[time-nuts] Fine frequency adjustment in 5370B

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Mon May 16 05:54:42 EDT 2005

Tom Van Baak wrote:

> A PRS10, for example, may have 100x or better
> accuracy than a typical 10811. Yet a 10811 can
> easily have 2x to 10x better short-term stability
> than a PRS10. You can indirectly measure this
> by seeing which reference, internal or external,
> gives the smallest standard deviation for the
> particular measurement you are making.


as you once said before, the phase noise on the PRS10


is poorer than the 10811A too.

Also interesting, is that the phase noise spec on the PRS10 (< -130dBc/Hz at 10 Hz) is sligthly 
misleading, when it is about 100x worst than that at *larger* offsets - see graph on that SRS page. 
However, the worst case which is -111dBc/Hz looks to be *about* 60 Hz (best I can tell from the log 
graph), so perhaps this is a measurement problem, rather than the oscillator itself. Although at about 
200Hz it is poorer than -130dBc/Hz too and I can't think of an explanation for that.

That behaviour is not what "the books" shows, although having never measured the phase noise of an 
oscillator, I don't know if this behaviour is typical or not.

I've been thinking about implementing:

GPS -> PRS10 -> Shera board -> 10811A

where the 1pps output from the PRS10 is used to lock the 10811A to 10MHz using Brooks Shera board. 
That *might* (and I'd be interested in opinions) give better short term stabilty and phase noise than 
the PRS10 itself, while retaining the long term stability of GPS.

Dr. David Kirkby PhD CEng MIEE,
Senior Research Fellow,
Department of Medical Physics,
Mallet Place Engineering Building,
Gower St,
University College London,
London WC1E 6BT.

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