hi all,

When swithcing ON a commercial FM receiver, we hear that clear noisy sound from the receiver speaker, but when changing to AM receiving, that noisy sound is not heared.

my quesions are:

1)Where is that noisy sound comming from? Is it internally generated in the receiver, or externalyy received?
2) Is it because FM section has an RF amp stage?
3) If we use RF amp stage fro AM section, will it have also that noisy sound?
4) Why AM section doesn't have that noisy sound?
5) When tuning to an FM station, that noisy sound is there any more. Why?

 

I suspect you mean the other way around, since FM is high fidelity.

With FM the discriminator stays locked onto the carrier wave. It tracks it, so if the carrier doesn't move neither does the audio output. This also tends to reject side bands.

With AM the carrier is a reference. The sidebands plus the reference make audio, but it has no ability to reject sidebands, unlike FM. So if another source of RF happens to fall adjacent to the AM signal it is merged with the audio.