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 (seeFLUCTUATIONS, 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 T₀ = 290°K (seeNYQUIST FORMULA):

(1) F = P_act/P_id = P_act/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 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_n ≈ 100°K), have the lowest noise figures. For amplifiers using traveling-wave tubes and tunnel diodes, F ≈ 3 to 10 and T_n ≈ 600° to 3000°K. Radio and television receivers have noise figures ranging from several units to several tens.