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

In a pulse-width modulation (PWM) system, the essential feature being varied is the width of the pulses in relation to the modulating signal. PWM works by encoding information in the width of the pulses; as the modulating signal changes, it effectively alters how long each pulse is held high versus low within a specific time frame.

When the amplitude of the modulating signal is higher, the pulse width increases, leading to a longer duration of the "on" state in the signal. Conversely, a lower modulating signal amplitude results in shorter pulse widths. This relationship allows the PWM system to control the average power delivered to a load, which is particularly useful in applications like motor control or dimming of lights.

Other parameters like pulse frequency and pulse amplitude do not uniquely characterize PWM systems. In PWM, the frequency of the pulses generally remains constant while the width varies. Pulse amplitude is not typically a parameter that is modulated in PWM techniques, as the focus is on varying the duration of the on-state in the pulse sequence itself.