Frequency Modulation is a Modulation Scheme where the incoming signal modulates the frequency of the Carrier Signal by a tiny fraction. Used e.g. in FM Radio.
A variation of Frequency Modulation is sometimes also used for digital Transmissions, this is then called Frequency Shift Keying or FSK-Modulation.
it would be nice to have a gnuplot picture here, is it possible to upload the gnuplot script for toying, too?
The output voltage of a FM modulator with input voltage uin is described by
where Δf is the frequency deviation from the center frequency at uin = 1V.
...can be determined using Carson's Bandwidth Rule: two times sum of the peak deviation Δf from the highest frequency occuring in the spectrum of the modulating signal (fm):
is higher than the one of Amplitude Modulation Schemes since athmospheric disturbances and noise usually don't shift the frequency of a signal but add their contribution to the amplitude of the transmitted signal. Amplitude variations don't affect FM much.