RF Thermal Noise: Johnson-Nyquist Noise

Thermal noise is one of the main sources of noise in RF and electronic circuits

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Thermal noise is always present in electronic circuits and it is one of the major sources of noise.

In RF circuits, it is often a critical parameter, especially for front end receiver circuits where it is key design parameters.

Noise as seen on oscilloscope
Noise as seen on an oscilloscope

Thermal noise basics

Thermal noise is referred to using a variety of names. Thermal noise is the most widely used, but it may also be called Johnson-Nyquist noise, Johnson noise, or Nyquist noise. This noise gained its various names because this noise was first detected and measured by John B. Johnson in 1926, and later explained by Harry Nyquist - both were Bell Labs and working together.

Thermal noise is generated as a result of thermal agitation of the charge carriers which are typically electrons within an electrical conductor. This thermal noise actually occurs regardless of the applied voltage because the charge carriers vibrate as a result of the temperature. This vibration is dependent upon the temperature - the higher the temperature, the higher the agitation and hence the thermal noise level.

Thermal noise, like other forms of noise are random in nature. It is not possible to predict the waveform and therefore it is not possible to reduce the effects by cancellation or other similar techniques.

Thermal noise in circuits

Thermal noise appears regardless of the quality of component used. The noise level is dependent only upon the temperature and the value of the resistance.

Therefore the only ways to reduce the thermal noise content are to reduce the temperature of operation, or reduce the value of the resistors in the circuit.

Other forms of noise may also be present, therefore the choice of the resistor type may play a part in determining the overall noise level as the different types of noise will add together.

In addition to this, thermal noise is only generated by the real part of any impedance, i.e. the resistance. The imaginary part does not generate noise.

Thermal noise is one of the main limiting factors in a number of areas. In particular it limits the sensitivity of radio receivers because there is a noise floor below which it is not possible to proceed. Some receiver techniques are able to provide signal reception below the noise floor, but the data rate and other factors may be limited. It is therefore useful to be able to calculate the noise for any given instance.

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