Radio transmission accomplished by symmetrical variation of the carrier frequency by an analog input signal. The amount of swing from center frequency is dependent upon the peak value of the modulating voltage, as well as upon its frequency. The frequency of the modulation signal governs the rate at which the changes in carrier frequency occur. Frequency-modulation (FM) sidebands are formed during the modulation process and are separated from one another by an amount equal to the audio frequency. The amplitude of the sidebands diminishes progressively as the sidebands occur farther and farther from the center frequency, and the number of significant sidebands depends on the amplitude of the modulating signal. The deviation ratio of the carrier-frequency variation to the highest signal frequency transmitted may be any selected value, from fractional to large values.
Because there is no amplitude change in the output of an FM transmitter, whatever the deviation ratio, this mode is an almost perfect cure for amplitude-related interference problems that plague radio-frequency reception. Another property of FM receivers is the relative freedom from interference between distant and local stations using the same channel; only the strongest signal is received, even if the wanted signal is only 3–6 dB stronger than the interfering one. This characteristic is known as the capture effect. In contrast, AM radio signals differing in strength by 35 dB result in noticeable interference.
Since most FM receivers are equipped with a muting circuit (squelch) that silences the audio channel when no signal is present, an irritating hiss noise is not emitted from the loudspeaker when the frequency (or channel) being monitored is not in use.
Frequency modulation is used mainly for transmissions above 25 megahertz (MHz). Typical uses are in broadcasting, television sound, mobile radio telephony, radio paging systems, space telemetry, intercity microwave relaying of all classes of public traffic including voice channels, teleprinting, facsimile, broadcast network programs, and television and computer data, and intercontinental telecommunications via satellite. Frequency modulation is used for both analog and digital communications, and phase modulation as well as frequency modulation is employed.
FM broadcasting
The frequency band 88–108 MHz is allocated to FM broadcasting in a large part of the world by international agreement. For a channel spacing of 200 kHz, there are 100 allocatable channels for transmission of an audio range of 50–15,000 Hz, with a frequency-deviation ratio of 5. This means that there are five significant sidebands above and below the carrier, the carrier is deviated a maximum of ±75 kHz, and the emitted spectrum is twice this value. Because of the relatively small signal power in the modulating frequencies above 4 kHz, the received signal-to-noise ratio is improved substantially by preemphasis of the audio signal in transmission, necessitating complementary deemphasis in the receiver to restore natural program balance. In fact, preemphasis produces a sort of hybrid form of modulation, being pure frequency modulation at the lowest audio frequencies, and gradually changing to phase modulation at the highest. See also Frequency modulation.
FM mobile transmission
Millions of land, maritime, and aeronautical mobile FM transceivers are employed by police, firemen, public safety agencies, industrial and commercial enterprises, private citizens, and radio amateurs who desire the benefits of enroute telephony. The intensity of such usage has grown exponentially, mainly because of the availability of reliable, small, low-power-consumption solid-state equipments that are economical, and also because of the public realization of the benefits of having such communications.
Radio relaying
Frequency modulation is used for microwave radio relaying over land, over water, and to great distances using satellites, sometimes carrying thousands of simultaneous telephone conversations or several television channels.
The advent of requirements for short-haul services, local distribution networks within cities, television relay, and a wide variety of optical communication services including high-speed computer communications, electronic mail, data transmission, and other services, where it may be cost-effective to avoid the local telephone loop, presents another application for FM radio relaying.
Telegraphy
Telegraphy, including teleprinting and binary digital data transmission, is based on shifting the carrier frequency or its phase between two limiting values, one of which represents a mark signal and the other a space signal. This frequency shift (or phase shift) is a form of FM signaling used over a wire, cable, or radio.
Facsimile
Black-and-white images (line drawings and typed copy) can be transmitted by employing the principles used in FM telegraphy; one limit frequency corresponds to black, and the other to white, on the image to be transmitted. A continuous gray scale can be transmitted and recorded if, instead of just two frequencies, a continuous frequency shift is employed between some low frequency (say, 1500 Hz) and some higher frequency (say, 2700 Hz), the exact frequency at any instant being proportional to the gray level of the image. See also Facsimile.
Telemetry
Frequency modulation is the preferred method for transmission of information or data from a remote or inaccessible location such as a rocket vehicle in flight. Each condition to be remotely observed actuates one subchannel, which, when multiplexed with other channels reporting other status conditions, modulates the radio carrier by frequency modulation.
