Frequency modulation

Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. (Contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant.)

In analog applications, the carrier frequency is varied in direct proportion to changes in the amplitude of an input signal. Digital data can be represented by shifting the carrier frequency among a set of discrete values, a technique known as frequency-shift keying.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM radio). A narrowband form is used for voice communications in commercial and amateur radio settings.

FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis, was popularized by early digital synthesizers and became a standard feature for several generations of personal computer sound cards.

Applications in radio

An example of frequency modulation. The top diagram shows the modulating signal superimposed on the carrier wave. The bottom diagram shows the resulting frequency-modulated signal.

Edwin Armstrong presented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935.

Frequency modulation requires a wider bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio" (although for many years the BBC insisted on calling it "VHF radio", which is quite logical, since commercial FM broadcasting uses a well-known part of the VHF frequency band; in certain countries, expressions referencing the more familiar wavelength notion are still used in place of the somewhat mysterious modulation technique name).

Analogue-tuning FM receivers inherently exhibit a phenomenon called capture, where the tuner's frequency drift or lack of selectivity will cause one station or signal to be suddenly overtaken by another on an adjacent channel. This is generally not the case with digital tuners, which use a PLL circuit to prevent this.

An FM signal can also be used to carry a stereo signal: see FM stereo. However, this is done by using multiplexing and demultiplexing before and after the FM process, and is not part of FM proper. The rest of this article ignores the stereo multiplexing and demultiplexing process used in "stereo FM", and concentrates on the FM modulation and demodulation process, which is identical in stereo and mono processes.

Technology

The harmonic distribution of a simple sine wave signal modulated by another sine wave signal can be represented with Bessel functions - this provides a basis for a mathematical understanding of frequency modulation in the frequency domain.

A rule of thumb, Carson's rule states that nearly all the power of a frequency modulated signal lies within a bandwidth of

$2(\Delta f+f_{m})$

where Δf is the peak instantaneous deviation of the carrier from the centre frequency and f_m is the highest modulating frequency.

Note that frequency modulation can be regarded as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal.

Frequency-shift keying refers to the simple case of frequency modulation by a simple signal with only two states, such as in Morse code or radio-teletype applications.

Manchester encoding may be regarded as a simple version of frequency shift keying, where the high and low frequencies are respectively double and the same as the bit rate, and the bit transitions are synchronous with carrier transitions.

When used in supervisory signaling in telephony, the term frequency-change signaling has been used to describe frequency modulation.

The phrase frequency-modulated, an adjective, should have a hyphen.

See also:

• Modulation, for a list of other modulation techniques
• History of radio