A possible alternative method of making traceable phase noise measurements

Establishing traceability for phase noise measurements is a task which has not been addressed to any significant extent. Some work was done at NPL which was based on a commercially available measurement system using the phase detector method of making phase noise measurements. This work established a relatively pessimistic value of ±0.95 dB which should be expanded by the normal coverage factor of k=2 to give ±1.9 dB. This work did not include the effects of the phase lock loop used to maintain phase quadrature which could give additional errors for close to carrier phase noise. This paper describes another method which could be used to provide traceable phase noise measurements and which is particularly suited to close to carrier measurements. There is also some doubt as to the value which should be ascribed to the uncertainty of the phase detector constant-a critical parameter of the measurement process. The value of the phase detector constant is usually obtained by measuring the slope of the beat frequency at the zero crossings and methods have been identified which are tolerant to asymmetric waveforms. However the degree of tolerance has not been quantified. The method described here does not use a phase detector and so this difficulty is circumvented