One type of pulse analog modulation is the pulse width modulation pulse varies in proportion with the amplitude of modulating signal, in PWM information is contained in the width variation, the generation of PWM is shown below:
A saw tooth generates a saw tooth waveform of frequency fs $\therefore$ saw tooth signal in this case is a sampling signal. It is applied to the inverting terminal of a comparator the modulating signal x(+) is applied to the non inverting terminal of same comparator. The comparator o/p will remain high as long as the instantaneous amplitude of x(-1) is higher than that of the ramp signal. This gives rise to a PWM signal at the compartor o/p, therefore this PWM signal is said to ve trail edge modulated PWM.
The PWM signal received at the i/p of the detection circuit is contaminated with noise which is applied to pulse generator circuit which generates PWM signal.
The regenerated pulses are applied to a reference generator.
The regenerated PWM pulses are also applied to a ramp generator.
The constant amplitude pulses at the o/p of reference pulse generator are then added to ramp signal.
The o/p of the adder is then clipped off at a threshold level to generate a PAM signal at the o/p of the clipper.
A LPF is used to recover original modulating signal back from PAM signal.