Noise, Receiver

the electrical fluctuations that occur mainly in the input circuits (for example, amplifiers and filters) of radio-receiving apparatus (seeFLUCTUATIONS, ELECTRICAL).

When combined with noises from external sources, such as atmospheric noise, terrestrial noise, and cosmic noise, a total noise is produced that acts as a natural threshold of the sensitivity of the radio receiver. The quantitative characteristic of receiver-noise intensity is its noise temperature. For example, until the 1950’s, the input devices used in superhigh-frequency radio receivers had noise temperatures of more than 2000°K, which was many times higher than the noise temperatures of the external sources. Subsequently, low-noise, superhigh-frequency amplifying devices were invented, such as tunnel diodes, parametric amplifiers, and quantum mechanical amplifiers, which made it possible to lower the receiver-noise temperatures, respectively, to values of the order of 300°, 30°, and 3 K, thereby improving the sensitivity of superhigh-frequency radio receivers by about 10 to 50 times.