
s integrated circuit chipsets become more sensitive to damaging ESDs and other transient voltage surges, the choice of circuit protection device becomes increasingly important. Surge protection devices suppress voltage transients and divert excessive current away from sensitive electronic components. For circuit designers, the first challenge is choosing the most appropriate solution from the many different types of devices available. Voltage surges can result from a variety of sources, typically associated with the handling of an end product or its connection to power and communications lines. Understanding these threats requires identifying the likely source of an over-voltage condition, and the circuits or components that need protection. Table 1 offers an overview of these threats and the types of protective devices typically used to prevent damage.
In environments where the relative humidity is low, an ESD event may produce a peak voltage as high as 15,000 volts. At 6,000 volts, an ESD event will deliver a painful zap. Although lower voltage discharges may initially go unnoticed, they can still produce catastrophic damage in electronic components and circuits. In less severe cases, ESD may lead to faulty circuit operation or produce latent defects that later lead to outright failures. The type of electrical threat involves determines the protection device characteristics required. For example, to provide ESD protection, suppression devices must offer very fast response times and the ability to handle repeated high peak voltages and currents for short durations. As Table 1 suggests, the most appropriate protective devices are often Polymer ESD Suppressors, silicon TVS Diode Arrays, and Multilayer Varistors (MLVs). If the circuit to be protected includes LEDs, special protective devices designed specifically for that application may also be needed. In contrast, lightning surge protection requires the use of crowbar devices, such as Gas Discharge Tubes and Protection Thyristors. These circuit protection devices can be used on communication lines where the short circuit current of the system is less than the holding current of the crowbar device (Protection Thyristor) or where the operating voltage of the circuit is much lower than the holdover voltage of the crowbar device (GDT).
Table 1: Typical circuit threats, protection devices, and selection criteria.
18
Visit: eeweb.com
A clamping device (Figure 1) essentially limits the transient voltage to a set value (above the operating voltage of the circuit being protected). When the applied voltage reaches the device’s breakdown value, it will switch to a much lower resistance value to minimize the transient energy the circuit will experience. This value is typically referred to as the dynamic resistance.
TECH ARTICLE A clamping device will stop conducting as soon as the transient voltage drops below the device’s breakover voltage, and the circuit will then be able to operate normally. Polymer ESD suppressors, TVS Diode Arrays, and MLVs are all examples of clamping devices. If the application is a high-speed digital I/O line or high-frequency RF line, it’s important that the protection device not add capacitance that could distort the signal. Polymeric ESD suppressors have ultra-low capacitance (typically around 0.05pF), act quickly, and have good voltage clamping capabilities. As a rule of thumb, polymeric ESD suppressors are best used when:
Figure 1: Operation of a clamping device for transient voltage protection.
•P rotection is required on data, signal, and control lines. • The circuit is not a power supply line. • Signal distortion can occur with very little added capacitance (i.e., high-speed data lines or RF circuits). • ESD is the only transient threat. TVS Diode Arrays, a different type of ESD protection device, are used on analog and digital signal lines, such as USB, HDMI, and Ethernet, and other signal lines associated with LCD modules, keypads, and electronic switch assemblies. When compared with other ESD protection devices, they provide the lowest peak and clamp voltages available, due to their low dynamic resistance values. They are available as discrete devices or in packages containing multiple devices to protect multiple lines (see example in Figure 2) within a small space. Generally, TVS Diode Arrays are used when: • The circuits being protected require the lowest possible clamp voltage (need low dynamic resistance in the protector). • L ow capacitance (typically, 0.40pF–30pF) and low leakage (around 0.02μA–10μA) are required. • Multiple lines must be protected and board space is limited, or when small discrete devices are preferred. • Transients other than ESD, such as EFT or lightning, must also be considered. TVS Diode Arrays can safely absorb severe (up to 30kV), repetitive ESD events without performance degradation. Multiple semiconductor technologies go into their
Figure 2: ESD multi-line protection using a TVS Diode Array for an indoor network and equipment. design, allowing them to provide optimum protection. In order to maintain both low capacitance and low clamping voltage, the diode arrays use steering diodes to direct the ESD transient into a central TVS diode. The steering diodes only operate in the forward bias direction to add minimal resistance and decrease the overall capacitance of the solution. Multilayer Varistors (MLVs) are another class of clamping devices, which are fabricated from ceramic-based materials (zinc oxide). MLVs are generally used for board-level protection against ESD, EFT, and other transients that can occur on power supply, data, and control lines that can tolerate higher capacitance. MLVs offer a cost-effective solution in a range of popular industry-standard discrete component
>>>>
Visit: eeweb.com
19
PULSE A wide range of other considerations must also be factored into the selection and application of devices for ESD protection, including parasitic capacitance, standoff voltage, ESD withstand, and many others. For guidance in choosing and using the most appropriate solution, Littelfuse offers a free ESD Design Guide. This valuable resource includes design consideration factors, example circuits, applicable standards, and recommended components. To download a copy, click here.
Figure 3: Example of MLV surge protection that clamps the S-video input line to 26VDC. sizes. Limited multi-line array configurations are also available. Some MLVs also offer low band-pass characteristics that filter high frequency noise from the circuit being protected. MLVs are typically used when: • Power supply line or low/medium speed data and signal lines are to be protected. • Transient currents or energy beyond ESD is expected (i.e., EFT and lightning-induced surges). • Designers looking to replace high-wattage TVS zener diodes (300W–1500W). • Added capacitance is desirable for EMI filtering (3pF–4500pF). • The operating voltage (up to 120VDC) is greater than silicon or polymer ESD suppressor ratings. Figure 3 illustrates a typical MLV application. The characteristics and location of these devices will protect the video system from most overvoltage surges that might occur on the S-video input lines.
Location, Location, Location Installing a circuit protection device in the appropriate location is just as important to ensuring effective ESD protection as is choosing the right device. The device should be placed as close to the connector or button/switch to be protected as practicable to ensure the ESD transient is clamped as soon as it enters the application. In addition, it should be installed as close as possible to the data/signal line as possible to eliminate the potential for an inductive overshoot voltage that would result in a peak voltage that could damage the circuitry.
>>>> 20
Visit: eeweb.com
Want to learn still more? For those interested in exploring and discovering more about the technical challenges associated with the space program, Littelfuse has developed a special NASA-oriented contest for the 2013 Speed2Design Program. Engineers can enter to win a two-day trip to go behind the scenes at NASA Ames Research Center (Moffett Field, California, on August 15) or Johnson Space Center (Houston, Texas, on October 24). Winners will have a chance to meet and talk with leading NASA scientists involved in developing the technologies of tomorrow. The trip includes a tour of the research center, presentations by NASA engineers, cocktail reception the night before the event, lunch and dinner the day of the event, hotel accommodations for two nights, ground transportation between the hotel and event, and a $500 debit card to use for travel expenses and other incidentals. There’s no cost to enter. Learn more about how win a trip to see NASA innovations first-hand at www. speed2design.com. ■
PULSE
Specifying So you need to specify an electronic enclosure. That seems simple enough. But, if you have done this before, you have learned the devil is in the details. Even a simple standard metal or plastic enclosure requires modifications to give access to the interior, stabilize and protect contents, and provide the right appearance. Enclosures typically require one cutout for a power cable, and they might need various holes, cutouts, tappings, gaskets, preassembly, and finishing as well.
22
Visit: eeweb.com
E
TECH ARTICLE
Bud Industries, Inc.
an
Electronic
?
Enclosure
M
that the mounting screws on the printed circuit board line up with the enclosure’s mounting bosses. It also gives you time to specify tapped inserts.
Where modifications are necessary, a good supplier can easily mill-out mounting bosses or card guides or add tapped inserts to allow mounting the printed circuit board closer to the front of the unit. Here, a bit of planning ahead gives you the flexibility to specify
Be sure to look for a supplier that carries a wide selection. This can sometimes let you eliminate modifications entirely. For example, a standard polycarbonate enclosure with a clear lid eliminates the need for cutouts for LEDs and displays. Also note that standard sizes are usually less costly than custom sizes. That said, because sheet metal boxes are formed by bending metal, the additional cost for a custom dimension might well remain within your budget. Custom plastic enclosures, though, entail high molding charges and long lead times.
any enclosure suppliers can turn around modifications in 10 to 15 days. But the most efficient companies can complete simple modifications in as few as five days at no extra charge. To best leverage their services, you should identify enclosure changes early in the design cycle. The upside of this approach is that it can let you use a stock enclosure that costs less and doesn’t need as much lead time as a custom enclosure.
Visit: eeweb.com
23
PULSE
Also look for an enclosure supplier that can preassemble enclosure components, including cable glands and mounting brackets, as well as apply decals and custom labels. A good supplier should also be able to preassemble racks and cabinets, helping slash your production time. That’s why it’s important that you carefully specify the locations of access panels, custom shelves, fans, vents, lock hasps, power strips, casters, and levelers. For components that need special EMI/ RFI protection, be sure the manufacturer knows the most cost-effective techniques. Whether it is a conductive coating in plastics or special gasketing, the manufacturer should be your resource for these design issues. When part appearance is important, choose a supplier that can provide custom powder-coat finishes or one that has many standard colors. This can help differentiate your product in the marketplace and give you a competitive edge. In addition, be sure also to nail down cutout locations and acceptable tolerances early in the cycle. Components such as switches might connect to the PCB with a flexible cable so the exact position of the cutout is not critical. However, components such as data ports mounted on the PCB require the cutout to be positioned precisely in relation to the PCB mounting bosses. Tolerances depend a lot on the precision of the enclosure manufacturer’s equipment, so make sure to ask a potential supplier what tolerances it can hold. Also, note that designs can be within tolerance but still not work for the application. Tolerances can accumulate and also component tolerances can vary more than the cut-out tolerances. Issues like this make it important to ask a supplier which cut has the most critical tolerance so the other cuts can be made in relation to it.
24
Visit: eeweb.com
Other design factors to consider include: Mounts. Are internal or external mounting provisions necessary? The appropriate supplier can add tapped holes or preassemble mounting brackets. Grounds. Will certain areas need to be masked for continuity? Are ground points or studs needed? Draft angles. Cast boxes typically have a 1 to 2 degree taper so the casting can be easily removed from the mold. Because draft angles can potentially affect the fit of mounted components, it’s wise to take them into account before making modifications. Wall thickness. Some components can only mount to walls of a certain minimum thickness. Other components can only mount through walls of a certain maximum thickness. Thinking about making modifications in house? The break-even quantity is typically about 50 to 100 pieces, depending on manufacturing processes. But there are hidden costs of doing the work yourself or jobbing it to someone other than the manufacturer. For example, you might be unaware that it’s bad practice to use the same drill bit for metal, plastic, and fiberglass. Also, you must determine parameters such as the correct torque speed to cut polycarbonate boxes to avoid cracking. These issues mean you must plan for a certain amount of scrap when making modifications in house unless you have specific expertise in this area. It’s therefore usually more economical to let an efficient enclosure supplier make the modifications, especially because they can often make these modification during production, reducing handling costs and improving accuracy. Also, the manufacturer will have specific equipment that improves the efficiency and accuracy of the process. Although job shops might be more cost competitive piece-wise on shorter runs, using a job shop incurs higher freight costs and additional shipping time, as well as necessitates dealing with multiple suppliers. When working with an enclosure supplier, you can download the DXF drawing from the supplier’s website, then import it into your CAD software and indicate on the drawing the locations of the modifications. The supplier works from this file and uses its expertise to verify your design, reducing the set-up and design costs for the project The most important take-away from these guidelines is that enclosures should be part of the design and not left as an afterthought. Discovering the ways an efficient supplier can modify enclosures frees you to focus on more critical parts of your job. ■
Get the Datasheet and Order Samples http://www.intersil.com
19MHz Rad Hard 40V Quad Rail-to-Rail Input-Output, Low-Power Operational Amplifiers ISL70444SEH
Features
The ISL70444SEH features four low-power amplifiers optimized to provide maximum dynamic range. These op amps feature a unique combination of rail to rail operation on the input and output as well as a slew enhanced front end that provides ultra fast slew rates positively proportional to a given step size; thereby increasing accuracy under transient conditions, whether it’s periodic or momentary. They also offer low power, low offset voltage, and low temperature drift, making it ideal for applications requiring both high DC accuracy and AC performance. With <5µs recovery for Single Event Transients (SET) (LETTH = 86.4MeV•cm2/mg), the number of filtering components needed is drastically reduced. The ISL70444SEH is also immune to Single Event Latch-up as it is fabricated in Intersil’s Proprietary PR40 Silicon On Insulator (SOI) process.
• Electrically screened to DLA SMD# 5962-13214
They are designed to operate over a single supply range of 2.7V to 40V or a split supply voltage range of ±1.35V to ±20V. Applications for these amplifiers include precision instrumentation, data acquisition, precision power supply controls, and process controls. The ISL70444SEH is available in a 14 Ld Hermetic Ceramic Flatpack and die forms that operate over the temperature range of -55°C to +125°C.
• Acceptance tested to 50krad(Si) (LDR) wafer-by-wafer • <5µs recovery from SEE (LETTH = 86.4MeV•cm2/mg) • Unity gain stable • Rail-to-rail input and output • Wide gain·bandwidth product . . . . . . . . . . . . . . . . . . . . 19MHz • • • • •
Wide single and dual supply range. . . . . . . . 2.7V to 40V Max Low input offset voltage . . . . . . . . . . . . . . . . . . . . . . . . . 300µV Low current consumption (per amplifier) . . . . . . . 1.1mA, Typ No phase reversal with input overdrive Slew rate - Large signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60V/µs • Operating temperature range. . . . . . . . . . . .-55°C to +125°C • Radiation tolerance - High dose rate (50-300rad(Si)/s). . . . . . . . . . . 300krad(Si) - Low dose rate (0.01rad(Si)/s) . . . . . . . . . . . . 100krad(Si)* - SEL/SEB LETTH . . . . . . . . . . . . . . . . . . . . 86.4MeV•cm2/mg * Product capability established by initial characterization.
Applications
Related Literature
• Precision instruments
• ISL70444SEH Evaluation Board User’s Guide AN1824
• Active filter blocks
• ISL70444SEH Single Event Effects Report AN1838
• Data acquisition
• ISL70444SEH SMD 5962-13214
• Power supply control
• ISL70444SEH Radiation Test Report
• Process control
RF 30
100kΩ
+ RSENSE
-IN
10kΩ RIN+
+IN
10kΩ RREF+
LOAD
-
+2.7V to 40V
V+ ISL70444 V-
+
Vs = ±18V
20
VOUT
10 VOUT = 10 (ILOAD * RSENSE)
100kΩ
VOS (µV)
RIN-
0
GROUNDED
-10
BIASED
-20
VREF
-30 0
50
100
150
200
250
300
krad (Si)
FIGURE 1. TYPICAL APPLICATION: SINGLE-SUPPLY, HIGH-SIDE CURRENT SENSE AMPLIFIER
June 14, 2013 FN8411.1
FIGURE 2. VOS SHIFT vs HIGH DOSE RATE RADIATION
Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2013 All Rights Reserved. All other trademarks mentioned are the property of their respective owners.
PULSE Emergence of
in Consumer Telecommunications
T
elecommunications as an industry is often bogged down by technological inertia. Various regulations
and dependencies mean that new innovations often don’t come to market for close to a decade, and can be slow to be adopted—especially in the stodgy television, phone, and internet industry. Fiber optic transmission of information has been around since the mid 1970s, following work done by researching at Corning Glass Works. Until very recently, however, most home telecommunications connections have remained as classic copper lines, limiting the average speed of internet in the US. According to Net Index, which pulls data from SpeedTest.net, the US is ranked 33rd in average download speeds, with average download speeds close to one third of the average speeds in the leader— Hong Kong. While dragging their feet and citing a lack of demands, providers in the United States have allowed
t
the infrastructure to slip behind the rest of the world.
26
Visit: eeweb.com
TECH ARTICLE
Alex Toombs Electrical Engineer
Visit: eeweb.com
27
PULSE Figure 1: Optical Fiber Layer Relative Size Diagram (Courtesy of Wikimedia user Mysid)
O
nly recently have some companies begun offering connections to residential addresses, shaking up the telecommunications picture in this country for the first time in many years. Verizon, Google, and AT&T all offer services in some areas that can allow both business and residences access to the high speed connections enjoyed worldwide. Google Fiber in particular, launched in Kansas City last year as Googleâ&#x20AC;&#x2122;s first foray into service provision, uses some particularly advanced new technology in order to deliver symmetrical 1 Gbps download and upload speeds. Much in the way that nationwide internet access changed the way businesses function and the way in which people interact and are entertained, widespread fiber (and the fascinating technologies that drive it) can only push our country forward.
Light Sources for Fiber Optic Communication Fiber optic communication begins with pulses of light that communicate information over long glass strands known as fibers. At the original end, electronic information is interpolated into light by the pulsing of an LED or laser diode at a specific wavelength used to minimize dispersion of light throughout the fiber. Light emitting diodes operate as forward biased p-n junctions that emit light through spontaneous emission with a certain bandwidth associated with the emitted light. Typically, LED sources are used in local area network connections where speed is not very critical and rates of 100 Mbps or lower are acceptable. The most commonly used semiconductor materials for these LEDs are indium gallium arsenide phosphide and gallium arsenide phosphide.
28
Visit: eeweb.com
Laser diodes, on the other hand, operate through stimulated emission, and modulated laser diodes can reach powers of hundreds of milliwatts. Lasers also produce light that is coherent, increasing the efficiency of light that is able to couple to the fiber over LEDs. Laser diodes are most often used in high power and high speed applications of fiber optic communications. Today, the most commonly used source are Vertical Cavity Surface Emitting Laser, or VCSELs, which offer greater power and speed than do LEDs at a similar cost, while coupling better to multi-mode fibers. VCSELs function with two distributed Bragg reflectors parallel to the wafer of the device where quantum wells that generate photons are fabricated. A diagram of a VCSEL is shown in Figure 2.
Fiber Cables Light produced at one end must then be coupled into a fiber, which is a difficult process wherein coherence of light is supremely helpful. These cables are made of glass, and transfer light over great distances due to the property of total internal reflection. Optical fiber cables generally consist of a core, cladding, and buffer material, much as a protected waveguide would. The refractive index of the core is lower than that of the cladding, meaning that light travels through there with minimal loss. The buffer protects the outside of the cladding from damage that could introduce imperfections. Sometimes optical fiber cables also have another layer called the jacket, which functions like the
TECH ARTICLE plastic wrapping around copper wires. A diagram of fiber optic cable layers is shown in Figure 1 to the left. Optical fibers are constructed into two major subtypes, multimode and single mode. Multimode fibers have a larger core and allow less coherent light to couple, meaning that they are often used in cheap applications. The downside to multimode fibers, however, is that they severely limit bandwidth and length of the connection, due to multimode dispersion. Single mode fibers allow for longer and faster optical links, while requiring more expensive materials and manufacturing techniques. Fibers must be cleaved with specialized tools in order to present a flat surface that light can couple into without scattering. Creating and installing high-performance fiber cables is very expensive, and is one of the largest prohibitive costs preventing more service providers from installing fiber to the home. For instance, transatlantic undersea communication cables installed in 2011 by Emerald Atlantis and Hibernia Atlantic cost around $300 million, but they are able to transmit data from New York City to London in under 70 milliseconds. These fiber optic cables can get even more expensive when amplification is needed, as optical amplifiers are made by doping lengths of fiber with expensive rare earth materials like erbium.
Fiber Optic Receivers On the other end of the line, photoreceivers detect information sent via fiber cables. Typically, these are semiconductor photodiodes that convert light into electricity via the photoelectric effect. As good emitters are also good absorbers, the materials used to make these diodes are generally similar to those in LEDs—indium gallium arsenide, most often. Various signal processing techniques are used in order to verify the information received, which is then converted back into electrical signals. Much like copper, fiber optic cables can carry many types of communication, and are currently used for phone, internet, and even television with Verizon and Google Fiber.
Fiber Optics vs. Cable Fiber optic communication and cable both have places in the world we live in. Copper cabling and electrical transmission are cheaper for close-range applications where
Figure 2: Simple VCSEL Diode Diagram (Courtesy of Wikimedia Commons user The Photon) bandwidth isn’t very important, while fiber optics are great at high bandwidth applications particularly with multiplexing. While copper wiring may only carry one signal at once, multiplexed signals of different wavelengths may travel in the same fiber optic cable, greatly increasing the total throughput one length of fiber has. Fiber optic cabling is best used where high voltages can be a problem, and light in fiber is immune to electromagnetic radiation while copper cabling is subject to any induced currents that some machinery can cause. Fiber creates no sparks, is difficult to tap into (unlike cable lines) and is largely non-corrosive. In short, fiber optics are great for secure, high-precision, harsh environment applications. Furthermore, in the world of increasing bandwidth, fiber is necessary to ensure that our information services continue evolving.
Why Fiber for the Future? For telecoms moving forward, information is almost certain to be carried largely by fiber. Better manufacturing processes are making the technology cheaper every day as consumers and businesses demand faster, more reliable internet and phone service. Existing phone, television, and internet networks can be altered to use fiber optics to achieve greater throughput, especially when multiplexed signals are introduced. Streaming of 4K and Blu-Ray quality videos is difficult (if not impossible) on our copper networks carried throughout the United States, as can be seen in our lagging internet speeds. Telecoms resistant to investing in the network claim that there are no applications that need symmetrical gigabit speeds—much in the same way that power companies insisted that a national power grid was not necessary in the earlier parts of the twentieth century. For innovation, entertainment, and overall progress, the United States needs to take advantage of the benefits that the technology behind fiber optic cabling can bring. ■
» CLICK HERE to comment on the article.
Visit: eeweb.com
29
PULSE
n part 1 of this article, the fi to be necessary in the e was selectivity and then sen this article to show how thes advanced until other factors These other factors are vario Dynamic Range, or D.R. for s in detail in part 3
32
Visit: eeweb.com
TECH ARTICLE
first two requirements found earliest of radio equipment nsitivity. It is the intention of se two important properties have become as important. ous aspects of what is called short. These will be covered 3 of this series.
Rodney Green Lead Inventor abd Head of R and D Mulpin Assembly Technology Visit: eeweb.com
33
PULSE To read Part 1 of this series, click the image below:
Constant Selectivity and the Superheterodyne Receiver As discussed in Part 1 of this series, as the frequency of a single or coupled tuned circuits is increased, the selectivity decreases to the point that there may be insufficient selectivity at the higher frequencies being tuned. This helps avoid several overlapping broadcast stations being heard at the same time. To combat this problem, manufacturers of radio receivers needed to turn their attention to the Superheterodyne Technique. This type of receiver converts the incoming frequency from the receiver’s antenna to a fixed frequency, no matter what the incoming frequency is. This fixed frequency is called the Intermediate Frequency, or simply the I.F. In order to do this, a special non-linear
Figure 1: Schematic of a mixer and oscillator typical of those used in valve Superheterodyne receivers.
34
Visit: eeweb.com
radio valve called a mixer was developed. The schematic for one such mixer is shown in Figure 1. All valves have non linearity, and the amount of non linearity depends on how the tube or valve is biased on its grids and on its construction. In a mixer valve of any type, the non linearity varies with the voltage applied to one of its control elements or grids, or even its cathode as is the case in Figure 1. Non linearity simply means that the output signal from the valve is not an exact copy of its input signal. Thus, if the gain or amplification of a valve were to change along with the voltage on the gain control element, then the output of the valve will be a combination of the gain control voltage and the input signal. Now, if the voltage on the gain control element was a large periodic signal such as that derived from a sine wave oscillator, and the input signal from the antenna was a periodic signal on a different frequency than that oscillator, the output of the valve would have predominantly three main components: • A frequency equal to that of the sum of the antenna frequency and the oscillator frequency. • A frequency that is the difference between the antenna frequency and the oscillator frequency. • The original input signal to the mixer. The Superheterodyne receiver takes advantage of this property found in valves (or semiconductors in modern sets). Now, let’s suppose we were to have a signal from the antenna of 4 Megahertz, and that an arbitrary I.F. was chosen as 450 Kilohertz. To convert the 4 MHz signal to 450 kHz (or 0.45 MHz), an oscillator frequency of 4.45 MHz would be needed. If the antenna circuits were tuned to 30 MHz, the oscillator would need to be on 30.450 MHz. The fixed frequency of the I.F. will have the effect of removing these overlapping signals at the higher frequency as they too will be converted if they get into the mixer. However, their frequency range will be outside the range of the I.F. filter tuned circuits. The oscillator frequency can be lower or higher than the received frequency by the same amount, but generally speaking, the oscillator is normally set to be higher than the desired incoming frequency. You should also note that the mixer output has both sum and difference frequency currents and input signal currents. However, the I.F. filter removes the
TECH ARTICLE unwanted frequencies and these do not get coupled on to the next stage of the receiver, which is the I.F. amplifier.
I.F. Amplifier Characteristics The I.F. amplifier is likely to have one or two amplifier tubes or valves, and each valve will have one or two tuned circuits on both input and output of each stage. These filter out all signals that are not required for a given communication purpose. In the high frequency (HF) radio bands, the required signal is generally either for speech, music, or both. The I.F. amplifier only needs to process these signals and reject any other signals. In the more expensive radio receivers, considerable effort has gone into making the I.F. filter have a flat top to its response, and steep sides so that the unwanted signals drop away very steeply. Another important feature of an I.F. amplifier stage is its ability to automatically adjust the amount of amplification of the signal applied to it so that the listener will hear a nearly constant volume no matter how strong the signal is. This feature is called Automatic Gain Control or AGC. This is also accomplished using a special I.F. amplifier tube called a remote cutoff pentode. These have specially made control grids which vary the amplification of the valve with a DC voltage applied to it, which is derived from rectifying the I.F. signal in the last stage of the I.F. amplifier. The AGC is applied to the control grid of the I.F. amplifier tube or tubes as shown in Figure 2.
Choice of Intermediate Frequency In Part 1 of this series, I indicated that it is easier to get good selectivity with fewer cascaded or coupled tuned circuits when the frequency is low. Thus, it would seem that the lower the intermediate frequency, the better. However, with superheterodyne design, there are a number of conflicting requirements that directly affect the I.F. chosen, so this is not an arbitrary decision. This decision also changes with technological advancements.
Image Response For every signal going into the mixer stage of a receiver, there are two signals that come out on each side of the local oscillator frequency,
Figure 2: A typical I.F. amplifier schematic of the 1940s. separated by the frequency of the I.F. It is also true that the same mixer will have an output at the intermediate frequency for two input frequencies at the antenna of the receiver. Looking at the same example as that above, supposing we want to receive a signal on 4.0 MHz, and that the I.F. is 450 KHz. This means that the local oscillator is on 4.450 MHz. The second (image) frequency at the antenna of the receiver which will also give the same I.F. response is a signal on 4.90 MHz. This is the difference between the input frequency (4.90 MHz) and the local oscillator frequency of 4.450MHz. In this instance the local oscillator is on the low side of the input signal. Now, the receiver cannot distinguish the image frequency from the wanted signal, unless the selectivity before the mixer can remove it. If you look at the image situation, it is always separated from the desired signal by twice the I.F. frequency. In the above example, the desired signal is 4 MHz, and the I.F. is 0.450 MHz. Thus the image is 0.90 (2 x 0.45) MHz above the wanted signal, which makes it 4.90 MHz. This relationship holds for all nonlinear mixers. A single tuned circuit in the front end of a receiver will remove the image 900 kHz from the wanted signal at this lower frequency. Thus, 450 KHz for the I.F. is fine for frequencies below about 10 to 15 MHz at the antenna.
Visit: eeweb.com
35
PULSE Ganged Tuning Capacitor controls local oscillator and two RF amplifiers
Introducing Tuned Circuit Q Double tuned I.F. transformer
Adjustable iron dust core
Figure 3: Internal view of a high performance radio receiver AR88 of the 1940s showing some of the features described in the text.
As the received frequency rises, the front end tuned circuits get less selective. When the wanted frequency goes up there is sufficient signal energy from the image frequency (if there is a signal there) that the image may be the predominant signal, especially if the image frequency has a strong signal on it and the wanted signal is a weak one. There are two ways that this can be avoided. In the earlier sets, there was an amplifier between the antenna and the mixer that had two ganged tuned circuits at the input frequency. The tuned circuits increased the selectivity and reduced the level of interference from the image frequency to an insignificant level. There are some very good examples of radio receivers at that time which have this feature and they could tune up to 30 MHz without significant image problems. Many of these were used in domestic short wave receivers. There were even receivers that had two RF amplifiers ahead of the mixer. One such receiver—the AR88—was a high performance receiver used by the military during and after World War 2, an example of which is shown in Figure 3.
Increasing the Intermediate Frequency Another way to reduce the effects of the image problem is to increase the frequency if the I.F. of the receiver. Remember that the image is separated from the desired signal by twice the intermediate frequency. Thus, the higher the I.F., the further away the image is, which reduces the requirements for sharper front end-tuned circuits. However, especially during the late 1920’s to the mid 1930’s, it was difficult to economically make an I.F. amplifier with enough selectivity at higher frequencies. The reason for this is that the coils of wire within the tuned circuits all had air cores. The inductance of a coil of wire increases approximately at a ratio of the square of the number of turns. Thus, a coil with 10 turns will have an inductance value 9 times lower than a coil of 30 turns. However, the resistance of the 30 turn coil will only be 3 times as much as that of the 10 turn coil. This meant that to make a higher frequency I.F. transformer, the inductance needed to be reduced, but relative to the inductance, the resistance was reduced by a lesser amount, making the inductance to resistance ratio lower, and thus lessening the selectivity of a tuned circuit. The I.F. of the earlier radio receivers needed to be low to maintain selectivity. I.F. amplifiers at the time were generally tuned between 50 kHz. and 455 kHz.
36
Visit: eeweb.com
A tuned circuit at resonance has equal and opposite reactance of its capacitor and its inductor. Tuned circuits have what is known as Q factor—Q meaning “quality.” It is measured as the ratio of coil reactance divided by its resistance. Capacitors have also a measurement of Q and that is also its capacitive reactance divided by its resistance. Now, because capacitors are made of metal plates separated by an insulating dialectic, there are no long coils of wire and thus capacitors have a much higher Q than inductors. For this reason, the Q of a tuned circuit is equal to its inductive reactance (XL) divided by its resistance. It should also be measured at the operating frequency. The higher the Q factor, the sharper the tuned circuit becomes. Practical tuned circuits can have a Q factor of up to about 200.
Figure 4: Shows five tuned circuits with different values of Q. Note that the lower the Q, the wider the frequency range (delta (F) becomes. See for example that the delta (F) is much narrower at the 1/2 power point as the Q gets higher. This occurs all the way down each slope.
Figure 5: Shows a typical Double Tuned I.F. Transformer shown without its capacitors and its cover, as used in valve radio receivers. The Iron dust tuning slugs are screw driver adjustable to allow for resonance adjustment.
TECH ARTICLE The ½ power (70% for voltage) bandwidth of a tuned circuit relates to its Q in that the bandwidth B is equal to Operating Frequency F divided by the Q. thus B=F/Q, where bandwidth is in the same units as the frequency. Figure 4 shows five tuned circuits with different values of Q.
Introduction of Iron Dust Cores Finely powdered iron, mixed in with an insulating medium concentrates the magnetic field around an inductor and vastly increases its inductance. Thus, for any given inductance value, considerably less wire can be used, which drastically reduces the resistance of an inductor. This, at last, made it possible to increase the coil Q and increase the intermediate frequency whilst maintaining proper selectivity. Figure 5 is a picture of a typical I.F. transformer with Pi windings with iron dust cores as used in valve radios from mid to the late 1930s onward.
Selectivity and Fidelity If a receiver has a low I.F. and is very selective due to cascaded tuned circuits, a received signal will have a very peaky, thin, and unpleasant tonal quality. This is because the selectivity can be so sharp that it cuts into the desired signal’s higher musical notes, if that is what is being listened to. The highest selectivity trace in Figure 4, if it were a low frequency I.F., could easily cut into the desired signal. So what can be done about it? Steep side skirts can be implemented to keep out adjacent stations, but also a flatter top to the band pass (top) area of the I.F. filter.
Tuned Circuit Coupling All of the tuned circuits we have looked at so far have had little or no coupling between them directly. Thus, the filters presented have simply had a frequency versus amplitude response which is a simple arithmetic sum of each response. There is a natural effect on tuned circuits if they are allowed to interact directly with each other that is called mutual coupling. Depending on the degree of coupling, the coupled tuned circuits exhibit a flattening of the top of the frequency response and further coupling causes a dip in the middle of the response, which is usually undesirable. The side
Figure 6: Shows the effect on bandwidth of a double tuned circuit with varying degrees of coupling.
skirts remain nearly as steep as for the two tuned circuits cascaded without mutual coupling. Figure 6 shows the effect of various degrees of the mutual coupling of two tuned circuits. The normal method of setting the required amount of coupling is to move two Pi wound coils of Figure 5 closer or further apart. The closer they are the more mutual coupling there is. There are several other methods shown in literature but are not covered in this article. A tutorial on mutual coupling can be found in Reference 2.
Solid State Receivers and Dynamic Range (DR) With the advent of Solid State (transistor) radio receivers in the 1960s and onward, it was found that there was another pressing issue with radio receiver design which needed addressing—the Dynamic Range (DR) of receivers. With the HF radio spectrum becoming more and more crowded with stations, some of which were transmitting power in the megawatt range, it became apparent that good selectivity was just not enough to remove interference generated within the early stages of a radio receiver before the I.F. filters. The interference generated could pass unwanted signals to the I.F. filter and pass through it. There are a number of processes within a radio receiver that contribute to this, which will be covered in Part 3 of this four part series.
References 1. Double Tuned Circuit a Tutorial. Go to the link below: http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/ hambladen/qst/1991/12/page29/index.html 2. A tutorial on tuned circuits and their Q: http://en.wikipedia.org/wiki/LC_circuit
» CLICK HERE to comment on the article.
Visit: eeweb.com
37
Hearing Bugs
Work Bribe
Good Ol' Days
Join Today eeweb.com/register