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noise figure(noise factor) See noise.
(or noise factor), the numerical characteristic of a radio receiver that indicates how much its sensitivity to an incoming signal is decreased by the effects of its internal noise (seeFLUCTUATIONS, ELECTRICAL).
The noise figure F is equal to the ratio of the total noise power at the output of an actual receiver Pact to the output noise power of a similar ideal (noiseless) receiver Pid under the condition that the only source of input noise in both cases is the thermal noise of the matched impedance (dummy antenna) at the temperature T0 = 290°K (seeNYQUIST FORMULA):
(1) F = Pact/Pid = Pact/kT0 Δ fG
Here, k is Boltzmann’s constant, Δf is the receiver’s passband in hertz, and G is the receiver’s power gain. The noise figure is also expressed in decibels: F (dB) = 10 log F. For an ideal receiver, F = 1 (or 0 dB), while for an actual receiver, F > 1. Often, the noise figure is described by the noise temperature Tn.
The noise figure is measured by means of standard noise generators or standard signal generators by determining how much the total power of the receiver’s output signal is increased with a calibrated signal Pgen at its input compared with the output power without the signal. When measuring by the “twofold excess” method, Pgen is adjusted so that the output-signal power is doubled. Then, Pact = GPgen, and the noise figure is calculated from equation (1).
Quantum mechanical amplifiers and cooled parametric amplifiers using semiconductor diodes, for which F ≈ 1.1 and Tn ≈ 30°K (when not cooled, F ≈ 1.3 and Tn ≈ 100°K), have the lowest noise figures. For amplifiers using traveling-wave tubes and tunnel diodes, F ≈ 3 to 10 and Tn ≈ 600° to 3000°K. Radio and television receivers have noise figures ranging from several units to several tens.
REFERENCESKuz’min, A. D. Izmerenie koeffilsienla shuma priemno-usilitel’nykh ustroistv. Moscow-Leningrad, 1955.
Sukhodoev, I. V. Shumy eleklricheskikh tsepei (Teoriia). Moscow, 1975.
I. T. TROFIMENKO