Angle modulation

Angle modulation is a class of carrier 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 phase or frequency of a carrier signal to encode the message signal. This contrasts with varying the amplitude of the carrier, such as in amplitude modulation (AM) transmission, the earliest of the major modulation methods used widely in early radio broadcasting.

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

The functional form of the cosine term, which contains the expression of the instantaneous phase $\omega t+\phi (t)$ as its argument, 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 instantaneous phase deviation $\phi (t)$ of the carrier is controlled by the modulating waveform, such that the principle frequency remains constant.

For frequency modulation, the instantaneous frequency of an angle-modulated carrier wave is given by the first derivative with respect to time of the instantaneous phase:

${\frac {d}{dt}}[\omega t+\phi (t)]=\omega +\phi '(t)\,\!$,

in which $\phi '(t)$ may be defined as the instantaneous frequency deviation, measured in rad/s.

Frequency modulation

Main article: Frequency modulation

Frequency modulation is widely used for FM broadcasting of radio programming, and largely supplanted amplitude modulation for this purpose starting in the 1930s, with its invention by American engineer Edwin Armstrong in 1933. FM also has many other applications, such as in two-way radio communications, and in FM synthesis for music synthesizers.

Phase modulation

Main article: Phase modulation

Major application areas for phase modulation include cellular and satellite telecommunications, as well as data networking methods such as in some Digital Subscriber Line systems, and WiFi.

See also

• Polar modulation