[time-nuts] Weird GPSDO behavior

Lars Walenius lars.walenius at hotmail.com
Sat Sep 30 02:43:05 EDT 2017

If I look on the TIC graph it looks like something happen at about + or - 500ns. Could it be that the software change algoritm? Like a ”Jam Sync”. Seems like it tries to get the TIC to zero quick by changing the DAC. Maybe it thinks it gets enough close in frequency and stops and goes back to the PI-loop with 20000 secs time constant? As the frequency seems to be off 0.7ppb the excursion to about 6-7 us with a 20000sec PI-loop make sense and it takes about a day to get back in phase (to + or - 500 ns and a new cycle begins).

If the above is correct the parameters for the PI-loop work but not for the other ”algoritm” that has a factor of 100 wrong oscillator sensitivity.

Changing to another time constant might give another cycle different from a day?? But will not solve the problem I guess.

Best Regards

From: Skip Withrow<mailto:skip.withrow at gmail.com>
Sent: den 28 september 2017 22:25
To: time-nuts<mailto:time-nuts at febo.com>
Subject: [time-nuts] Weird GPSDO behavior

Hello Time-Nuts,

I have a NTGS50AA GPSDO (close cousin to the NTBW50AA and Thunderbolt)
with the OCXO removed and a SRS PRS-10 rubidium oscillator in its
place.  I have been running Lady Heather 5.0 and have changed the
damping, gain, and time constant to give me a 20,000 second time
constant with a damping of .6.  I have attached a Lady Heather screen
shot of the weird behavior.  You can see that my GPS antenna is in a
very none ideal location (window on the west side of the building).

Once per day (about 8am) something disturbs the system.  So, the GPSDO
spends much of its time recovering and never gives me anywhere near
the performance that this system is capable of.  I would think that it
is not the PRS-10 as it has no knowledge of time.  I would also think
that it is not the GPS system or receiver, since the GPS constellation
repeats twice per day.

Kind of the two things that I am left with are a glitch by the power
company every morning (there is some large industrial machinery across
the street (but then I would kind of expect glitches at 8am and 5pm),
and perhaps Lady Heather doing something funny.  This system has been
running for quite some time, I have not tried restarting Lady Heather

Anybody seen anything like this, or have any good ideas?

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