In PPM the amplitude and width of the pulses is kept constant but the position of each pulse is varied accordance with the amplitudes of the sampled value of the modulating signal.

The position of the pulses is changed with respect to the position of reference pulses.

The PPM pulses can be derived from the PWM pulses. With the increase in the modulating voltage the PPM pulse shift further with respect to reference.

The vertical dotted lines drawn in Fig are treated as reference lines to measure the shift in position of PPM pulses. The PPM pulses marked 1, 2 and 3 in fig go away from their respective reference lines.

This is corresponding to increase in the modulating signal amplitude. Then as the modulating voltage decreases the PPM pulses 4,5,6,7 come progressively closer to their reference lines.