[time-nuts] Re: Cross correlation for analogue PN measurement

[Bob Camp]

Hi

If quadrature is the objective, just use quadrature splitters. Don’t bother with all 
the PLL stuff, it just will mess everything up. The phase detector noise floor, and
loop characteristics are just the start of the mess …. 

Bob

> On Sep 2, 2023, at 3:17 PM, alan bain via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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
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