pass filter, the average voltage across the filter will vary in the same way as the original modulating
voltage. This varying voltage will closely approximate the original modulating voltage.
Pulse-position modulation (ppm), pulse-frequency modulation (pfm), and pulse-code modulation
(pcm) are most easily demodulated by first converting them to either pdm or pam. After conversion these
pulses are demodulated using either peak detection or a low-pass filter. This conversion may be done in
many ways, but your study will be limited to the simpler methods.
Ppm can be converted to pdm by using a flip-flop circuit. (Flip flops were discussed in NEETS,
Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.) Figure 3-22 shows the
waveforms for conversion of ppm to pdm. View (A) is the pulse-modulated pulse train and view (B) is a
series of reset trigger pulses. The trigger pulses must be synchronized with the unmodulated position of
the ppm pulses, but with a fixed time delay from these pulses. As the position-modulated pulse is applied
to the flip-flop, the output is driven positive, as shown in view (C). After a period of time, the trigger
pulse is again generated and drives the flip-flop output negative and the pulse ends. Because the ppm
pulses are constantly varying in position with reference to the unmodulated pulses, the output of the flip-
flop also varies in duration or width. This pdm signal can now be applied to one of the circuits that has
already been discussed for demodulation.
Figure 3-22.Conversion of ppm to pdm.
Pulse-frequency modulation is a variation of ppm and may be converted by the same method.