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AM-FM commercial receivers noise

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froten

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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?
 
It's simply because of the differences between AM and FM, the noise actually comes from the frontend of the receiver, and is amplified massively giving this effect.
 
1)Where is that noisy sound comming from? Is it internally generated in the receiver, or externalyy received?
A bit of both.
The gain within the various stages of the receiver means that there is a wide spectrum of noise (within the IF passband of the receiver) at the detector, the best that the FM detector can do is to "decode" this as audio noise.

2) Is it because FM section has an RF amp stage?
That will contribute to the noise at the detector, but is not the root cause of the noise, that is due to the way the FM detector works.

3) If we use RF amp stage fro AM section, will it have also that noisy sound?
If you put more and more gain at the front of an AM receiver, yes the noise will increase. But, there is no point in doing this as there is a limit to the minimum useful signal which can be received. Google for "thermal noise" if you want to know more.

4) Why AM section doesn't have that noisy sound?
AM and FM detectors work in different ways, if the noise was there on AM, the detector would not "quieten" when the wanted signal was received, you would just have a noisy signal.

5) When tuning to an FM station, that noisy sound is (not) there any more. Why?
As I said earlier

there is a wide spectrum of noise (within the IF passband of the receiver), the best that the FM detector can do is to "decode" this as audio noise

When there is a wanted signal within the IF passband, the amount of noise becomes small compared with the wanted signal and the output of the detector starts "quieting" as the wanted signal increases.

JimB
 
The noise you hear is due primarily due to the transistor and resistor thermal noise of the receivers, which you can hear when there is no signal. I believe the difference is noise may also be related to the difference in bandwidth between the two signal. AM has a signal bandwidth of 10KHz whereas FM has a signal bandwidth of near 200Khz. This high FM bandwidth allows for a high S/N when receiving a good signal, but has a much higher noise bandwidth when not tuned to a station. Since noise is proportional to the square-root of the bandwidth, the FM receiver would naturally have more noise then the AM receiver even if the receiver noise levels were otherwise equal.
 
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