FFT, Frequency Modulation.

Cuthbert Nyack
Applet shows the FFT of an FM signal with carrier frequency w1, modulating frequency w2 and modulating index wd/w2. wd is the frequency deviation. As the modulating frequency is changed the bandwidth, which is determined by the frequency deviation, remains roughly constant.

In order to get a reliable spectrum the FFT line sep should be equal to or e a submultiple of the modulating frequency.
eg(20.0, 0.5, 0.01, 12.55, 512, 9.0, 80, 375.0, 34.5, 0.05)
line sep = 0.5, m = 1.21 gives a modulating index of ~2.4 and zero amplitude carrier. As m is increased, one set of sidefrequencies after another goes to zero. With m = 10.0, the modulation index is 20.0 and the spectrum extends over a range w1 ± ~22 w2.

eg(20.0, 0.5, 0.01, 25.09, 512, 9.0, 160, 375.0, 34.5, 0.05)
line sep = 0.25.

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COPYRIGHT 2006 Cuthbert Nyack.