FFT, Pulse Position Modulation.

Cuthbert Nyack

A Pulse Position signal with pulse carrier frequency w1 and sinusoidal modulating frequency w2 and its spectrum is shown below.

This spectrum can have a high harmonic content eg (8.0, 1.0, 0.006, 12.56, 1024, 0.0, 256, 750.0, 15.0, 0.08)
As the modulating index m is increased, the spectrum does not contain a component at the modulating frequency and so cannot be demodulated by a low pass filter unlike PAM and PWM. In fact a low amplitude component does occur in the applet but this is an artifact arising from how the PPM signal is generated.

eg parameters (8.0, 1.0, 0.006, 12.56, 1024, 0.15, 256, 750.0, 15.0, 0.08)
As the modulating index is increased, the spectrum changes differently from a PAM signal because a different set of lines appears around different harmonics. In fact a PM spectrum appears around each harmonic with the modulating index increasing with harmonic number. The spectra around the higher harmonics therefore spreads out much faster than the spectra around the lower harmonics as m is increased.

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