Noise figure

Noise figure (NF) and noise factor ("F") are measures of degradation of the signal-to-noise ratio (SNR), caused by components in a signal chain. It is a number by which the performance of an amplifier or a radio receiver can be specified, with lower values indicating better performance.
The noise factor is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature "T"0 (usually 290 K). The noise factor is thus the ratio of actual output noise to that which would remain if the device itself did not introduce noise, or the ratio of input SNR to output SNR.
The noise "figure" is simply the noise "factor" expressed in decibels (dB).
General.
The noise figure is the difference in decibels (dB) between the noise output of the actual receiver to the noise output of an “ideal” receiver with the same overall gain and bandwidth when the receivers are connected to matched sources at the standard noise temperature "T"0 (usually 290 K). The noise power from a simple load is equal to "kTB", where "k" is Boltzmann's constant, "T" is the absolute temperature of the load (for example a resistor), and "B" is the measurement bandwidth.
This makes the noise figure a useful figure of merit for terrestrial systems, where the antenna effective temperature is usually near the standard 290 K. In this case, one receiver with a noise figure, say 2 dB better than another, will have an output signal to noise ratio that is about 2 dB better than the other. However, in the case of satellite communications systems, where the receiver antenna is pointed out into cold space, the antenna effective temperature is often colder than 290 K. In these cases a 2 dB improvement in receiver noise figure will result in more than a 2 dB improvement in the output signal to noise ratio. For this reason, the related figure of "effective noise temperature" is therefore often used instead of the noise figure for characterizing satellite-communication receivers and low-noise amplifiers.
In heterodyne systems, output noise power includes spurious contributions from image-frequency transformation, but the portion attributable to thermal noise in the input termination at standard noise temperature includes only that which appears in the output via the principal frequency transformation of the system and excludes that which appears via the image frequency transformation.
Definition.
The noise factor of a system is defined as
where and are the input and output signal-to-noise ratios respectively. The quantities are power ratios. 
The noise figure is defined as the noise factor in dB:
where and are in decibels (dB).
These formulae are only valid when the input termination is at standard noise temperature , although in practice small differences in temperature do not significantly affect the values.
The noise factor of a device is related to its noise temperature :
Attenuators have a noise factor equal to their attenuation ratio when their physical temperature equals . More generally, for an attenuator at a physical temperature , the noise temperature is , giving a noise factor
Noise factor of cascaded devices.
If several devices are cascaded, the total noise factor can be found with Friis' formula:
where is the noise factor for the -th device, and is the power gain (linear, not in dB) of the -th device. The first amplifier in a chain usually has the most significant effect on the total noise figure because the noise figures of the following stages are reduced by stage gains. Consequently, the first amplifier usually has a low noise figure, and the noise figure requirements of subsequent stages is usually more relaxed.
Noise factor as a function of additional noise.
The noise factor may be expressed as a function of the additional output referred noise power formula_4 and the power gain formula_5 of an amplifier.
Derivation.
From the definition of noise factor
and assuming a system which has a noisy single stage amplifier. The signal to noise ratio of this amplifier would include its own output referred noise formula_4, the amplified signal formula_8 and the amplified input noise formula_9,
Substituting the output SNR to the noise factor definition,
In cascaded systems formula_12 does not refer to the output noise of the previous component. An input termination at the standard noise temperature is still assumed for the individual component. This means that the additional noise power added by each component is independent of the other components.