[time-nuts] Measuring gpsdo vs itself

[Azelio Boriani]

Try this setup: feed the GPSDO into A and B inputs but not to the
reference. That is, use the counter internal reference to time the
difference so that you have an uncorrelated source that can span all the
interpolator's nonlinearities.

On Mon, Nov 5, 2012 at 11:42 AM, Bill Dailey <docdailey at gmail.com> wrote:

> Thanks guys,
>
> Like usual more complicated than I thought.   I was hoping that this would
> cancel any stability issues common to both the reference and the signal
> thus giving me best case ability.  I seem to be getting numbers too good to
> be true so there must be a hitch.  I get an ADEV 5x10-13 at 1 s mostly
> linear to 7x10-16 at 10,000 s with a small hump at 20s-80s.  Figured there
> was some kind of gotcha.
>
> Doc
>
>
> On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson <
> magnus at rubidium.dyndns.org
> > wrote:
>
> > On 11/05/2012 06:30 AM, Tom Van Baak wrote:
> >
> >> Ah, I see what you mean now. Yes, that setup can give you a rough
> >> estimate of the counter's noise floor.
> >>
> >> I can't give you specific numbers but one danger with this sort of test
> >> is that the input and the timebase are artificially locked together
> (i.e.
> >> fixed phase relationship) through the common reference. Your
> measurements
> >> may thus show artificially less noise than a real-life case of
> independent
> >> input(s) and reference.
> >>
> >> This can happen if your sub-ns counter is based on interpolators.
> Because
> >> the input and the timebase are locked in phase, the counter lands near
> the
> >> same point of the interpolator scale on every single measurement, rather
> >> than experiencing the noise (and non-linearity) of the entire scale.
> >>
> >
> > It's a little more complex than interpolator non-linearities alone. You
> > also need to include cross-talk between the signals. This cross-talk is
> > usually higher between A and B inputs than from reference, but never the
> > less.
> >
> > You would need to sweep the trigger input delays to illustrate these
> > non-linearities. From a single measurement you can get both a better or
> > worse number compared to the average which is what you would expect to
> see
> > for free-running signals.
> >
> > So, you can get a rough idea about the baseline, but it is not a
> > sufficient method.
> >
> > See the SR620 manual for a plot of non-linearities.
> >
> > Cheers,
> > Magnus
> >
> >
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>
>
> --
> Doc
>
> Bill Dailey
> KXØO
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