All of these radios work by mixing the antenna signals with a signal from a crystal which has the effect of moving the received signal down in frequency where its easier to process and easier to filter. This explains why receiver and transmitter crytals are not interchangable -- the receiver crystal will be offset from the actual channel frequency by the desired intermediate frequency. This intermediate frequency is amplified and filtered and detected to get the actual control signal.
The choice of the intermediate frequency involves trading off two things -- you need significant separation between the signal you receive and the crystal to get the mixer to work properly but the wider the difference the more difficult it is to make a very sharp filter for the channel you want. The way round this is to first filter and amplify at a higher intermediate frequency, then convert to a lower frequency and then apply a very sharp filter -- i.e. dual conversion.
It sounds more complicated than it is. The basics are really straightforward to grasp, the skill and the subtlety in the design comes from knowing what tradeoffs to make -- there's no such thing as a perfect receiver (at least, not one that would fit in a model sailplane). Small and light may not make for sensitive and selective, for example -- but if the manufacturer built the radio for a tiny indoor model then they would quite reasonably assume that nobody would be flying it at a half-mile range.
Digital radios -- "software defined radios" -- are the next big thing but I won't expect to see them in R/C for some years. By that time we won't be using frequency as the primary means of selecting signals......the whole idea of channels will be obselete.
Martin Usher
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