# How does fm actually suppress noise

1. Sep 20, 2011

### wukunlin

So I have been taught about FM having noise supressing characteristics, but in that particular course the lecturer told us not to worry about it as it is out of the course syllabus.

So I tried to look up on the details on the internet but all I keep seeing is that FM supresses noise but I just can't find the how, I mean the mathematics and all

would someone be kind enough to explain to me on FM noise quieting (or point to a tutorial or something)?

2. Sep 20, 2011

### Janus

Staff Emeritus
Put simply:

AM, being amplitude modulation, is dependent on the strength of the signal. It is the variation of the signal amplitude that carries the information. If there is noise at the frequency of the transmission, Say a spike from some outside source, the receiver treats it just like a variation in the amplitude of the original signal because it can't tell one variation of amplitude from another.

FM works by varying the frequency of the signal while the amplitude is not a factor. The same amplitude spike from an outside source would be ignored by the receiver as it is only looking for variations of frequencies of the signal, not amplitude.

3. Sep 20, 2011

### vk6kro

The noise on an FM signal is mostly amplitude modulation. That is, the level of the signal changes randomly due to electrical interference.

This signal is converted to a lower frequency, but it still has this noisy variation in amplitude.

So it is passed through a Limiter. This is sometimes an amplifier stage that is being severely overdriven so that the positive and negative peaks of the signal, including the noise, are clipped off, just leaving the central, noiseless, middle part of the signal.

This signal is then passed on to the FM detector which only reacts to the frequency of the signal and is not affected by the clipped nature of the signal.

4. Sep 20, 2011

### DaveC426913

What an awesomely clear explanation.

Noise tends to manifest as a change in amplitude, not as a change in frequency. FM's signal is based on frequency modulation not amplitude modulation, so it is essentially immune to that source of noise.

I learned something new today.

5. Sep 21, 2011

### wukunlin

hmm, I think i kind of get it now.

So when a FM received is tuned at say, 100MHz. Without any transmitters operating at 100MHz, the receiver will pick up the noise due to the infinite amount of side bands from neighbouring stations (@ 99 and 101 MHz for example.)
But when a transmitter is powered on in the range of the receiver, broadcasting at 100 MHz, without any modulating signals, the receiver will pick up the 100MHz carrier but since of S/N ratio is so large the noise at 100.01MHz and 99.99MHz will not be picked up, hence at the receiver it will be practically silent.

Is this right?

6. Sep 21, 2011

### vk6kro

No, the noise comes from electrical equipment, cars, thunderstorms etc.

This superimposes itself on the FM signal which should be constant amplitude but becomes variable amplitude due to the noise.

The purpose of the limiter is to remove these variations in amplitude so that a constant level of signal is delivered to the FM detector.

This is not to say that FM receivers do not receive noise. When signals are absent or weak, a lot of noise can be produced in the speaker.
For this reason, FM receivers usually have a "mute" control to disconnect the audio amplifier when there are no signals or only very weak signals.

7. Sep 21, 2011

### wukunlin

ooohhhhh I get it now, thanks a lot for clearing that up :)