AM radios were easy to understand. The strength of the signal goes up and down, and the audio follows. FM radio is a little more difficult. [AllAmericanFiveRadio] has an old tube FM set and takes us on a tour of how the FM discriminator works. You can see the video below.
The first step is to look at the IF signal on the scope. It is hard to see, but the frequency is changing, and that’s the basis of modulation that the discriminator has to resolve.
The next step is to feed a fixed signal into the IF tuned circuits. The scope shows that at the center frequency, the signal passes through with no issue. However, as the frequency changes, the tuned circuit attenuates the output, converting the frequency change into an amplitude change. Unsurprisingly, once you have the analog change, you can demodulate it like an AM signal.
There is a bit more, of course, but the basic idea is just that simple. The video redraws the tube schematic with diode symbols, and the explanation is clear.
You should know that there are several other ways you can decode FM. Also, of course, there are many other ways to modulate a carrier with information.
