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Electrical noise is created naturally by almost any type of device. In circuits that need to amplify and process very low level signals, noise can interfere with the desired signal, causing degraded or complete loss of reception. This is significant for any wireless receiver, often being the limiting factor on reception quality and range. Much of the interfering noise can be generated by the components in the receiver circuit itself.

There are two approaches that can reduce the effect of noise. The first is to increase the transmitted signal level so that the receiver noise becomes insignificant. However, this is usually not practical because of cost, power, size, weight, safety, or regulatory limitations. The other approach is to reduce the noise generated by the receiver circuitry. This is generally the only cost-effective and practical option available.

Noise figure is a measure of noise generated by a circuit. It is defined as the input signal-to-noise ratio divided by the output signal-to-noise ratio. It can be expressed as a ratio, sometimes called noise factor, or in dB, as shown in Equation 1 and illustrated in Figure 1. For an ideal, noise-free device, the input and output signal-to-noise ratios would be equal, giving a noise figure ratio of one. In any real device, some noise is added, so the noise figure ratio is always greater than one. One of the objectives for circuit designers is to make the noise figure as low as possible to minimize the adverse effects of noise in the system.

At microwave frequencies, the primary type of noise is thermal noise, which is generated by any resistance in the circuit, or by the active devices used to amplify signals. Resistance is due to discrete resistors or the resistivity of lossy components. The available noise power produced by a resistance is given by Equation 2:

where
P_n = available noise power
k = Boltzmann’s constant
T = temperature in Kelvin
B = bandwidth of the system