[time-nuts] Cross correlation for analogue PN measurement

[alan bain]

I have been curious as to whether it is possible to reduce the noise
floor below that of the reference oscillator when doing a PN
measurement HP3048 style if the DUT feeds a power splitter which feeds
two identical PLLs locking (separate) low phase noise oscillators in
quadrature (e.g. two copies of the 8662A + 11848A setup) with the two
phase detector outputs feeding Ch1 and Ch2 respectively of the dynamic
signal analyser, but with the latter set to compute the cross spectrum
(which the old 3562A can do).

Clearly one needs to repeat the process multiple times and average. If
the two reference oscillators are assumed uncorrelated to the DUT and
each other (maybe they need to live in separate racks - or piles - to
avoid vibration coupling) then this cross spectrum reduces to the
power spectrum of the DUT noise.  Clearly the two oscillators need
independent time bases - not a connection to the house 10MHz standard,
but that's probably fine as the phase lock loop would normally tweak
the EFC of the reference in each synthesiser.

The cross correlation approach seems used commonly in the digital
domain as part of IQ type analysers, but does it work in the purely
analoge one or does some other factor appear and become the dominant
one and wipe out the gains?

I seem to have almost enough heavy HP lumps to try this, but not quite
enough, hence the question!

Alan