Angle modulation

 ( Radio Modulation Modes ) 

Angle modulation is a class of signal modulation that is used in telecommunication transmission systems using . 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.

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

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

$A(t)$ represents the time-varying amplitude of the sinusoidal carrier wave and the cosine-term is the carrier at its angular frequency $\backslash omega$, and the instantaneous phase deviation $\backslash phi(t)$. 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 $\backslash omega\; t\; +\; \backslash phi(t)$ 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 instantaneous frequency. These variations are controlled by both the frequency and the amplitude of the modulating wave. In phase modulation, the instantaneous phase deviation $\backslash phi(t)$ of the carrier is controlled by the modulating waveform, such that the principal frequency remains constant.

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

$\backslash omega\_I\; =\; \backslash frac\{d\}\{dt\}\; =\; \backslash omega\; +\; \backslash phi\text{'}(t)\; ,$

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

For frequency modulation (FM), the modulating signal $s(t)$ is related linearly to the instantaneous frequency deviation, that is $\backslash phi\_\{FM\}\text{'}\; =\; K\_\{FM\}\; s(t),$ which gives the FM modulated waveform as

$m\_\{FM\}(t)\; =\; A\; \backslash cos\; \backslash left(\; \backslash omega\; t\; +\; K\_\{FM\}\; \backslash int\; s(\backslash tau)\; d\backslash tau\; \backslash right).$

For phase modulation (PM), the modulating signal $s(t)$ is related linearly to the instantaneous phase deviation, that is $\backslash phi\_\{PM\}(t)\; =\; K\_\{PM\}s(t),$ which gives the PM modulated waveform as

$m\_\{PM\}(t)\; =\; A\; \backslash cos\; \backslash left(\; \backslash omega\; t\; +\; K\_\{PM\}\; s(t)\; \backslash right).$

In principle, the modulating signal in both frequency and phase modulation may either be analog in nature, or it may be digital. In general, however, when using digital signals to modify the carrier wave, the method is called keying, rather than modulation.Whitham D. Reeve, Subscriber Loop Signaling and Transmission Handbook- Digital, IEEE Press (1995), , p. 5. Thus, telecommunications use frequency-shift keying (FSK), phase-shift keying (PSK), or amplitude-phase keying (APK), or various combinations. Furthermore, another digital modulation is line coding, which uses a baseband carrier, rather than a passband wave.

The methods of angle modulation can provide better discrimination against interference and noise than amplitude modulation.Simon Haykin, Communication Systems, John Wiley & Sons (2001), , p. 107 These improvements, however, are a tradeoff against increased bandwidth requirements.

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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 synthesizer.

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Phase modulation 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.

The combination of phase modulation with amplitude modulation, practiced as early as 1874 by Thomas Edison in the quadruplex telegraph for transmitting four signals, two each in both directions of transmission, constitutes the polar modulation technique.

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Further reading

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