i. The timer IC555 is operated in monostable mode.
ii. The negative going carrier pulses are to the differentiator formed by R1 & C1. The differentiator produces sharp negative pulses which are applied to trigger input pin
(2) of IC 555.
iii. These triggering decides the starting instants (leading edge) of the PWM pulses.
The PWM pulses go high at the instants of arrival of these triggering pulses.
iv. The termination of the pulses is dependent upon,
a) R2, C2 discharge time
b) The modulating signal applied to control input pin (5)
v. The modulating signal applied to pin no (5) will vary the control voltage to IC 555 in accordance to the modulating voltage.
vi. As this voltage increases, the capacitor C2 is allowed to charge through R2 upto a higher voltage & hence for a longer time (as R2 C2 time constant is fixed). The width of the corresponding output pulse will increase due to this action. As soon as VC2 is equal to the control voltage, the PWM pulse goes to zero.
vii. Thus PWM signal is generated at the output pin (3) of IC555 as monostablemultivibrator.