In 1933,

Edwin H. Armstrong Edwin Howard Armstrong (December 18, 1890 – February 1, 1954) was an American electrical engineer and inventor, who developed FM (frequency modulation) radio and the superheterodyne receiver system. He held 42 patents and received numerous awar ...

patented a method for generating

frequency modulation Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing. In analog fre ...

of radio signals. The Armstrong method generates a double

sideband In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands c ...

suppressed

carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...

phase shift In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it ...

s this signal, and then reinserts the carrier to produce a frequency modulated signal.

Frequency modulation Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing. In analog fre ...

generates high quality

audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound * Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum * Digital audio, representation of sou ...

and greatly reduces the amount of

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

on the channel when compared with amplitude modulation. Early broadcasters used amplitude modulation because it was easier to generate than frequency modulation and because the receivers were simpler to make. The electronics theory indicated that a frequency modulated signal would have infinite

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

; for an amplitude modulated signal, the bandwidth is approximately twice the highest modulating frequency. Armstrong realized that while a frequency modulated signal would have an infinite bandwidth, only the first few sets of sidebands would be significant; the rest could be ignored. An amplitude modulated

voice channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for informa ...

bandwidth would be approximately 6 kilohertz; a common frequency modulated voice channel bandwidth could be 15 kilohertz.

How it works

The Armstrong method begins by generating a

at a very low

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

, say 500 kilohertz. This frequency is below the AM broadcast band and much below the current

FM broadcast FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...

band of 88 to 108 megahertz. This carrier signal is applied to two stages in the

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...

: a balanced modulator and a mixer. To understand how a balanced modulator works it is necessary to understand amplitude modulation and how it works. Most people describe amplitude modulation as a method of changing the strength of the carrier (amplitude) in sync with the modulating audio. This is true, the power output does change with modulation, but it changes because any AM modulator generates two sidebands, one above and one below the carrier. As power goes into these sidebands, the power output increases. The amplitude modulated signal, then, consists of a constant strength carrier and two sidebands. The sidebands carry the information and the carrier just goes along for the ride. The carrier can be removed at the transmitter and reinserted at the receiver to allow the transmitter to put all the power in the sidebands. A frequency modulator also generates sidebands, but instead of one sideband on each side of the carrier, it generates many sidebands on each side of the carrier. The FM bandwidth is wider because of the many sidebands. The power output from an FM transmitter is constant with modulation, so as power goes into the sidebands, the carrier power is reduced. A balanced modulator mixes the audio signal and the radio frequency carrier, but suppresses the carrier, leaving only the sidebands. The output from the balanced modulator is a double sideband suppressed carrier signal and it contains all the information that the AM signal has, but without the carrier. It is possible to generate an AM signal by taking the output from the balanced modulator and reinserting the carrier. In the Armstrong method, the audio signal and the radio frequency carrier signal are applied to the balanced modulator to generate a double sideband suppressed carrier signal. The phase of this output signal is then shifted 90 degrees with respect to the original carrier. The balanced modulator output can either lead or lag the carrier's phase. The double sideband signal and the original carrier signal are then applied to the mixer, and the original carrier—90 degrees out of phase—is reinserted. The output from the mixer is a frequency modulated signal. Reinserting the carrier without the phase shift produces an AM signal. Reinserting the carrier with the 90 degree phase shift produces a PM signal. If the intelligence is integrated before being applied to the resulting phase modulator, this equivalent to an FM signal. One of the problems with the Armstrong method is that the

frequency deviation Frequency deviation (f_) is used in FM radio to describe the difference between the minimum or maximum extent of a frequency modulated signal, and the nominal center or carrier frequency. The term is sometimes mistakenly used as synonymous with fre ...

—the amount of modulation—must be kept small to minimize

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...

.U.S. Patent 1,941,068 The maximum deviation is a fraction of 1 kilohertz, but FM broadcast requires 75 kilohertz deviation and a typical FM voice channel deviation is 5 kilohertz. To solve this problem, Armstrong multiplied the signal many times to a higher frequency to obtain the necessary deviation. For example, to generate an FM signal with 5 kilohertz deviation at 146.94 megahertz, the transmitter would generate a signal at 6.1225 megahertz with only 0.2 kilohertz deviation, and then multiply the signal 24 times.

Legacy

The Armstrong method is no longer used commercially. Frequency modulation is most commonly generated at the operating frequency with the required deviation. While the system was being used in the 1930s and 1940s it provided a high quality FM audio signal

References

{{DEFAULTSORT:Armstrong Phase Modulator Radio modulation modes Radio technology Broadcast engineering