Editing Noise figure (section)

== General ==
The noise figure is the difference in [[decibel]] (dB) between the noise output of the actual receiver to the noise output of an "ideal" receiver with the same overall [[Gain (electronics)|gain]] and [[Bandwidth (signal processing)|bandwidth]] when the receivers are connected to matched sources at the standard [[noise temperature]] ''T''<sub>0</sub> (usually 290&nbsp;K). The noise power from a simple [[Electrical load|load]] is equal to ''kTB'', where ''k'' is the [[Boltzmann 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&nbsp;K.  In this case, one receiver with a noise figure, say 2&nbsp;dB better than another, will have an output signal-to-noise ratio that is about 2&nbsp;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&nbsp;K.{{sfnp|Keysight|2019|p=9}} In these cases a 2&nbsp;dB improvement in receiver noise figure will result in more than a 2&nbsp;dB improvement in the output signal-to-noise ratio. For this reason, the related figure of ''[[Effective input noise temperature|effective noise temperature]]'' is therefore often used instead of the noise figure for characterizing satellite-communication receivers and [[low-noise amplifier]]s.

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.