In 1933, Edwin H. Armstrong patented a method for generating

frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...

radio signal Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...

s. 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 periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...

phase shift In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other 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 expressed in such a s ...

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

Frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...

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 sound ...

and greatly reduces the amount of

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

on the channel when compared with

amplitude modulation Amplitude modulation (AM) is a signal modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the instantaneous amplitude of the wave is varied in proportion t ...

. 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 bandwidth would be approximately 6

kilohertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base ...

; 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. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

, say 500 kilohertz. This frequency is below the

AM broadcast AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmi ...

band and much below the current FM broadcast band of 88 to 108

megahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base ...

. This carrier signal is applied to two stages in the

transmitter In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...

: a balanced modulator and a mixer. To understand how a balanced modulator works it is necessary to understand

and how it works. Most people describe

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 Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The process encodes information in form of the modulation or message ...

, 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 An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals or a series of binary numbers for Digital signal (signal processing), digital signals. Audio signals have frequencies i ...

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 signal ...

.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 (the so-called "Serrasoid" method, which was created by Radio Engineering Labs (REL), and was endorsed by Armstrong).

Legacy

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

References

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