Angle modulation

Angle modulation is a class of analog modulation that is used in telecommunications transmission systems. The class comprises frequency modulation (FM) and phase modulation (PM). The technique is based on altering the angle (or phase) of a carrier signal to transmit data. This as opposed to varying the amplitude of the carrier, such as in amplitude modulation transmission.

Formalism

In general form, an analog modulation process of a sinusoidal carrier wave may be described by the following equation:

$m(t)=A(t)\cdot \cos(\omega t+\phi (t))\,$.

Here A(t) represents the time-varying amplitude of the analog carrier wave, and the cosine-term is the carrier at its instantaneous phase angle $\phi (t)$. This description directly provides two major groups of modulation, amplitude modulation and angle modulation. In amplitude modulation, the phase term is held constant, while in angle modulation the term A(t) is constant and second term of the equation has a functional relationship to the modulating message signal.

The functional form of the cosine term provides the distinction of the two types of angle modulation, frequency modulation (FM) and phase modulation (PM). In FM the message signal causes a functional variation of the carrier frequency. These variations are controlled by both the frequency and the amplitude of the modulating wave. In phase modulation, the phase term $\phi (t)$ of the carrier is controlled by the modulating waveform.

See also

• Polar modulation