What is PM, Phase Modulation

Phase modulation, PM can be used for both analogue and digital data, but it is for data & phase shift keying that it is most widely used.

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Phase Modulation Includes:
Phase modulation Phase shift keying, PSK

Modulation formats: Modulation types & techniques Amplitude modulation Frequency modulation Quadrature amplitude modulation

As the name implies, phase modulation, PM uses variations in phase for carrying the modulation.

As phase and frequency are interrelated, this relationship carries forwards into phase modulation where it has many commonalities with frequency modulation. As a result the term angle modulation is often use to describe both.

Phase modulation, PM is sometimes used for analogue transmission, but it has become the basis for modulation schemes used for carrying data. Phase shoft keying, PSK is widely used for data communication.

Phase modulation is also the basis of a form of modulation known as quadrature amplitude modulation, where both phase and amplitude are varied to provide additional capabilities.

Phase modulation basics

Before looking at phase modulation it is first necessary to look at phase itself. A radio frequency signal consists of an oscillating carrier in the form of a sine wave is the basis of the signal. The instantaneous amplitude follows this curve moving positive and then negative, returning to the start point after one complete cycle - it follows the curve of the sine wave.

Sine wave plotted as intensity against time — A Sine Wave

The sine wave can also be represented by the movement of a point around a circle, the phase at any given point being the angle between the start point and the point on the waveform as shown.

Also the phase advances as time progresses so points on the waveform can be said to have a phase difference between them.

Phase difference between two signals — Phase difference between signals

Phase modulation works by modulating the phase of the signal, i.e. changing the rate at which the point moves around the circle. This changes the phase of the signal from what it would have been if no modulation was applied. In other words the speed of rotation around the circle is modulated about the mean value.

Phase modulation showing variations or modulation of the steady progression of the phase angle — Phase modulation concept

To achieve this it is necessary to change the frequency of the signal for a short time. In other words when phase modulation is applied to a signal there are frequency changes and vice versa. Phase and frequency are inseparably linked as phase is the integral of frequency.

Frequency modulation can be changed to phase modulation by simply adding a CR network to the modulating signal that integrates the modulating signal. As such the information regarding sidebands, bandwidth and the like also hold true for phase modulation as they do for frequency modulation, bearing in mind their relationship.

Forms of phase modulation

Although phase modulation is used for some analogue transmissions, it is far more widely used as a digital form of modulation where it switches between different phases. This is known as phase shift keying, PSK, and there are many flavours of this. It is even possible to combine phase shift keying and amplitude keying in a form of modulation known as quadrature amplitude modulation, QAM.

The list below gives some of the forms of phase shift keying that are used:

PM - Phase Modulation
PSK - Phase Shift Keying
BPSK - Binary Phase Shift Keying
QPSK - Quadrature Phase Shift Keying
8 PSK - 8 Point Phase Shift Keying
16 PSK - 16 Point Phase Shift Keying
OPSK - Offset Phase Shift Keying

These are just some of the major forms of phase modulation that are widely used in radio communications applications today. With today's highly software adaptable radio communications systems, it is possible to change between the different types of modulation to best meet the prevailing conditions.

Since the introduction of digital or data communications, the use of phase modulation in the form of phase shift keying has grown very significantly. Previously there were few advantages to its use. Now it is widely used along with quadrature amplitude modulation which incorporates phase elements to its operation as well as amplitude. With data communications only set to increase various forms of phase modulation, or forms of modulation that use phase elements will continue to rise.