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

Tom Van Baak tvb at LeapSecond.com
Tue Jan 20 23:46:11 EST 2009


Hi Bruce,

Couple of questions. To me the plot looks like it is dominated
with measurement noise (negative slope on the left) and OCXO
frequency drift (positive slope on the right).

You mention "GPS receiver noise" but I don't see that in the
plot. Is this a free-running TBolt? If so, what 1pps source are
you comparing it to?

The title of the plot says, "Undisciplined Thunderbolt OCXO vs.
Thunderbolt GPS receiver"; so is this maybe a plot between
two different Thunderbolts; one locked, the other unlocked?
What TIC was used to collect the data?

/tvb

----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, January 20, 2009 6:04 PM
Subject: Setting loop TC using thunderbolt internal data.


> If one turns of the disciplining of the Thunderbolt's internal OCXO,
> then one can log various parameters such as the 1 pps error.
> A plot of the resulting OADEV (all tau) using such data is attached.
> 
> For short tau the GPS receiver noise dominates.
> For large tau the OCXO noise dominates.
> 
> If the OADEV plot is symmetric about the minimum then the value of tau
> at the Allan intercept coincides with the value of tau at the minimum in
> the OADEV plot.
> In this case the plot has greater slope after the minimum than before so
> the value of Tau at the Allan intercept is about 500 sec rather than the
> 400sec corresponding to the minimum in the OADEV plot.
> The optimum  loop time constant for this particular Thunderbolt is thus
> around 500sec.
> 
> In the absence of any other means of measuring the performance of the
> Thunderbolt as a function of the loop time constant, this technique
> allows the loop time constant to be set near the optimum for each
> particular Thunderbolt.
> 
> Bruce
>





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