The class comprises frequency modulation (FM) and phase modulation (PM), and is based on altering the frequency or the phase, respectively, of a carrier signal to encode the message signal.
This contrasts with varying the amplitude of the carrier, practiced in amplitude modulation (AM) transmission, the earliest of the major modulation methods used widely in early radio broadcasting.
In general form, an analog modulation process of a sinusoidal carrier wave may be described by the following equation:[1] A(t) represents the time-varying amplitude of the sinusoidal carrier wave and the cosine-term is the carrier at its angular frequency
The functional form of the cosine term, which contains the expression of the instantaneous phase
[2] In FM the message signal causes a functional variation of the instantaneous frequency.
These variations are controlled by both the frequency and the amplitude of the modulating wave.
of the carrier is controlled by the modulating waveform, such that the principal frequency remains constant.
may be defined as the instantaneous frequency deviation, measured in rad/s.
is related linearly to the instantaneous phase deviation, that is
In general, however, when using digital signals to modify the carrier wave, the method is called keying, rather than modulation.
Furthermore, another digital modulation is line coding, which uses a baseband carrier, rather than a passband wave.
[2] These improvements, however, are a tradeoff against increased bandwidth requirements.
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.
Phase modulation is important in major application areas including cellular and satellite telecommunications, as well as in data networking methods, such as in some digital subscriber line systems, and WiFi.