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The subject of low-power transmitters has always been a fascinating one for experimenters. They enable you to operate your own radio station, broadcast music, control devices remotely without wires, do surveillance work, eavesdrop on private conversations, and other things such as remote sensing. Current Federal Communications Commission (FCC) regulations allow the operation of very low-power unlicensed transmitters subject to various restrictions. These restrictions generally regulate operating range and frequency of operation, as well as possibly types of emission and duration of transmissions. The effort entailed in the construction of these devices will be well rewarded, and the knowledge and experience gained can lead to other pursuits such as ham radio, broadcasting careers, and electronic engineering.
Currently, it appears that the low-power FM broadcast transmitter is a popular project. These devices broadcast in the 88-108 MHz band and can have a range of several hundred feet when a sensitive receiver is employed. FCC regulations limit range and field strength at a certain distance. No explicit restrictions are imposed on power, but a single transistor operating at 5 volts or less will easily generate a signal sufficient to accomplish all that is legally allowed for range and field strength. Modern low-power transmitters of this nature are a far cry from the one transistor approach that used to be popular years ago. This approach used an oscillator that is directly modulated with a microphone. FM is obtained by modulating the operating voltages of the oscillator, which in turn affects frequency via a change in transistor parameters.
Although this hit-and-miss approach can be made to work, the modern low-power transmitters may employ varactor modulators, microprocessor-controlled frequency synthesis using a phase locked loop (PLL), audio preemphasis, audio mixing and control facilities, and stereo modulator circuitry that generates a "real" baseband multiplex signal. Integrated circuit (IC) devices make these tasks easily implemented with reasonable amounts of circuitry. The audio quality and the frequency stability of transmissions are usually good enough so that it can be difficult to tell the signal from one of these low-power units from that of a commercially licensed station.
AM transmitters that have similar performance characteristics are also available.
Currently, AM low-power transmitters operating in the standard AM broadcast band (530-1710 kHz) are allowed 100 milliwatts (mW) into a 10-foot antenna. This allows usable ranges of several hundred feet, with a few miles not impossible with a well-designed antenna system. These transmitters are a far cry from the "phono oscillators" of the 1950s. Radio amateurs using 10-20 watts in the 1.8-MHz (160 meter) ham band have gotten ranges of 50 miles or more using 8-foot whip antennas mounted on automobile bodies, so 100 mW should give a range of 5 miles under identical conditions. The inevitable presence of co-channel interference and noise, however, as well as the desire for a good-sounding, clear signal, generally limits this range to less than 1 mile under the best of circumstances.
Several approaches can be taken to the exact system used for low-power transmitter architecture, but these range from simple oscillators to sophisticated synthesized units. The following factors should be considered:
1. Cost: Many low-power transmitters are designed for hobby use or for experimentation. They might be considered as expendable in some cases, as for wild life tracking, surveillance, or remote sensing applications, where they will eventually be lost or destroyed. The low-power transmitter may be sold in kit form, to be assembled by a hobbyist, or as a completed assembly.
2. Physical size: This may be an important factor, as in hidden surveillance or as a payload in a model rocket or aircraft, or even attached to an animal collar.
Obviously, such transmitters and their accessory components (e.g., battery, microphone or other sensor, and antenna) have to be kept small, requiring surface-mount construction techniques. Other applications may have few size restrictions.
3. Power considerations: Available power might be limited, such as a watch or hearing aid battery, or continuously available but limited, such as from a telephone line, or unlimited for all practical purposes, as from a 120- or 240-volt AC power line. The low-power transmitter will have to derive all of its operating voltages from a specific source.
4. Type of emission (FM, AM, video, pulse, etc.): The transmitter design is deter mined by these factors. Simple audio modulation or tone signals are relatively easy, but video and other complex waveforms may require more complicated circuitry with a higher component count.
5. Quality of transmission desired: Attributes such as audio or video quality, RF output power, frequency stability, and range dictate the need for certain circuit features. It may be possible or necessary to omit certain circuit elements and features to obtain a certain goal, such as size, cost, or power consumption.
Certain factors, such as range of transmission or frequency stability, may man date other requirements. Often, compromises must be made.
6. Legal aspects: FCC Part 15 rules dictate certain power, emission type, range, antenna, RF field strength, and frequency restrictions; however, in government, police, or military work, these restrictions may not apply or are often ignored. For export and in countries other than the United States, different rules may apply. Specific exceptions for power, frequency, and emissions are available to authorized agencies on individual cases, but the experimenter must comply with FCC Part 15 regulations or Part 97 regulations for the amateur radio service, as applicable.
An almost unlimited number of approaches can be taken to low-power transmitter design, but the basic architectures may be classified as follows:
- Basic oscillator
- Master oscillator-power amplifier
- Heterodyne mixer-power amplifier
FIG. 1 Basic Transmitter Architecture: HETERODYNE MIXER WITH RF POWER
AMPLIFIER
Basic Oscillator
The basic oscillator is simply what it says (see FIG. 1). An oscillator circuit generates the RF carrier and outputs it to a load, generally the antenna. A modulator circuit may be coupled to the oscillator to apply modulation and/or control output power.
This approach has certain advantages and disadvantages:
Advantages: Low cost, simplicity, small potential size, low power consumption Disadvantages: Poor frequency stability and susceptibility to frequency pulling from changes in antenna characteristics, which are determined by proximity and environmental effects. Oscillators are not easy to modulate without producing undesirable effects. Crystal control is difficult to use without creating undesirable effects. An FM modulation scheme may also produce AM and vice versa. Oscillators may drift with temperature, battery voltage, and so forth. This precludes or makes difficult narrowband operation with sensitive receivers and is best used with wideband modulation modes.
Master Oscillator-Power Amplifier
The master oscillator-power amplifier (MOPA) approach uses an oscillator to drive a separate amplifier, which acts as a buffer and ideally isolates the oscillator from load changes produced by the antenna. In the simplest cases, this may be the output, or radio-frequency power amplifier (RF PA); however, more stages might be used as follows: The oscillator output can be fed to a frequency multiplier stage(s) to multi ply the oscillator frequency by a number generally two or three times, but four or even five times is possible with good filtering. A somewhat pessimistic but practical rule of thumb is that a frequency multiplier cannot be more efficient than the reciprocal of the multiplying factor squared. For a doubler, this is 25 percent; for a tripler, 9 percent. This means high-power consumption for the multiplier stages, with no contribution to the output power; however, this approach is time tested and works well, and for many years was a primary approach to HF, VHF, and UHF transmitter design. We use this approach in several projects in this guide because it is simple and reliable. The oscillator is only modulated in the case of FM, and this is relatively easy, the AM being removed in the nonlinear multiplier stages. The RF PA, some times called the "final," is modulated in the case of AM (audio or video).
Advantages: Simplicity, reasonable cost, excellent modulation characteristics, and good to excellent frequency stability (especially if crystal control is used). Time proven approach and useful for reaching high frequencies, up to more than 1000 MHz. By including extra controls, it can be used to make a multiband transmitter if the desired outputs are harmonically related (i.e., integer multiples). For many years, ham radio operators used transmitters in which an oscillator operating in the 160-meter band (1.8 MHz) was multiplied to 3.5, 7, 14, 21, and 28 MHz by switching in or out multiplier stages and tuned networks. Of course, the transmitter required tuning when the output frequency was changed, but this was of little concern because it allowed one transmitter to cover the entire HF spectrum.
Disadvantages: Can be complex, may need several stages if a high multiplication factor (4 times) is needed, can be easily mistuned to wrong output frequencies, and generates harmonics and spurious frequencies that need to be filtered out.
Skilled technical personnel and some test equipment may be needed to set up and tune properly. The need for good RF filtering makes small physical size relatively difficult to obtain if high spectral purity is desired. Power consumption is some what high relative to RF output, especially in low-power (less than 1 watt) work because of the overhead of the non-output-producing multipliers.
Heterodyne Mixer-Power Amplifier
The heterodyne mixer-power amplifier approach uses the principle from super-heterodyne receivers in reverse. A signal with the desired characteristics (AM, FM, single sideband, video, etc.) is generated at one frequency called the intermediate frequency (IF). This can be done with a high level of performance because it is generally all done at one relatively low frequency. Excellent, high-performance circuitry and filtering can be used because only one frequency is to be handled.
This produces a clean signal. The signal is then mixed with a local oscillator (LO) signal in a mixer, and the mixer output is filtered to suppress all but the desired output signal. The LO can be a crystal oscillator, variable-frequency oscillator (VFO), or a phase locked loop (PLL) synthesizer. Most modern transmitters use a PLL source for the LO.
This output signal is then amplified in a broadband amplifier to the final output power. Some harmonic filtering generally is necessary, but broadband, no-tune transmitters result from this approach. Bandwidths as great as 1.8 to 1 can be achieved without filter switching or tuning of any kind, and with automatic filter switching the entire HF spectrum (generally 1.5-30 MHz) can be covered with uniform performance. All modern single sideband (SSB) and amateur radio transceivers use this approach, and it is used at VHF and UHF for military and commercial purposes.
Advantages: Wide frequency coverage, broadband, and any type of modulation usable with the proper linear amplifier. Generally, uniform performance over a wide frequency range with no retuning or adjustment.
Disadvantages: High cost, complexity, large physical size compared to other approaches (LSI and VLSI devices and surface-mount technology are a great help here), extensive filtering and sometimes shielding required, linear amplifiers needed for analog modulation schemes, and possibility of spurious non-harmonically related emissions. Careful design is needed to reduce these emissions to a satisfactory level. Generally unsuitable when very small, low-cost, low-power transmitters are required.
Of course, new technologies make more possible in less space and cost. The wireless revolution, with cell phone and digital personal communication services (PCS), has brought many new RF devices that can provide solutions to design problems; how ever, a kind of "double-edged sword" problem exists here. The trend toward surface mount and wireless devices has also brought the discontinuance of many good but somewhat large devices (meaning here that you can see and handle them without aid and can work with them without possessing the skills of a fine watchmaker) that are "perfect" for the experimenter and hobbyist. These devices are not technically obsolete, just too large for surface mount. In some cases, the chips or transistors have just been repackaged in surface-mount packages.
One large manufacturer (Motorola) has discontinued practically all of their discrete RF bipolar 7.5- and 12-volt power devices in TO-92 and X style packages, many of which are industry standards and excellent experimenter devices. Another company (Microsemi) is picking up the manufacture and supply of many of these devices, for aftermarket and replacement service, but the trend is disturbing from an experimenter's viewpoint. Although you can work with many of the "larger" surface-mount packages with the aid of small tools, somehow it just isn't as much fun.
A very small surface-mount chip component dropped on a carpet is probably lost forever. To reach higher frequencies with good performance, however, and to keep size down, this technology really is the way to go. Surface-mount components have very small parasitic capacitance and inductances compared to traditional through hole components. It is amazing to see how well surface-mount circuits work at UHF after working with conventional components. Fewer parasitic effects and the improved bypassing and reduced stray coupling achieved make for better operation, with far fewer UHF "dogs." For experimenters and hobbyists, working in the 500-1300 MHz range with surface mount can be as easy as working in the 100-200 MHz VHF range with conventional components. Of course, the price paid is the necessity of working with very small components. In this guide, surface-mount components are used together with conventional through-hole components in several projects. We assume some familiarity with RF and audio circuitry and that you have built a few circuits before. We also assume that you are computer literate to the point of being able to save and recall files and look things up on the Internet. Although it is not totally necessary, it also would be helpful for you to be able to write some simple assembly language programming software for microprocessors, such as the 8051, or the Microchip PIC chips, such as the PIC16F84 or similar devices.
Many of the newer VLSI devices, such as frequency synthesizers, require serial programming inputs that are most easily generated with microcontrollers and micro processors. If you decide to use these new chips for your projects, you will have to be somewhat comfortable with microcontrollers. This is the direction RF technology is going in-smaller size, higher frequencies-and if this phase of electronics is to be your hobby, you may as well get used to it. Alternate approaches are to take up antique radio repair, audio, computer programming, or gardening instead. In addition, you should get a ham license if you do not already have one. The days of struggling with Morse code are gone, and a code-free technician class license will open many doors to interesting opportunities to put transmitters on the air, get practical experience, and meet new people. If you can understand the contents of this guide and learn a few rules and regulations, you are well on your way to obtaining a ham license. Check out the American Radio Relay League's (ARRL) Website at www.arrl.org for details on obtaining a ham license.
Some of the projects in this guide use what we call the "new technology approach," where LSI IC devices perform many of the functions in a system. An example are the frequency synthesizer IC devices used in our FM stereo transmitters.
This approach allows a drastic reduction in the component count. The new technology approach can be a little too "black box," and many circuit points and waveforms are inaccessible for tests and observation. The necessary components may be avail able from only one or two manufacturers and can be discontinued at any time because of poor sales records, corporate mergers or buyouts, or other factors not related to technical performance. This scenario renders the project obsolete because parts can no longer be obtained. A 28-pin LSI IC offers little to teach about its internal workings, whereby a discrete circuit can be observed, tested, and probed to satisfy your curiosity. New technology can tie you to a manufacturer and a specific approach and offers little in the way of education about how things work at a fundamental level.
Today, people seem to be becoming more and more dependent on increasingly sophisticated devices of which they have less and less understanding about how they work. For example, as children, we used to use a needle and a paper horn to extract sound from a record on a turntable. Try to do this with a compact disc. We made radios from a razor blade, a safety pin, and scrap wire, and with the aid of a 50-foot antenna and a pair of earphones, could pick up local radio stations. Try this with FM stereo. The basics were easier to learn with the old technology because we could "get to it" hands on. It wasn't hidden in an expensive little box containing a bunch of chips. If this trend continues, and it probably will, the operation of common house hold devices and appliances will be as big a mystery to most people as the existence of life after death. Yet, basic principles still and always will apply, and one of the best ways to learn basics is through observation and experimentation.
It is hoped that the projects in this guide will help provide this opportunity. Some of the projects use what we call the "old technology approach," where most circuit functions are performed by discrete semiconductors. The ATV transmitter for 440 MHz is an example. A crystal oscillator drives a multiplier chain and power amplifier, and a video amplifier built with discretes serves as the modulator. The "old technology" might be better called the "traditional approach" because nothing is really that old. The 440-MHz ATV transmitter could have been built 25 or 30 years ago and performed just as well, assuming that the same components were available then; however, it has the advantage of being easy to understand and work with, and it is relatively immune from obsolescence as a result of part discontinuances. Industry standard parts that are generic in nature are used, and many alternate replacement transistor devices exist that will function in each stage. Considering the surplus market availability of parts, this design will probably still be useful 25 to 30 years from now as a ham or experimenter project. But the "new technology" approach will likely be obsolete by then because some of the required parts undoubtedly will have been discontinued years before. We could have used a PLL IC device, microprocessor, and integrated RF power module, but why bother? This application doesn't need any of it and can be simply implemented with traditional "old technology." The basic needs and requirements do not necessarily change with time, only the implementation methods.
Therefore, traditional approaches still have merit as teaching aids and in simple applications where freedom from obsolescence and easy serviceability by technicians without specialized training are important. This is especially true in developing nations and remote areas, where sources of supply are scarce or nonexistent. So we make no apologies for the "old technology" approaches in some of the projects in this guide. Not all of us are engineers or work for large electronics companies, or know someone who does. By the way, vacuum tubes are still being manufactured in Russia, China, and other nations and are in demand in some parts of the world. A vacuum tube radio or TV can be fixed with little more than a voltmeter and schematic. Try this with a solid-state LSI TV or a modern stereo receiver. If you person ally haven't seen it or done it before, any type of "old technology" is new technology to you because you probably know nothing about it. We also find it amusing that some of the critics of old technology have problems repairing a simple transistor radio or even a table lamp. We hope that the projects in this guide will be a good learning experience for the builder.