80 Watt stereo transmitter

Posted on Aug 24, 2012

This transmitter was designed from the ground up to provide very high sound quality, coupled with excellent frequency stability, reliability, etc. It can be used as a standalone transmitter to serve a medium-sized town, or as an exciter to drive a kilowatt-class power amplifier to serve a large city. It is designed to work from 13.8V nominal voltage, so that it can be run from a common communications power supply in parallel with a backup battery. In the event of a power cut, the transmitter can keep operating from the battery, at slightly reduced power as the voltage drops. It consists of four modules, the three most important of which are ready, tested, and described below. The fourth module has not yet been built, and might never be built, but I will describe its basic functions so that you can design it, if you want.

80 Watt stereo transmitter
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The RC4200 analog multiplier used in this stereo encoder seems to have gone out of production. Before considering the construction of this circuit, make sure you can actually find this IC, or else be sure you know how to modify the circuit to use a different analog multiplier, such as the AD633 or other one you might find. The textbook way of processing and encoding a stereo signal for FM transmission goes like this: 1) Take both channels and low-pass-filter them at 15kHz, with steep rolloff; 2) Apply pre-emphasis. Depending on the part of the world, it should have either a 75µs or a 50µs time constant; 3) Strictly limit the audio level to ensure that overdeviation cannot happen; 4) Create a stable, clean 38kHz sine wave; 5) Subtract the right channel from the left channel, and multiply the result with the 38kHz carrier; 6) Create a clean 19kHz sine wave, phase-locked to the 38kHz one; 7) Add the left channel, right channel, the (L-R)*38kHz signal, and the 19kHz signal, with specific amplitudes. There are several ways to implement this algorithm. Modern factory made transmitters often do the whole thing digitally, in a DSP. But it's still less expensive and simpler to do in the analog domain. That can be done in various ways too, and far too many transmitters these days use ultra cheap, mediocre methods like hard-switched multipliers based on CMOS switches. They do work, but are very noisy! My design...

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