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 recipro- cal 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 con- tribution 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 book 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).

WThere are two defining characteristics of artificial intelligence robots that you must be aware of. First of all, AI robots learn and adapt to their environments, which means that they change behaviors over time. The second characteristic is emergent behavior, where the robot exhibits developing actions that we did not program into it explicitly. We are giving the robot controlling software that is inherently non-linear and self-organizing. The robot may suddenly exhibit some bizarre or unusual reaction to an event or situation that seems to be odd, or quirky, or even emotional. I worked with a self-driving car that we swore had delicate sensibilities and moved very daintily, earning it the nickname Ferdinand after the sensitive, flower loving bull from the cartoon, which was appropriate in a nine-ton truck that appeared to like plants. These behaviors are just caused by interactions of the various software components and control algorithms, and do not represent anything more than that.

Heterodyne Mixer–Power Amplifier

The heterodyne mixer–power amplifier approach uses the principle from superhet- erodyne 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 Low-Power Transmitters, General 5 (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 uni- form performance. All modern single sideband (SSB) and amateur radio transceiv- ers use this approach, and it is used at VHF and UHF for military and commercial purposes.

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 recipro- cal 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 con- tribution 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 book 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 transmit- ter 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 nonoutput-producing multipliers.

Heterodyne Mixer–Power Amplifier

conventional throughhole components in several projects. We assume some familiar- ity 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 pro- gramming 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 addi- tion, you should get a ham license if you do not already have one. The days of strug- gling 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 book 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 book 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 technol- ogy 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 inter- nal workings, whereby a discrete circuit can be observed, tested, and probed to sat- isfy 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 funda- mental 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 book 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

Basic Building Blocks

Low-power transmitters are made up of the same basic circuits as any other transmit- ters. Some of the circuitry, though, such as modulators, power supplies, and power amplifiers, differ in size and operating voltages and currents. Generally, certain fea- tures 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 book, 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 engi- neers to be the technology of choice. In the areas of reliability, fault tolerance, effi- ciency, 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 transmit- ters and their relative merits and drawbacks. For detailed theory of these circuits, we refer you to any good reference text on the subject.

Aurther

Rudolf F. Graf

William Sheets

Downlaod Book