In electronics, noise temperature is a temperature (in kelvins) assigned to a component such that the noise power delivered by the noisy component to a noiseless matched resistor is given by
in watts, where:
- is the Boltzmann constant (1.381×10−23 J/K, joules per kelvin)
- is the noise temperature (K)
- is the noise bandwidth (Hz)
Engineers often model noisy components as an ideal component in series with a noisy resistor. The source resistor is often assumed to be at room temperature, conventionally taken as 290 K (17 °C, 62 °F).
A communications system is typically made up of a transmitter, a communications channel, and a receiver. The communications channel may consist of any one or a combination of many different physical media (air, coaxial cable, printed wiring board traces…). The important thing to note is that no matter what physical media the channel consists of, the transmitted signal will be randomly corrupted by a number of different processes. The most common form of signal degradation is called additive noise.
The additive noise in a receiving system can be of thermal origin (thermal noise) or can be from other noise-generating processes. Most of these other processes generate noise whose spectrum and probability distributions are similar to thermal noise. Because of these similarities, the contributions of all noise sources can be lumped together and regarded as thermal noise. The noise power generated by all these sources () can be described by assigning to the noise a noise temperature () defined as:
In a wireless communications receiver, would equal the sum of two noise temperatures:
is the antenna noise temperature and determines the noise power seen at the output of the antenna. The physical temperature of the antenna has no affect on . is the noise temperature of the receiver circuitry and is representative of the noise generated by the non-ideal components inside the receiver.
Noise factor and noise figure
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