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Name: david
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

I am having difficulty understanding how a signal is an FM signal modulated onto a carrier wave. My text book does not explain it to my satisfaction. AM modulation is fairly easy to understand, so maybe we can relate the two somehow.

AM modulates the AMPLITUDE of a fixed frequency carrier; FM modulates the FREQUENCY of a fixed amplitude carrier. Actually, it does not have to be fixed amplitude: one of the main reasons for developing FM technology was to become immune to amplitude changes; like lightening produces static on AM but not on FM.


First, let's look at AM. You can find diagrams of what an AM wave schematically looks like on the internet. The wave envelope gets larger or smaller according to the size of the modulating signal signal. A circuit in the receiving radio containing a diode (in simplest form) can follow the amplitude. The zero point is the average amplitude of the signal. When the amplitude of the signal is a little larger than the average, then the audio output from the detector is positive. When the amplitude of the signal is a little smaller than average, then the audio output from the detector is negative.

[Side note: so what's all this frequency sideband stuff? Who needs sidebands and extra frequencies for AM? It looks like you are only changing the amplitude of the wave, not its frequency. Well, it turns out that if you modulate a carrier wave, physics says that this is really as if there is another frequency present. Think about what happens when we combine two sine waves of slightly different frequency. When the signals are in phase, the signals add to each other and the amplitude gets large. When the signals are out of phase, the signals subtract from each other and the amplitude gets small. For example, suppose you combine a 1,000,000 Hz wave with a 1,000,100 Hz wave. The resulting amplitude rises and falls at 100 Hz. It is actually not perfect. To get a perfectly in-phase wave that rises and falls at 100 Hz you need 1,000,100 Hz and 999,900 Hz. If you have a large carrier wave at 1,000,000 Hz, and add small amounts of the previous waves, you get small and perfect 100 Hz modulation.]

There are also nice diagrams of an FM modulated signal on the internet. In FM, the frequency shifts a little higher or lower depending on the signal audio signal. The amount the frequency deviates from the carrier represents the amplitude of the modulating. When the waves get scrunched closer together (higher frequency) then that signifies a positive audio signal. When the waves expand a bit (lower frequency) then that signifies a negative audio signal.

The detection circuit for an AM radio "follows" the envelope of the radio wave and demodulated audio signals come out. That's easy to see.

The detection circuit for an FM radio is more complicated and has a circuit that locks onto the average frequency of the FM signal, and another circuit that looks at how far the signal frequency deviates from the average frequency. Higher in frequency positive. Lower in frequency negative (or vice versa) Again, demodulated audio signals come out. I hope that helps.

Bob Erck

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