Nick, professional GPSDO's achieve precision much less than one 10MHz count, by doing sub-nanosecond level time interval measurements between GPS PPS and the OCXO-divided-by-10M-PPS. During initial lock there is some software smarts to reset the divide-by-10M chain at the right point so the initial lock is a bit more like a frequency lock rather than a phase lock - these smarts also kick in if there is a substantial disturbance to phase to be resolved (that substantial difference probably doesn't require a real EFC correction but instead is some kind of glitch ridden over by going to frequency lock.)
Google things like "z3801a tic" (tic = time interval converter) to see past discussions about the state of the art a few decades ago. Tim N3QE On Sun, Aug 16, 2015 at 2:47 PM, Nick Sayer via time-nuts < [email protected]> wrote: > I’ve designed and make and sell a GPSDO on Tindie ( > https://hackaday.io/project/6872-gps-disciplined-tcxo). It’s brand new - > I’ve sold a handful of them so far. So as to make this post not *entirely* > self-serving, what I would like is some further guidance on how I can > better characterize its performance. > > The GPS reference is a 1 pps signal (It’s the Adafruit Ultimate GPS module > - a PA6H). The manufacturer claims an accuracy of ±10 ns, but that's > accuracy relative to the true start of the GPS second. They don’t make any > claim for stability. > > The oscillator itself (Connor Winfield DOT050V 10 MHz) has a short-term > (though they don’t say how short that term is) stability of 1 ppb. The > absolute accuracy of it is (I assume) irrelevant, because it’s a VCTCXO and > the control voltage is steered by GPS feedback. > > The feedback loop takes samples over a 100 second period. That gives me an > error sample with a granularity of 1 ppb. I keep a rolling sample window of > 10 samples to get an error count over 1000 seconds. I've kept track of both > of these values for extended periods (days) as well as logging the DAC > value (the number that's proportional to the control voltage). The 1000 > second sample window error averages zero, and it almost never exceeds ±7 > (every once in a while if I physically move it, it will show a momentary > error glitch, but that shows up in the short term feedback sampling too). > The 100 second samples are almost all 0 or ±1, with an occasional ±2 > showing up. As I said before, if I bonk the oscillator, it may briefly show > a ±6 or so for one sample. > > If I pit two of them against each other on a scope and take a time lapse > video (http://www.youtube.com/watch?v=9HkeCI90i44), you can see that they > stay mostly locked with occasional periods of drift. I sort of assume that > that represents periods where the two GPS receivers disagree as they decide > differently how to select among the available satellites. > > I've been saying out loud that the oscillator is ±1 ppb from GPS over the > 1000 second window. I know of Allan variance, but I don't have anything > else handy I can use for comparison. I also can't really afford to send one > off for testing to a proper lab. In looking at > http://tf.nist.gov/general/pdf/2297.pdf, it suggests that my results are > relatively poor compared to what a GPSDO can achieve (more like 10^-12 > rather than 10^-9), but I assume that they’re able to use a higher > frequency GPS reference than just 1 PPS (and they’re a lot pricier). > > What else can I do to try and characterize the performance? If mine is > performing far more poorly than the same price ($175) can buy elsewhere, > then what am I doing wrong? > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
