HighPlains

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A lot of incomplete information in this thread. But in post 9, Rodney pretty much hit it out of the park. Crystals have certain circuit responses that the RF designer must understand and design for in order for them to work properly. Besides the type of crystal, quite often they are used in circuits that respond to the harmonics of the crystal, since cutting a crystal for the actual operating frequency would result in a very fragile device.

On the question of heterodyne circuits (where an incoming signal is mixed with a local oscillator), this was first done in RC systems around 1958 when the FCC allowed additional frequencies for our hobby. Traditionally, the receiver L.O. frequency is offset from the transmitter frequency by 455 kHz, either above or below. This worked fine in the early days of the hobby, including the original stuff on 72 MHz (the good old days with two color frequency flags). But as the RF spectrum started getting used up by other operators, we eventually got into the 50 channels of frequency, while they put pagers in between our RC frequencies.

So receiver design had to be upgraded to dual conversion. This meant that the first conversion had the L.O. offset by 10.7 MHz, while the second conversion was offset by the original 455 kHz. What this did was to make the receiver very selective in a very noisy RF environment. One of the problems with a single conversion receiver is image rejection. While the designer puts a filter ahead of the first L.O. conversion, when it was only 455 kHz away from the desired signal, the receiver will also pick up signals on the other side of the L.O. frequency, and since they may be much stronger, they show up as an over riding signal and you end up crashing. Trying to reject those signals with a simple front end filter is just not possible because the filter is not sharp enough when you are only a half MHz and you are operating at 72 MHz. But with the duel conversion working at 10.7 MHz, suddenly the crappy front end filter works like a champ.

So you can see, there is a lot that goes into the design of a receiver. I could talk for hours on the subject and not even hit the highlights of subject. Heck, there are tomes written on just the antenna, or book after book on filter design, and quite a bit more on oscillators. The engineering you can buy for a few dollars is amazing, and the technology developed the past 120 years is stunning. RC is now 115 years old this year, and Tesla is not a car. He is the man that created the future.