1 Thermal Noise

Thermal noise goes under a number of names including Johnson-Nyquist noise, Johnson noise, or Nyquist noise. This electrical or radio frequency (RF) noise is generated as a result of thermal agitation of the charge carriers (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, therefore, the thermal noise level. Thermal noise is random in nature and not possible to predict its waveform. Therefore, it is not possible to reduce the effects by cancellation or other similar techniques. The only ways to reduce the thermal noise content are to reduce the temperature of operation or by selecting components with more favorable temperature coefficients (such as thin film versus thick film resistors).

An example of this problem is demonstrated in the figure below. Here, the shaded band between the red and black curves represents the range of different phase noise measurement results of the same oscillator at 100MHz. This measurement was made with the same test setup, but we changed some resistors on the PCB. The results shown below might also have been affected by uncontrolled AM noise leakage or electromagnetic interference (EMI) that is now reduced from having selected different components. This illustrates why, in most conversations, thermal noise, EMI, and AM noise leakage are grouped together.