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

[Bruce Griffiths]

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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