[time-nuts] Setting loop TC using thunderbolt internal data.

Bruce Walker bruce at w1bw.org
Mon Jan 26 16:31:08 EST 2009


The data I posted came from a 10 hour run collecting data every
second.  I could certainly set up other longer experiments.

We quickly arrive at the limits of my experience...I had never looked
at Hadamard deviation before.  I was curious to see what I could learn
about the Tbolt without having anything better to measure it against.
For the time being, this is my "lab" reference.

Let me see whether I understand what I've measured.  The Allan
intercept for my tbolt's undisciplined oscillator as measured by the
onboard GPS solution is on order tau=800s.  The Hadamard intercept
under the same conditions appears to be more like 1600s.   Does that
tell me that if the disciplining model includes a drift compensation,
the optimal disciplining time constant is close to the Hadamard
intercept tau, but if there were no drift model, the Allan intercept
would be more appropriate?

And a primer or reference to the other statistical measures you
mentioned (Theo_1, etc.) would be appreciated.

--bruce W1BW

On Mon, Jan 26, 2009 at 3:36 PM, Bruce Griffiths
<bruce.griffiths at xtra.co.nz> wrote:
>
> Bruce
>
> What was the record length?
>
> TOTDEV and TOTHadamard should give more reliable estimates over a
> greater tau range.
>
> Bias corrected Theo_1 and the Hadamard equivalent should be even better
> at longer tau.
>
> Bruce
>
> Bruce Walker wrote:
> > Thanks for the suggestion.  Here is a composite plot with Allan and Hadamard
> > deviations for my tbolt run last night with disciplining disabled.  I think
> > I computed Hadamard correctly.
> >
> > OADEV (blue) has a minimum of 9.6e-12 around tau=790
> > OHDEV (red) has a minimum of 4.9e-12 around tau=1660
> >
> > --bruce W1BW
> >
> >
> > On Mon, Jan 26, 2009 at 2:05 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> >
> >> wrote:
> >>
> >
> >
> >> Some idea of the shift in the minimum can be gleaned by plotting the
> >> Hadamard deviation vs tau.
> >>
> >> Bruce Walker wrote:
> >>
> >>> I ran the same test as Bruce Griffiths on my tbolt; that is, I turned off
> >>> osc disciplining altogether and created an OADEV plot from the internal
> >>> phase measurements taken from its GPS solution.  The plot is attached.
> >>>
> >>  It
> >>
> >>> has a minimum of about 1e-11 at tau=800, and it has the sane general
> >>>
> >> shape
> >>
> >>> as Griffiths'.
> >>>
> >>> The next experiment I plan to do is to repeat the same thing using manual
> >>> holdover mode rather than "Disable disciplining".  That should continue
> >>>
> >> to
> >>
> >>> try to compensate for temperature and long-term drift (just not steered
> >>>
> >> by
> >>
> >>> GPS), whereas the data taken last night use a fixed DAC.
> >>>
> >>> --bruce W1BW
> >>>
> >>>
> >>>
>
>
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