[time-nuts] Trueposition behavior

[djl]

Is this effectively dither on the control voltage?

On 2020-03-10 10:30, Skip Withrow wrote:
> 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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-- 
Dr. Don Latham  AJ7LL
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304