# noise figure

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## noise figure

(noise factor) See noise.## Noise Figure

(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 (*see*FLUCTUATIONS, ELECTRICAL).

The noise figure *F* is equal to the ratio of the total noise power at the output of an actual receiver *P*_{act} to the output noise power of a similar ideal (noiseless) receiver *P*_{id} 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 *T*_{0} = 290°K (*see*NYQUIST FORMULA):

(1) *F* = *P*_{act}/*P*_{id} = *P*_{act}/*kT*_{0} Δ *fG*

Here, *k* is Boltzmann’s constant, Δ*f* 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 *T _{n}*.

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 *P*_{gen} at its input compared with the output power without the signal. When measuring by the “twofold excess” method, *P*_{gen} is adjusted so that the output-signal power is doubled. Then, *P*_{act} = *GP*_{gen}, 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 *T _{n}* ≈ 30°K (when not cooled,

*F*≈ 1.3 and

*T*≈ 100°K), have the lowest noise figures. For amplifiers using traveling-wave tubes and tunnel diodes,

_{n}*F*≈ 3 to 10 and

*T*≈ 600° to 3000°K. Radio and television receivers have noise figures ranging from several units to several tens.

_{n}### REFERENCES

Kuz’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