# FM Amplitude vs. Frequency Spectrum

1. Apr 16, 2012

### elemis

I know that an AM Amplitude vs. Frequency Spectrum has a central frequency = carrier frequency and two side bands of carrier frequency ± signal frequency.

Now what would a similar spectrum look like for an FM wave ?

My textbook just says that '.... further side frequencies that are multiples of the audio frequency are produced'.

2. Apr 16, 2012

### Floid

It is going to look like an AM signal in that the FM wave's spectrum will be centered about the carrier frequency.

But what you will end up seeing is spikes on the spectrum that are at the carrier frequency +-the instantaneous frequency of the signal you are modulating as well as a (hypothetically speaking) infinite number of sidebands that are at multiples of that frequency. Of course, the higher order sidebands have very small amplitudes so in reality they fall into the noise level.

3. Apr 16, 2012

### elemis

Do you have a diagram describing the above ? I'm having trouble visualising it...

4. Apr 16, 2012

### Floid

http://www.microwaves101.com/encyclopedia/spectrumanalyzer.cfm#fm

Figure 8 shows what you would see on a spectrum analyzer. You have the carrier frequency in the center, the modulated signal would be the first spikes on either side of it and then the other sidebands moving outward in both directions. The sidebands are just your harmonics of the modulated signal.

5. Apr 16, 2012

### sophiecentaur

There are two extremes of FM and their spectral patterne look very different. Real signals tend to have the two patterns 'mixed up'.
FM with so-called wide deviation sweeps the carrier frequency over a wide range of frequencies but relatively slowly. The resulting spectrum, for a low frequency modulating signal, tends to look like "angel's wings". d. The raised bits are because the carrier spends more time per Hz of modulation at the extreme frequencies (gradient of the sine wave is low).

The power spectrum for very low deviation FM looks almost identical to that of AM. The difference is, however, that the Phases of the upper and lower sidebands are different by 180 degrees. There is still an identifiable central carrier, here.