[time-nuts] Measure GPSDO stability with minimum resources?

Bob Stewart bob at evoria.net
Fri Oct 7 15:41:29 EDT 2016

Thanks to everyone who responded to my post.  I put in a 5 minute DAC value averager and saved that once every 15 minutes to a 12 hour circular queue last night.  It just started giving me responses, and I've gotten -139 seconds, -4800, -7200, and -2400.  The -139 is probably because I started the thing up too early in the lock process.  I'm going to have to give some thought to this as I get more results.  I expected to get values around -3600, and actually, the last three fit that criteria with a dividend of only 43,200 (12 hours).  I notice that I had an OCXO burp in the middle of the night.  So, I'll have to give some thought about saving a number of the most recent values and throwing away any that don't seem to fit in with the rest.  Once I'm satisfied with the code, I'll increase the queue size to at least 48 hours.  I'm not sure if more than that is really useful, but a multiple of 24 may bias it toward environmental conditions at startup, so perhaps 64 hours would be better.



      From: Bob Stewart <bob at evoria.net>
 To: Discussion of Precise Time and Frequency Measurement <time-nuts at febo.com> 
 Sent: Wednesday, October 5, 2016 12:37 PM
 Subject: [time-nuts] Measure GPSDO stability with minimum resources?
For my GPSDO, I need to calculate the OCXO aging for holdover projection purposes as well as get some figure of merit for the recent past of the OCXO stability.  The latter is so that I can determine that the PLL has (or soon will have) a good lock.  I'm developing on a dfPIC33FJ128MC802, and I've used about 70% of the code space,  I could probably set aside 4K bytes of data space for this calculation.  

I have a rather primitive way of doing part of this, but I was hoping someone would steer me to something a bit better.

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