Many of the issues around reception these days, is getting the signal into the radio and being able to amplify it.
The way CB radio works, is to use a process similar toy hetrodyning. at least it's how the process is done...
It's also why you see service manuals with charts showing PLL, channel, and IF frequencies the radio uses.
Take 27MHz, and you have several Xtals that you can use to obtain it - directly, or use a method where the PLL can generate a known frequency and keep it accurate - mix in a known signal it can make, into the signal - and when the two mixed or combined, you have the main CB frequency, the PLL's own frequency, and "images" - one above higher, in frequency - which is the ADDED result of the mixing, another is LOWER than both the CB frequency,and the PLL's frequency - it's an image.
To save bandwidth of discussion, Uniden, Cobra, Tandy - others took to use the LOWER set of frequencies to use and design circuits with a lot of gain So they use heterodyne to grab a signal (or channel - a narrow range of frequencies) by mixing in one, to obtains products or mix's of the main CB "channel" - and then they can design amplifiers with high gain around them.
So, if you look at the service manual - let's say - Channel 19. 27.185MHz, IF - 16.945MHz - ok, what does that mean?
It means the PLL can generate an IF frequency, and if you look further into the service manual - you'll see the PLL uses a 10.240MHz xtal as a time base reference - this is important to know. For now you have 3 frequencies you can design amplifiers around and for to obtain good signal using heterodyne and image.
Ok, here's' an accelerated way to see this...
27.185MHz = PLL will send to the 1st FET a signal of 16.490MHz, leaving two workable images, one at 43~44MHz (not too many transistor parts work well that high without serious impedance and matching input admittance issues) so we take the low-road and use methods to obtain the other LOWER image...or one close to 10.695MHz.
We can use 10.695 for the DERIVED IMAGE - the one we wish to extract.
So here's where your Crystal Filter selection comes into play.
You can mix into the FET (1st IF) and obtain the 10.695MHz (or 10.7MHz if so desired) using you 10.695MHz Xtal - then amplify it really well.
But we're not done yet!
We can now use the PLLs' 10.240 Xtal timebase as a means again to Heterodyne the 10.695MHz down to 455kHz by mixing in 10.240 MHz directly from the timebase Xtal. It can be amplified so that it can be used not just for the PLL but for the Radios' 2nd IF stage to obtain 455kHz image and amplify that even more and better than at 10.695MHz.
Then 455kHz can be your AM Detector stuff you get for receive - AM - but you can do this same process for FM but then FM doesn't need the 2nd IF - you can use the 10.695, and 10.240 and DISCRIMINATE against the signal to obtain FM that is internally referenced a 450kHz local oscillator signal and get audio out that way.
Hope the above can show others how a CB with a simple 10.240 Xtal -can generate an IF signal in a freerunning oscillation around 16.400~17MHz that's kept in check by the PLL referencing it to the 10.240 Xtal time base it has. Then obtain 10.695MHz and then use the PLL's 10.240 clock against that - to obtain 455kHz and then detect that signal as your AM channel station...
So instead of making a xtal vibrate by applying power, you can send power thru it, as a signal - and it will only pass the ones (signals) closest to it's resonate frequency.
:+> Andy <+:
