RF Thermal Noise Tutorial

- an overview or tutorial about the basics of RF or thermal noise or Johnson-Nyquist noise in electronic circuits: their cause & their effects.

Thermal noise is always present in electronic circuits to a lesser or greater degree.

Thermal noise is a critical parameter within many RF circuits.

Thermal noise is obviously of great importance within receiver circuits where this form of noise along with other forms of noise limit the sensitivity of the receiver.

Thermal noise basics

Thermal noise goes under a number of names including 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.

This electrical or RF 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.

Noise signal as seen on an oscilloscope trace

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.

By Ian Poole

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