FFT, Pulse Amplitude Modulation.

Cuthbert Nyack

Applet illustrates the frequency spectrum of a PAM signal. The carrier is a rectangular pulse train with frequency w1 and the modulating signal is a sinusoid with frequency w2. The frequency spectrum can be derived by multiplying the spectra of the 2. There is a component at frequency w2, components at (2n + 1)w1 and at (2n + 1)w1 ± w2. The component at the modulating frequency w2 means that the signal can be demodulated by a low pass filter.

eg (8.0, 1.0, 0, 6.28, 512, 0.7, 512, 375.0, 20.0, 0.06)
This shows the spectrum from 0 to 32w1.

eg(8.0, 1.0, 1, 6.28, 512, 1.0, 512, 375.0, 20.0, 0.17)
Here narrower pulses are used. The spectrum now has both odd and even terms up to 32w1.

eg(8.0, 1.0, 2, 6.28, 512, 1.0, 512, 375.0, 14.4, 0.05)
Uses rectangular pulses. The spectrum is similar to the first but flattening the pulses has introduced frequencies at 2nw1 ± w2.

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