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Low-power transmitters are made up of the same basic circuits as any other transmitters. Some of the circuitry, though, such as modulators, power supplies, and power amplifiers, differ in size and operating voltages and currents. Generally, certain features found in larger transmitters, such as metering, safety and protection circuits, modulation limiting, and other monitoring functions, may be omitted or are provided in simplified form. Safety and protection are usually not issues because the power is too low to result in damage to components under fault conditions, and operating voltages are 1.5 to possibly 24 volts, with 6 to 12 volts being the most commonly used DC supply voltages. Ten watts of RF output would mean about 1 to 2 amps at 12 volts, so the current levels are not high either; however, at power levels of 1 watt, painful RF burns to the skin are possible, and even lower powers may be hazardous under certain conditions, but for most Part 15 applications, we are working with a few milliwatts at most.
Solid-state circuitry is almost universally used, but some experimenters may work with vacuum tubes, especially at the 1- to 10-watt RF power levels, which pose a shock hazard from the 100- to 250-volt plate (B+) supplies necessary for vacuum tube work. In this guide, we are not concerned with vacuum tubes, but we mention them anyway because they still have some application in transmitter work. At very high RF power levels (1000 watts or more), they are still considered by many engineers to be the technology of choice. In the areas of reliability, fault tolerance, efficiency, physical size, and cost, vacuum tube technology still has the edge for high power RF work. A 1000-watt vacuum tube RF amplifier can generally be made smaller, lighter, and cheaper than a 1-kW solid-state amplifier because no bulky heat sinking is needed for the transistors.
The basic building blocks to be discussed are as follows:
- · Oscillators
- · Amplifiers
- · Multipliers
- · Modulators
- · Frequency synthesizer (PLL) circuits
These components are discussed regarding their application in low-power transmitters and their relative merits and drawbacks. For detailed theory of these circuits, we refer you to any good reference text on the subject.
Oscillators
Oscillators are necessary in any low-power transmitter because they are a means for generating the necessary RF signal. Oscillators come in many forms, but we are mainly concerned with those suitable for RF signal generation at frequencies higher than 100 kHz. A good oscillator-as far as low-power transmitter use is concerned- has the following desirable characteristics:
1. Good frequency stability: Ideally, temperature and voltage variations, together with inevitable circuit component tolerances, should not have too serious an effect on the oscillator frequency. Only certain components should affect the frequency of oscillation. The load placed on the oscillator should not change or "pull" the oscillator frequency.
2. Adequate power output: Signal output should be adequate to drive the load or following stage, even at the minimum expected supply voltage. The oscillator should start up reliably at low supply voltages and temperatures.
3. Spectral purity: This means that the oscillator output should have a single, monochromatic spectral line, with absolutely no energy present at any frequency other than that desired. This characteristic can be related to frequency stability.
4. Ability to be modulated: Direct FM is often used, in which an audio or other modulating waveform is applied to a varactor diode, which is connected to the frequency-determining elements in the oscillator circuit. This requirement conflicts with those of stability and purity, and in a specific application, design compromises must be made.
5. Simplicity: To keep size and cost down, circuits with as few components as possible are preferred, but other performance requirements may dictate the exact circuitry needed.
Frequency-determining elements are those elements in an oscillator circuit that affect or are intended to determine the oscillator frequency, such as an LC tank circuit or a crystal. These elements must have as high a Q factor as possible. The stability of an oscillator is proportional to the rate of phase change with frequency in the oscillator loop, which consists of an amplifier and a feedback network. Stability is often broken down into long term and short term. Long term is, for example, the ability of an oscillator to maintain its set frequency within satisfactory limits over a long period, such as a week, month, or year. Component aging and long-term physical changes in components determine this ability. Short-term stability consists of such factors as warmup drift, drift caused by temperature changes normally incurred during operation of the circuit, on-off cycling, and so forth. Timeframes of up to one day would be applicable. Oscillator phase noise consisting of frequency changes of very short duration, less than 1 second, can be considered as a very short-term drift.
All oscillators exhibit phase noise caused by spurious modulation of instantaneous frequency by various forms of noise effects in circuitry and individual components, such as flicker noise in transistors and the inevitable thermal noise in resistors.
This noise may have spectral components out to several hundred kHz or more, but it generally falls off with increasing frequency, and a typical figure might be 60-100 dB down or better at 1 kHz away from the main frequency. This performance figure is normally specified in dBc, meaning the number of decibels the noise power is down with respect to the carrier (oscillator frequency), generally at 1 kHz or another specified frequency away from it, as measured in (usually) a 1-hertz bandwidth. The better oscillators in this regard have high Q frequency-determining elements that dominate the loop characteristics. Crystal-controlled oscillators are the best, and simple RC oscillators, such as NE555 timers, are the poorest.
In low-power transmitter work, crystal control is preferred, either directly in the master oscillator or indirectly as in a PLL synthesizer. This is especially true if narrowband modulation is to be used, such as AM, narrowband FM, or single sideband (SSB). These emissions have occupied bandwidths of 15 kHz or less. The receiver should ideally have no more bandwidth than necessary for the particular type of modulation, for the best signal-to-noise ratio, which translates into the best range, lowest usable transmitter power, and smallest antennas, all of which are important considerations. This has a direct bearing on the frequency stability requirements for the low-power transmitter. Typical transistor LC oscillator circuits that would be used in low-power bugs and surveillance transmitters, and also in consumer-grade AM and FM receivers, might have an overall stability (combined long and short term) of 0.1 percent or so, and as poor as 1 percent of the nominal (center) oscillator frequency. For an AM receiver and a small surveillance transmitter operating at, say, 1500 kHz, this would be 1.5 kHz. Because the AM receiver has about 10 kHz of bandwidth, a transmitter drift of 1.5 kHz would at worst cause a little audio distortion, depending on the exact shape of the receiver bandwidth curve (i.e., sharp or broad response). This problem is not serious and is easily corrected by a slight retuning of the receiver, if this is possible. A poor transmitter oscillator with 10 or 15 kHz of drift would be a nuisance, but the signal would still be easily found because at worst it would have moved up or down one channel, as long as the channel it drifted into was unoccupied by a strong station.
If the transmitter was operating at, say, 150 MHz, however, this would be another story. The transmitter now may have moved as much as 150 kHz or even 1.5 MHz with a poor design. The signal would now be much harder to find initially, and once found, it may keep wandering around, making it difficult to keep the receiver tuned to it. This would be a pretty bad situation when a recording is being made or in any other situation in which signal interruption is not allowable. In this case, crystal control of the transmitter frequency would be mandatory.
The better free-running LC oscillators can achieve about 0.01 percent short-term stability (i.e., 1 part in 10000). This stability is adequate for standard AM and short wave broadcast reception over the 150 kHz to 30 MHz range. This was also typical of the AM ham receivers, transmitters, and variable-frequency oscillators (VFOs) built in the 1950s and early 1960s, but it proved to be inadequate for serious continuous wave (CW) and SSB work. With careful design and choice of components, volt age and temperature compensation, and rigid mechanical design with temperature compensated corrected materials, somewhat better than 1 part in 100,000 (0.001 percent) can be achieved. This feat of engineering is not often achieved, though, and is almost impossible in any type of transmitter we will be working with. Good amateur radio VFO designs for SSB use achieve, after a warmup period, 100 Hz short term stability at frequencies typically in the 5-MHz range. This is a stability of .002 percent. In modern ham gear (post 1985), this stability is still not very good, especially when digital frequency readouts to 10 Hz are commonly used and expected. A mediocre crystal oscillator circuit, on the other hand, can easily perform an order of magnitude better than this in the area of short-term stability.
In the previous example, to get a short-term stability of 1.5 kHz at 150 MHz, we would need 1 part in 10 million, or .0001 percent stability. This goal is easily achieved with a crystal and a properly designed oscillator circuit. Modern scanner receivers cover up to as high as 2000-2500 MHz, and frequencies this high are increasingly being used for narrowband work. Therefore, high-frequency stability is important. The use of PLL techniques allows variable-frequency operation with one or sometimes two fixed crystal oscillators. Therefore, free-running oscillators should be used only in low-power transmitter work, where the occupied bandwidth of the transmitted signal is 0.1 percent or more, and if some frequency drift can be tolerated. These instances include garage door openers, FM wireless mikes, and certain toy applications. A few projects and circuits in this guide use this approach. Some representative oscillator circuits that have application in low-power transmitters are shown in FIG. 1.
FIG. 1 Typical Oscillator Circuits for Low Power FM Transmitter Use
Amplifiers
Amplifiers used for low-power transmitters may be either of IC or discrete transistor construction. For most small low-power transmitter applications, small-signal bipolar and FET discrete transistors of the appropriate type, having Ft ratings of 3 to 10 times the operating frequency, are adequate. Modern RF silicon NPN types come in both conventional plastic, metal, and surface-mount packages, and even audio types sometimes have Ft ratings of more than 100 MHz. We use the 2N3563 for many RF applications, but many other types, such as the 2N918, 2N2857, 2N5179, and 2N2369, are also usable. These types have been around for more than 20 years, are widely available from the surplus market, are inexpensive, and have been manufactured by several sources. Many other types are suitable, but these are our personal favorites, not because they are necessarily the best, but because of our long experience with them. For amplifiers that must supply a little more power, the 2N3866 and 2N5109 are excellent, and for UHF, the MRF559 X lead package is good for a half watt at 800 MHz, or the BFR96. In addition, higher-powered TO39 types, such as the 2N4427 and MRF630/SD1444, are good for more than 1 watt at 150 and 450 MHz, respectively, and are easy to work with, with no fragile beryllium oxide pack age to worry about damaging.
RF power ICs (actually monolithic modules) that take all of the work out of designing RF power amplifiers are available from several manufacturers. They are flat assemblies that operate from popular supply voltages and frequency ranges, and some have provisions for RF power output control, which can be used for amplitude modulation. They normally must be mounted on a heatsink; however, these modules generally are designed for specific frequency ranges and power levels, and using them may tie you down to a specific manufacturer. In case of failure, they are more expensive to replace than a discrete RF device, and the manufacturer may discontinue them, with no future availability of a replacement unit. They are useful, but they have associated tradeoffs, both in design freedom and future availability.
Today, RF transistors with 500-MHz Ft ratings are inexpensive, and ratings of more than 5 GHz are common, with devices available to 25 GHz and higher. Typical 3-5 GHz types in this category are the MRF901, BFR90, and the NEC25139 dual gate FET. It does not pay to use a transistor with a much higher frequency rating than you need, and this practice may actually cause you trouble because of parasitic effects. It is common to have a 5-GHz transistor generate spurious UHF signals because of stray inductances and capacitances of common components intended for use at lower frequencies. We had an amplifier that was intended for 220 MHz generate a 1-GHz signal (1000MHz), with the 220-MHz tuned circuit having no effect whatever. The collector-tuning capacitor had a resonance at 1 GHz, with the emitter inductance and collector emitter capacitance of the transistor acting as a feedback network at 1 GHz. This problem was cured by using chip components for the capacitors, but without a spectrum analyzer, the problem would not have been so easily spotted. A 500- or 1000-MHz transistor (2N3563) did not exhibit this effect and worked fine.
Several IC devices are sold for RF amplifier use. The Motorola MC1350 is an 8 pin IC that works well up to 100 MHz. The MAR-1 (Mini Circuits Corporation) is a monolithic microwave IC (MMIC) that works well if limited output power is satisfactory, and is good to 2000 MHz with gains of around 8-17 dB, depending on frequency. The ERA series from the same company works up to 8-10 GHz. These ICs use a 4-pin X package and a single 5- to 12-volt supply, and are 50 ohms in and out, simplifying interfacing. For the common 10- to 20-dB gain stage usually needed, however, ICs offer little or no advantage and save very little. They are best used when special requirements or very high RF gains are needed that are unavailable from a single amplifier stage using discrete transistors, or when several functions must be performed, such as in an FM receiver limiter-detector or a multistage gain controlled IF system that must supply automatic gain control (AGC) AFC, meter and squelch functions, and so forth. ICs of this type often provide performance far superior to an equivalent discrete component design, with much less circuitry, fewer adjustments, and simpler setup and testing. They may also offer "deluxe" features that are not easily supported by traditional discrete component designs, such as logarithmic meter output for signal-strength monitoring, synchronous AM detection, digital volume and tone controls, multi-standard operation capability, and the opportunity for microcontroller interfacing.
Many RF devices in conventional TO92 plastic and X lead packages are being phased out by their primary manufacturers because they are being replaced with surface-mount types; however, other manufacturers are picking up many of these discontinued types, so they will be available in the future and can be obtained surplus.
There will always be a need for these devices for maintenance and replacement purposes because they have become "industry standards." The 2N2222, 2N2905, and 2N3055 devices, for example, which date back to the 1960s but have been improved, are still useful in new designs and are still popular for experimenters. In many regards, some transistors are fairly "generic" because proper circuit design using feedback techniques can cancel the effect of wide variations in parameters. With 10 or 20 well-chosen devices, you can do almost anything you want to. A few representative RF amplifier circuits are shown in FIG. 2.
FIG. 2 Representative RF Amplifier Circuits
Multipliers
Frequency multipliers are basically nonlinear circuits that produce harmonics of the operating frequency. A filter, which usually comprises tuned circuits, picks out the desired harmonic and rejects the input frequency and all other harmonics. Multipliers are normally used to double or triple the input frequency, but higher orders of multiplication are often performed. Multipliers may consist of discrete transistor amplifier stages, Schottky (hot carrier) diodes, varactors, and snap diodes. In low power transmitter work, the discrete transistor stage is often used. The output net work is generally a double-tuned circuit tuned to twice or three times the input frequency. This allows higher frequency output from a lower-frequency oscillator and acts as a buffer to reduce interaction of the load and oscillator stages.
A multiplier stage must be driven into nonlinearity to produce harmonics, and the output power is often no more than the input power, and sometimes less, especially when multiplications of 4 or 5 are performed. A multiplier stage should be driven from a low-source impedance, both at the input and output frequencies, for best performance. RF efficiency (the ratio of RF out to DC power input) tends to be poor, with 20 percent for a doubler and 5-10 percent for a tripler being typical. Some circuit approaches use specific transistors that exhibit a collector "varactor effect," and these circuits exhibit much better efficiency as frequency multipliers. These circuits use idler-tuned circuits in the input networks and work well when properly tuned, but they are prone to spurious outputs and instability. A spectrum analyzer is necessary when working with these circuits. In the case of multipliers that are strictly overdriven amplifiers, however, the rest of the (unwanted) output is dissipated in the transistor and output network loss resistances as heat.
In addition, the fundamental component appears in the output along with unwanted harmonics and is, along with lower-order harmonics, usually larger in amplitude than the desired harmonic output. The output network must reject everything except the desired harmonic. Double-tuned circuits are a must for doublers and triplers, with triple- or quadruple-tuned circuits often needed for higher orders of multiplication. Therefore, it may be better to use two doubler stages instead of one quadrupler. The same transistors that are used for RF amplifiers can be used as multipliers, and output frequencies approaching the Ft of the transistor can be produced with reasonable efficiencies. You should not use frequency multipliers to pro duce power outputs of more than 50 mW, and less than 20 mW is even better, to reduce levels of unwanted harmonics. You should never use the output stage of a transmitter as a frequency multiplier-as was often done back in the vacuum tube days-because this is asking for problems with spurious emissions. Representative multiplier circuits are shown in FIG. 3.
FIG. 3 Representative Frequency Multiplier Circuits
Modulators
A modulator, for our purposes, is a circuit used to superimpose information on a carrier waveform. The amplitude of the carrier (usually, but not necessarily, a sine wave) may be modulated with this modulating waveform (AM or SSB), or the instantaneous frequency (frequency modulation, FM) or phase (phase modulation, PM) of the carrier wave can be modulated with it. Another possibility is that the carrier wave may be simply turned on and off (pulse or CW). Combinations and variations of these methods may be employed as well. The modulating waveform can be audio, video, digital pulses, or combinations of these methods. In most low-power work, audio is used, and video is used for TV transmissions.
Digital waveforms are used for control purposes or data transmission and for dig ital audio and video. In addition, a hand-operated key switch may be used to turn on and off the carrier to form Morse code characters, the oldest form of digital communication, and the only digital communication method readable by humans without special equipment. Morse code is one of the best weak-signal communications modes in use, with performance exceeded only by sophisticated digital techniques such as binary phase shift keying (BPSK) requiring sophisticated equipment and phase coherent transmitters and receivers operating at slow data rates.
Morse code still gives the best results for weak-signal long-distance communication with low-power equipment. Amateurs have made two-way intercontinental contacts with 100 mW or less power on the HF bands, between 2-30 MHz unit cost of all modulation modes using Morse code (CW). Despite the easing and elimination of the code requirements for a ham license, Morse code is still often used and is popular, often getting through when all else fails. The ability of Morse code to use 100 Hz receiver bandwidths really helps because it allows signals 10-20 dB weaker than SSB voice to be plainly audible. It is not and will never be obsolete, contrary to what anyone says, until the day we humans are obsolete.
Modulators for AM are generally audio amplifiers that superimpose their output on the DC supply to the output amplifier or another amplifier in the transmitter signal path. The instantaneous voltage supplied to the stage by the modulator deter mines the output of that stage. In many cases, the DC supply to the RF stage is in series with the modulator output. The modulator must provide a power output (audio or video) equal to half of the DC input to the RF stage being modulated, for 100 percent modulation. For low-power transmitters, powers of at most a few watts are needed, and simple transistor or IC audio amplifiers make good audio AM modulators. Video modulators for conventional (AM) National Television Standard Committee (NTSC) video need to handle bandwidths of up to 4-5 MHz, and most audio ICs will not handle this high of a frequency, so a discrete audio amplifier design using high-frequency transistors must be used. Clamping circuits are also needed to ensure proper DC video-level relationships because the sync pulse tips must always produce constant maximum transmitter RF output levels, regardless of the rest of the video waveform. In addition, support is needed to add a low-level audio subcarrier (FM) at 4.5 MHz if audio transmission is needed.
Frequency modulation is generally accomplished with a varactor diode connected to the frequency-determining elements of a low-power transmitter. In addition to a varactor (also called varicap) diode, several other approaches, such as a reactance modulator, are possible but are seldom used in low-power transmitter work and therefore are not mentioned. The varactor diode method cannot be beat for simplicity and reliability. The audio, pulse, or video waveform is coupled to the varactor through RF isolation components (RF chokes or high-value resistors). This setup varies the instantaneous capacitance of the diode, causing the frequency of the oscillator circuit to change in accordance with the modulating signal. FM can be narrow band (deviation 15 kHz or less), such as commercial two-way radio and 2-meter amateur FM or wideband (25 kHz or more), such as in commercial FM audio broad casting.
The deviation is a measure of how much the carrier frequency varies under modulation and must not be confused with the modulating frequency, which determines how often the carrier frequency is varied as a result of modulation. The ratio of the deviation to the modulation frequency is called the modulation index and is often designated by the Greek letter ß (beta). The values of ß vary instantaneously. In an FM stereo transmission with a 1-kHz audio signal and a 75-kHz deviation, ß is 75.
Increasing the audio modulation to 15 kHz produces a ß of 5. The larger the value of ß, the better signal-to-noise ratio that an FM system can potentially produce com pared to an equivalent AM system. This is called the FM improvement factor, but it is only valid at signal levels over a certain threshold, in which the carrier-to-noise ratio of the received signal is 7-10 dB, depending on system design and other factors. Below this level, AM is superior, but in practice, we make sure the received signal is above threshold at all times, so FM is quieter and cleaner sounding than AM, as any radio listener knows. FM modulation is also used by satellite television broad cast services, for microwave video links, and for some amateur TV work, especially in Europe, on the 23-cm (1300 MHz) band and higher. Deviations of several MHz are used for video, with carrier frequencies usually in the microwave range (more than 1000 MHz).
SSB is basically AM with the carrier and one sideband removed, leaving a signal whose makeup can be likened to an audio signal arithmetically shifted to a much higher frequency. Most SSB work is done in the HF range, with some in the VHF and UHF bands, mainly amateur and military. Because the carrier is not transmitted, it must be reinserted at the receiver. Either the lower sideband (LSB) or the upper sideband (USB) can be transmitted, and it is possible to use both for two different audio channels (independent sideband [ISB]). In addition, audio subcarriers can be used for even more audio channels. The sidebands are RF just like the carrier and therefore do not need the carrier to be transmitted. The carrier has two-thirds of the total power of an AM signal with 100 percent modulation, and the remaining one third is equally divided between the LSB and USB. Therefore, each sideband has one-sixth the total power.
A 40-watt carrier AM signal has 10 watts in each sideband, and because only one sideband is needed, a 10-watt SSB transmitter would be equally effective as a 40 watt AM transmitter. The advantage gained here is 6 dB, and because only half the receiver bandwidth is needed, another two times (3 dB) advantage is gained at the receiver, for a total of eight times advantage. A 40-watt SSB transmitter would theoretically have an eight times advantage (about 9 dB) over a 40-watt AM transmitter, and is smaller because of the elimination of the required heavy AM modulator.
Therefore, SSB has largely replaced AM in HF communications work; however, SSB poses some strict frequency stability requirements. To sound natural, the carrier frequency must be reinserted within 50 Hz of the original, with 10 Hz preferred. As the carrier frequency increases, this goal becomes increasingly difficult to achieve, and the extra circuitry needed to generate and receive SSB, together with special filters to remove the unwanted sideband, limit audio quality and frequency response.
AM is still used for broadcasting where strong signals are normally available and reasonable audio quality is important, and simple low-cost receivers must be made available. AM is still used at VHF (118-136 MHz) and UHF (225-400 MHz) for ground-to-air communications by the military and commercial airlines and is universally used worldwide for this purpose. It also avoids the FM "capture effect," where a somewhat strong signal may completely suppress another signal that is only 6-10 dB weaker. This situation would be undesirable in aviation communications because a weaker emergency signal or a signal from an aircraft, say, 10-20 miles out from the airport could be totally suppressed by a stronger signal from an aircraft on downwind or final approach. The air traffic controller (ATC) could be totally unaware of and miss a transmission from a large jet airliner carrying a hundred or more passengers, or from another aircraft in trouble. This situation could obviously result in a serious accident. With AM, the ATC still hears that someone else is attempting to communicate, even if the message is garbled or unreadable, and they can request another transmission. This is a good reason to stay with AM. (SSB is covered further in a later section because it is a rather complicated subject.)
Frequency Synthesizer (PLL) Circuits
Frequency synthesizer circuits are feedback servo loops in which a voltage controlled oscillator (VCO) generates a signal whose frequency is locked to a fixed reference signal (see FIG. 4). A signal sample from the VCO feeds a frequency that is suitable for the frequency divider circuit, which can be made to have a fixed or variable divide ratio (generally called N) and is compared to a crystal oscillator in both frequency and phase. This is done with a frequency and phase detector, whose output can go in either a positive or negative direction. The output-a DC voltage at lock-controls the VCO to achieve a steady-state detector output. Because the divider can be made variable, the VCO frequency varies and equals the reference frequency multiplied by N, which can have almost any value, and frequency resolution equals the reference frequency. Therefore, if the reference frequency is made 1 kHz, and the value of N is 10000, the VCO will produce a 10-MHz signal. If N is increased to 10001, the VCO output will be 10001 kHz.
FIG. 4 Basic PLL Frequency Synthesizer System
All of the divider and phase detector circuitry, and often the reference oscillator circuitry, is contained in an LSI IC chip, and surprisingly few components are needed to build a simple frequency synthesizer. Motorola makes chips specifically designed for this purpose, some operating as high as 2 GHz. The VCO design is critical to good performance. Because this is a feedback loop, loop constants must be carefully chosen in the same manner as any other feedback system. In order to achieve low-phase noise, good spectral purity of output, and stable lockup with reasonable settling times (the time it takes for the frequency to settle within specified limits), some design decisions and compromises must be made.
The programming of the variable divider and the reference frequency is generally done via digital logic lines going to the synthesizer chip. Data may be in parallel for mat, such as in the Motorola MC145151-2, which allows easy programming via logic levels that can be obtained from manually set external switches or a microcontroller. Data may also be in serial format, which vastly reduces the number of data lines to the synthesizer chip to only three-a clock, data, and a chip enable line. The Motorola MC145170-2 is an example of this kind of PLL synthesizer chip. Although requiring a microcontroller to interface to it, this chip results in a simple synthesizer with little circuitry. A microcontroller such as an 8051 or 8751 type or a PIC chip such as the 16C84 or 16F84 can easily provide this functionality and still have plenty of room left to manage the digital display generally used with a synthesizer and per form other overhead tasks. The serial approach using a microcontroller is therefore preferable, and most of the commercially available PLL synthesizer chips seem to use this approach. It is also possible to generate the required serial data in a PC and use its serial port to program the synthesizer, allowing computer control of the synthesizer frequency of the receiver or transmitter, while providing a display of frequency on the monitor screen, assuming the appropriate software is installed. Cards with wideband receivers covering 500 kHz to 1300 MHz are available for installation in your PC and contain a synthesizer chip similar to the ones previously mentioned.
A PLL chip may also be set up to act as a frequency multiplier, enabling elimination of a discrete transistor multiplier chain. The Motorola MC13176 one-chip transmitter does just this, and allows a crystal at 1/32nd of the desired output frequency to control a VCO operating in the VHF or UHF band. If a transmitter at, say, 440 MHz is wanted, you can use a crystal at 13.75 MHz, add a few components, and get a few milliwatts at 440 MHz. An FM and an AM modulator is built in, allowing production of a modulated signal that only needs to be amplified to a suitable level for trans mission. The advantages of this chip are its small size, simple operation, and low cost. The disadvantages are its susceptibility to mechanical FM modulation of the VCO caused by physical disturbances of sensitive components, low power output, and rapid obsolescence because of discontinuance of the chip by the manufacturer. It is also a rather small surface-mount device that some experimenters may find difficult to use; however, it would make a good small transmitter with crystal stability for surveillance work. This chip and many others that have been introduced for use in wireless devices are nice to experiment with, but we wonder if they will still be avail able several years from now. These so-called solutions may become problems at some future date. We would not stake our life on any of these chips or make any long-term production commitments unless a second source becomes available. For a unique experimenter project or a limited production throwaway item, however, it makes sense to consider using these chips.
The subject of PLL synthesizers is an extensive one that could occupy an entire guide by itself. Our intent here is merely to introduce them, and further specific details must be left to the construction projects in which they are used. A representative circuit is shown in FIG. 4.