[time-nuts] Checking accuracy of Rubidium standards

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Nov 10 02:38:32 UTC 2008


Joe
J. L. Trantham wrote:
> I have been enjoying this discussion.  
>
> Since the original question was the desire to 'compare' the frequency of an
> LPRO to a Z3801, it seems that you could consider that from two (at least)
> perspectives.
>
> Before I begin, I confess that I am a novice in this arena and please
> correct me in any area that needs it.
>
> The first perspective is the issue of frequency.  That seems to me to be the
> issue of the average frequency of the LPRO versus the average frequency of
> the Z3801.  Assuming that there is no gross difference of the 10 MHz
> signals, a lissajous figure (X-Y display) on a scope with the appropriate
> bandwidth amplifiers would be a reasonable initial approach.
>
>   
The limitation with using Lissajous figures is that eventually the noise
in the relatively wide oscilloscope bandwidth (due to amplifier noise
and signal phase noise) limits the figure rotation rate that can
reliably discerned. Also the lack of a record of changes limits the
ability to see long term trends/drifts.

However it can be a useful/instructive starting point particularly when
the frequency difference is relatively large but still small enough that
the figure rotation can be seen.
> Assuming that they are both near 10 MHz and you do not know which is the
> most accurate (although the Z3801 would seem to be the default standard), if
> it takes 10 minutes for a single cycle of the lissajous figure to complete,
> then it is 1 cycle per 600 seconds difference between the two and therefore
> the two are within 1/600 Hz or 1.67 mHz of each other.  If we assume that
> they are both close to 10 MHz, then that is 1.67 parts in 10E-10 difference
> between the two.  Is my logic faulty?
>
> The other perspective is the issue of 'purity'.  That is to say, what is the
> 'frequency modulation' of the source?  This, I think, is the issue of phase
> noise.  Correct?
>
> That is something that I have not yet had a chance to contemplate as far as
> how to measure.  It would appear to require a particularly stable (pure)
> source as a reference though.  Various multiplying or dividing protocols
> would seem to introduce a host of other variables that would seem to be
> difficult to account for though they might accentuate an impurity in the
> signal in question.  I have read Bruce's comments and I still do not
> understand the basics of time stamping or how a sound card might provide
> this.
>
>   
One technique is to use a three cornered hat technique.
If the phase fluctuations of 3 oscillators are statistically
independent, then it is possible to determine the statistical
fluctuations in each of them.
> I would appreciate any direction for further reading regarding this and I
> would appreciate any direction/correction/etc. in the thoughts above.
>
> Joe
>
>   
Bruce



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