[time-nuts] Looking for info/help on TruePosition GPSDO behavior

[ziggy9+time-nuts at pumpkinbrook.com]

Not seeing any replies, I started up one of my TrueTime units here. I’m not seeing that behavior.  (Last 96 min from an 18 hour run below.) I see my software version is slightly different (12.1.1) but the GPS version is the same. I have other units (TrueTime and UCCM variants) with the same Furuno receiver and not noticed any anomolies. 

I did see PRN 138 come and go but there was no change in DAC correlated with that. I set the delay to 27ns to match yours, and interestingly there was a positive going DAC glitch then. Setting the cable delay back to 0 resulted in a negative going glitch. Curious. 

Q: Are you sure about the time period? It looks to me more like 33 min, 20 seconds. 
Q: Are you feeding the unit with > 12V? Apparently it will drop regulation if it isn’t given ~13V or more. No idea if that’s the issue but just a thought.  




> On Feb 17, 2020, at 5:13 PM, Skip Withrow <skip.withrow at gmail.com> wrote:
> 
> Hello Time-Nuts,
> I'm looking for a GPSDO for a project and have found some interesting
> behavior in the TruePosition GPSDO.
> 
> My requirements for the GPSDO are to be relatively small and that Lady
> Heather knows about the unit.  I have several of the TruePosition
> GPSDOs around, so started playing.  I first had to update my LH from
> ver. 5 to ver. 6, but that was pretty easy.  After firing up the
> TruePosition and doing a survey I noticed that the EFC decides to
> take a 4mV jump every now and then and quickly settles back to where
> it was (looks like time constant is 60-100s or so).  I have tried 3 or
> 4 units so far and they all do the same thing.
> 
> Some of my questions are:
> Have other people seen this behavior?
> LH displays an oscillator gain and the number seems to be there from
> the beginning. Is this jump something that either the TruePosition or
> LH uses to get that gain number?
> Could this behavior be caused by something LH is doing to the unit?
> 
> I have also noticed that there is a very regular pattern to the number
> of satellites.  This is due to PRN138 (a WAAS bird) going in and out
> (about a once per 2 min. period).  I would have to think that this is
> a function of the FURUNO GT-8031 GPS receiver.
> 
> I suppose my next step is to look at the 10MHz output and see if the
> jumps are there (I would bet they are).  Then seeing if they go away
> when LH is shut down.
> 
> I have included a LH screen shot to show these behaviors.
> 
> Regards,
> Skip Withrow
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