[time-nuts] Phase Detectors/Mixers for DMTD and PN measurements

[Tobias Pluess]

Hi colleagues

In parallel to my GPSDO project I am also thinking about a DMTD measurement system which should also be capable of doing phase noise measurements. This will be necessary to measure the stability and phase noise of my own GPSDO.

There is the paper from W. J. Riley, "A small DMTD system". He uses TUF-R3SM+ mixers. The HP 11729C carrier noise test set uses a custom HP part as phase detector, and a lot of other publications I found use the HP 10514A, either as mixer or as phase detector.
I wonder, which criteria are relevant to select a mixer for this application (besides the frequency range). Are there devices which are better when used as mixers, or phase detectors, or are there even devices which are good for both purposes? Of course, we want low noise, but for most if not all commercial mixers, only the isolation and conversion loss is specified, but I never saw a mixer datasheet having information about the mixer's noise contribution. So there must be other criteria people use when selecting a mixer for a DMTD or phase noise measurement system.

Further. in this paper "OPTIMIZATION OF DUAL-MIXER TIME-DIFFERENCE MULTIPLIER" from Sojdr I even found a reference "The mixer output has a capacitive loading (22 nF) that increases the zero-crossing slope". The "Small DMTD system" from Riley also does have this capacitive loading. Can somebody explain why this does help to improve the zero crossing slope?


Tobias
HB9FSX