The circuit that generates PWM wave is as follows, Here in this circuit Op-Amp works in comparator mode.It compares two voltages, modulating voltage with Saw-tooth Voltage. Saw-tooth voltage is taken as reference voltage.
|condition||Output voltage Vo(t)|
from the graphs whenever modulating voltage dominates saw-tooth voltage corresponding output is low.
Similarly, when saw-tooth voltage dominates modulating voltage corresponding output is High.
Then the resultant output voltage is a PWM signal.
Now, a PPM signal has been generated by passing the PWM signal through a Mono-stable Multi vibrator . Here the resultant signal is a PPM signal with the pulse starting with respect to trailing edge of PWM signal.
The width and Amplitude of Pulse remains constant only the position of the pulse changes with respect to m(t) .