Noise temperature

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Noise temperature: At a pair of terminals, the temperature of a passive system having an available noise power per unit bandwidth at a specified frequency equal to that of the actual terminals of a network.

The noise temperature of a simple resistor is the actual temperature of that resistor. The noise temperature of a diode may be many times the actual temperature of the diode. That is, we could replace the diode with a resistor and place that resistor to the equivalent temperature to generate the same level of the noise. Similarly, we can model a terrestrial antenna to have a noise temperature of 290K if the noise level of the resistor at 290K (=16.8C) is the same as the noise level of the antenna.

Specifically, the noise temperature is defined by T = N/kB, where N is the noise power within bandwidth B, and k = 1.38 × 10−23J K−1 is Boltzmann's constant. A radar system is characterized by several noise temperatures: the antenna temperature Ta, the receiver temperature Tr, and the transmission line temperature Tl. The transmission line temperature is a measure of the noise power within the receiver bandwidth generated by the resistive losses in the transmission line or waveguide between the antenna and the receiver. The transmission line temperature is frequently combined with either the antenna or the receiver temperature, depending on the reference point for the measurement. The total system temperature is T = Ta + Tl + Tr.