When the superheterodyne-type radio receiver became popular in the early 1930s, one of the things listeners noticed was that powerful broadcasting stations tended to overload the receiver and weak stations could barely be heard. As the listener tuned across the standard AM broadcast band, station volume levels consequently could be anywhere from a whisper to a roar.
The difficulty could be managed, of course, by setting the volume control to a low level and then listening carefully, bringing it up when the desired station was located. This only solved part of the problem, however. Amplitude modulated signals are prone to fade in and out, which changes the available radio frequency energy entering the receiver, and thus the audio volume level. Correcting this condition would require the listener to keep one hand constantly on the volume control – not exactly conducive to a relaxing evening in front of the radio!
The problem was solved with a slight redesign of the demodulator, or detector circuit. Normally, this circuit’s only function was to recover the audio voltage from the transmitted signal and pass that on to the audio output amplifier. Engineers realized, with a little trial-and-error research, that a portion of this voltage could be used in a self-correcting feedback loop.
In practice, a small amount of the recovered audio voltage was filtered using a series resistor and parallel capacitor. This combination also provided the necessary time constant, governing the rise and decay of the controlling voltage. Through careful selection of the resistor and capacitor, the time constant could be adjusted to compensate for the effects of fading.
Then, the controlling voltage was fed into the radio frequency section of the receiver. The voltage was used in an inverse relationship to control the gain of the circuits: as the signal grew stronger, so did the controlling voltage and the gain of the controlled circuits was reduced. The opposite condition held as the signal grew weaker. The overall result was a manageable receiver volume level and blasting was kept at a minimum.
As applied to consumer receivers, the circuit was known as automatic volume control (AVC), and soon became standard on almost all commercial receivers, even to the present day. In communications receivers
, it was known as automatic gain control (AGC) and the circuit was slightly more elaborate, usually featuring a selection of time constants.
Rider, John F. Servicing Superheterodynes. New York: John F. Rider Publisher, 1934.
Rohde, Ulrich L. and Bucher, T. T. N. Communications Receivers. New York: McGraw-Hill, 1988.
Ghirardi, Alfred A. Modern Radio Servicing. New York: Radio & Techincal Publishing, 1937.