That is the power that would be absorbed from the component or source by a matched load.
Noise temperature is generally a function of frequency, unlike that of an ideal resistor which is simply equal to the actual temperature of the resistor at all frequencies.
A noisy component may be modelled as a noiseless component in series with a noisy voltage source producing a voltage of vn, or as a noiseless component in parallel with a noisy current source producing a current of in.
This equivalent voltage or current corresponds to the above power spectral density
, and would have a mean squared amplitude over a bandwidth B of: where R is the resistive part of the component's impedance or G is the conductance (real part) of the component's admittance.
Speaking of noise temperature therefore offers a fair comparison between components having different impedances rather than specifying the noise voltage and qualifying that number by mentioning the component's resistance.
It is also more accessible than speaking of the noise's power spectral density (in watts per hertz) since it is expressed as an ordinary temperature which can be compared to the noise level of an ideal resistor at room temperature (290 K).
Thus it does not make sense to talk about the noise temperature of a capacitor or of a voltage source.
The noise power spectral density generated by any source (
as defined above:[1] In an RF receiver, the overall system noise temperature
equals the sum of the effective noise temperature of the receiver and transmission lines and that of the antenna.
The composite noise temperature of the receiver and transmission line losses
represents the noise contribution of the rest of the receiver system.
In other words, it is a cascaded system of amplifiers and losses where the internal noise temperatures are referred to the antenna input terminals.
On the other hand, a good satellite dish looking through the atmosphere into space (so that it sees a much lower noise temperature) would have the SNR of a signal degraded by more than 6 dB.
The noise temperature of an amplifier is commonly measured using the Y-factor method.
If there are multiple amplifiers in cascade, the noise temperature of the cascade can be calculated using the Friis equation:[3] where Therefore, the amplifier chain can be modelled as a black box having a gain of
In the usual case where the gains of the amplifier's stages are much greater than one, then it can be seen that the noise temperatures of the earlier stages have a much greater influence on the resulting noise temperature than those later in the chain.
has been quadrupled, in addition to the (smaller) contribution due to the attenuator itself
(usually room temperature if the attenuator is composed of resistors).