Short note: PAM, PWM and PPM

Analog pulse modulation uses a periodic pulse train as the carrier and varies one pulse parameter in step with the sampled message.

m(t)  ____/‾‾‾‾\____/
PAM   |  |‾| |‾‾| |‾|  |   height varies
PWM   ▌  ▐█  ▐██ ▐█  ▌     width varies
PPM   | |   |    | |       position varies

PAM (Pulse Amplitude Modulation)

• The amplitude of each pulse is proportional to the sample value; width and position are fixed.
• Produced by natural or flat-top (sample-and-hold) sampling.
• Simplest; first step of PCM and TDM.
• Poor noise immunity, since noise adds directly to amplitude.
• Demodulated with a low-pass filter (plus equalizer for flat-top).

PWM (Pulse Width / Duration Modulation, PDM)

• The width of each pulse varies with the sample; amplitude and starting position are fixed.
• Generated by comparing the message with a sawtooth/triangular wave in a comparator (or a monostable).
• Better noise immunity (amplitude can be clipped), but power varies with width, so transmitter power is wasted.
• Used in motor speed control, class-D amplifiers, SMPS.

PPM (Pulse Position Modulation)

• The position (time shift) of each pulse from its nominal position varies with the sample; amplitude and width are fixed.
• Generated from PWM: the trailing edge of each PWM pulse triggers a short fixed pulse.
• Constant pulse power and best noise immunity of the three, but needs accurate synchronization.
• Demodulated by converting back to PWM and low-pass filtering.