What is Noise Temperature?

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- everything RF

May 12, 2023

Noise Temperature (Tn) is a way to express noise of a system in terms of an equivalent temperature. It is not the physical temperature of a system but is a theoretical value that defines an equivalent temperature required to produce a specific amount of noise power in that system. It is defined as the equivalent temperature of the noise power per unit bandwidth of an active or passive system at any given frequency. 

Noise is generally a random, undesirable signal that impacts the performance of the system, and is a critical component in areas like telecommunications. It can be generated either from inside or outside the system. The noise that is generated within the system is usually in the form of semiconductor noise, thermal noise, and the noise current resulting due to the movement of charge carriers (electrons and holes). Noise due to external sources can be from extraterrestrial radiation bodies such as galactic or cosmic noise, absorption of electromagnetic energy from the atmosphere, interference due to the surrounding receivers, and the noise temperature of the Earth itself.

While these and other types of noise exist, most of the noise processes that occur in nature typically generate noise for which the spectrum and probability distributions are very similar to thermal noise, an effect that arises due to the random, rapid motion of electrons in various components of a system. Thermal noise is generated due to certain lossy components of an antenna and hence, is picked up as radiation and transferred to the receiver subsystems, in addition to the thermal noise generated by each of the subsystems. Due to these reasons, thermal noise is regarded as the main source of noise that limits the performance of a system. This is an important parameter when evaluating the level of noise present in typical telecommunication systems such as RF receivers. The equivalent noise temperature of the noise power generated within such a system can be given as,

Noise temperature can also be represented in terms of two other equally important factors used to characterize different types of noise. They are noise factor and noise figure.

Noise temperature can be defined in terms of noise factor as,

where T0  is the absolute temperature of the environment and equals 290 K.

The relationship between noise temperature and noise figure is given as,


Noise temperature is widely used to characterize and evaluate many electronic circuits and systems. For instance, it is used in measuring the level of noise in radio astronomy, front-end amplifiers, and in cascaded amplifier stages. Other emerging applications where this is used include microwave/millimeter wave radiometers and radar receiver systems.