PAM vs PPM; generation of PWM

PAM vs PPM

Generation of PWM

Pulse width modulation (PWM) keeps amplitude and position of the leading edge fixed and varies the width of each pulse in proportion to the message sample.

             +-----------+
m(t) ------->|  (+)      |       +------------+
             |   sum     |------>| Comparator |--> PWM
saw-tooth -->|           |       | (vs. Vref) |
(fs)         +-----------+       +------------+

saw  /|  /|  /|      m(t) + saw compared with Vref
    / | / | / |
   /  |/  |/  |
PWM  ___    _____    __
    |   |  |     |  |  |      width ~ m(nTs)
 ___|   |__|     |__|  |__

Working (comparator method):

1. A sawtooth (ramp) generator runs at the sampling frequency $f_s$.
2. The message $m(t)$ is added to the ramp (or compared directly with the ramp).
3. A comparator outputs HIGH while the ramp is below the message (or the sum exceeds the reference) and LOW otherwise.
4. The time at which the crossing occurs depends on the message value, so the trailing edge moves and the pulse width is proportional to $m(t)$.

Alternative: a monostable multivibrator triggered at $f_s$ whose timing is controlled by the message voltage (e.g. 555 timer with the message on the control pin).

PPM from PWM: differentiate the PWM and use the trailing-edge spikes to trigger a fixed-width monostable; pulse positions then follow the message.