[time-nuts] Trueposition behavior

[Skip Withrow]

Hello Time-Nuts,
A couple of weeks ago I posted regarding some weird behavior of the
Trueposition GPSDO that I was seeing.  I have now been able to get
back to the problem and have further results to report.

I have several of these GPSDOs that are version 12.0.1 (I believe the
latest is 12.1.1) and they all seem to behave the same.

I have attached two Lady Heather plots.  One is the GPSDO output of
the control voltage, the other is the 10MHz output monitored by
another NTBW GPSDO.  The bumps in the control voltage are what was
bothering me.

It appears that the way this GPSDO operates is to let the oscillator
free run until there is 50ns of accumulated time error (LH reports
'Normal' during this phase),  Then it goes into control mode, you can
see more noise on the DAC signal during this time plus the big spike,
and yanks the phase back to zero (LH reports 'Acquiring' during this
phase).

The unit must model the OCXO drift as the DAC does change during the
drift period.  So, the longer the unit runs the longer between upsets
(hopefully).  These plots were taken shortly after start up, I am in
the process of letting it run for a while now.  BTW, the very periodic
dropping and acquisition of the WAAS bird (PRN138) is still present.

This behavior is a good news/bad news situation.  On one hand, during
the drift period the output of the GPSDO is not influenced by GPS and
getting yanked around second by second (like the Thunderbolt).  On the
other hand, if the 50ns yank happens when you are making a measurement
it kind of trashes things.  I haven't tried seeing if LH can disable
disciplining (for use during measurement periods).

Regards,
Skip Withrow
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