Correlation-based noise measurements below the thermal noise floor

In the characterisation of the phase noise of a component, it is a common practice to measure the cross spectrum at the output of two phase detectors that simultaneously compare the component output signal to a common reference. This technique, which is based on correlation and averaging, allows the rejection of the phase detector noise. Working in the VHF band with an improved version of the above idea, we realized that, under certain hypotheses, the correlation and averaging mechanism makes the instrument insensitive to thermal noise. This feature makes the instrument suitable to investigate noise phenomena below the thermal floor, but-if not understood-is a potential cause of experimental mistakes. Both theoretical explanations and experimental proof are given.