In a pulse-width modulation system, what parameter does the modulating signal vary?

• A. Pulse frequency.
• B. Pulse duration.
• C. Pulse amplitude.
• D. Pulse intensity.

Answer: (B)

In pulse-width modulation, the modulating signal dictates how long each pulse stays high within a fixed carrier cycle, i.e., it changes the pulse width or duration. This variation in on-time within each cycle adjusts the duty cycle, which is how much of the time the output is on and thus the average output level. The pulse repetition rate is set by the carrier frequency, not by the modulating signal, and the pulse amplitude is essentially fixed, with only the width encoding the information. So the parameter being varied by the modulating signal is the pulse duration.