PAM vs PWM vs PPM: Understanding Pulse Modulation Techniques

This article breaks down the distinctions between PAM (Pulse Amplitude Modulation), PWM (Pulse Width Modulation), and PPM (Pulse Position Modulation). These are all analog pulse modulation techniques, each encoding information in a different aspect of a pulse train.

PAM (Pulse Amplitude Modulation)

In PAM, the amplitude of a carrier pulse is modulated according to the amplitude of the modulating signal. Essentially, the instantaneous amplitude of the message signal directly influences the amplitude of the transmitted pulse.

As the name suggests, the amplitude of a train of high-frequency carrier pulses is varied in accordance with the sampled values of the message signal. The message information is, therefore, encoded in the amplitude of a series of signal pulses.

Demodulation is achieved by detecting the amplitude level of the carrier at each symbol period.

Fig. 1: PAM - Illustrating the relationship between the analog message signal and the PAM modulated signal.

The figure above shows the time-domain representation of PAM, clearly depicting the analog message signal and the resulting PAM-modulated signal.

PWM (Pulse Width Modulation)

The PWM signal is a pulse signal whose width is proportional to the amplitude of the modulating analog signal. In other words, the duration of each pulse changes based on the message signal’s amplitude.

Fig. 2: PWM - A time-domain representation of Pulse Width Modulation.

The figure illustrates the time-domain representation of PWM.

A common application of PWM is in the speed control of DC motors. By varying the pulse width, you can effectively control the average voltage applied to the motor, thereby adjusting its speed.

PPM (Pulse Position Modulation)

The PPM signal is a pulse signal whose position is proportional to the amplitude of the modulating signal. Instead of changing the amplitude or width, PPM alters the timing of the pulses.

Fig. 3: PPM - Showing the time-domain representation of Pulse Position Modulation.

The figure depicts the time-domain representation of PPM.

The ubiquitous 555 Timer IC is frequently used to generate waveforms for both PPM and PWM. PWM is typically generated using the 555 timer in monostable multivibrator mode. PPM, on the other hand, can be generated by using a PWM signal as a trigger in the monostable multivibrator mode.

PAM vs PWM vs PPM: Key Differences Summarized

Here’s a table summarizing the key differences between the three modulation techniques: