We have seen that in analog communications, the two most widely used modulation techniques are amplitude and frequency modulation. However, there is a third fundamental quantity that can be used to carry information: phase. This principle underlies phase modulation (PM, phase modulation), a technique that, although less commonly used in its pure analog form, plays a central role in modern digital communications.

In phase modulation, information is impressed onto the instantaneous phase of the carrier, while both amplitude and the average frequency remain constant. Intuitively, this means that the transmitted signal does not vary in “intensity” or average frequency, but instead experiences time shifts—advances or delays—i.e., phase shifts relative to a reference. The modulating signal directly controls these phase variations, determining the final shape of the transmitted waveform.

From a conceptual standpoint, PM is closely related to frequency modulation. Since instantaneous frequency is linked to the rate of change of phase, phase modulation inevitably implies frequency variations as well. In other words, PM and FM belong to the same family of angular modulations and share many properties, even though they differ in how the modulating signal acts on the carrier.

As in FM, the spectrum of a PM signal consists of multiple sidebands distributed around the carrier frequency. The occupied bandwidth depends both on the amplitude of the modulating signal and on the speed of its variations over time. In general, these techniques require more bandwidth than amplitude modulation, especially when high signal quality or high-fidelity transmission is desired.

A characteristic parameter of phase modulation is the modulation index, which expresses the maximum extent of phase variation induced by the information signal. Unlike frequency modulation, this parameter depends directly on the amplitude of the modulating signal and not on its frequency. As a result, higher-frequency signals produce faster phase variations and therefore a broader spectral content.

From a reception standpoint, PM demodulation requires circuits capable of detecting phase variations in the carrier. In practice, phase-locked loop (PLL) systems are often used, allowing the receiver to track the phase of the incoming signal and accurately reconstruct the modulating signal. As in FM, the constant amplitude of the carrier makes PM relatively insensitive to amplitude noise, improving overall system robustness.

Although analog phase modulation is less widely used than FM in practical applications, its principle forms the basis of many modern digital modulation techniques. In particular, schemes such as PSK (Phase Shift Keying) and its variants use discrete phase changes to represent binary or multilevel symbols. In this sense, PM represents a natural bridge between analog modulation techniques and digital methods, in which information is encoded discretely but still transmitted through analog carrier properties.

In summary, alongside amplitude and frequency, phase represents the third fundamental dimension that can be exploited to carry information: less intuitive to visualize, but extremely powerful and today essential in the most advanced communication systems.