Nick: >From your description, it sounds like you have a frequency locked loop with a maximum frequency sensitivity of 1 ppb, as opposed to a phase locked loop.
At the end of the 100 second count period, do you start over with a counter set to zero, or do you carry forward any error from the previous period? --- Graham == On Sun, Aug 16, 2015 at 2:31 PM, Tom Van Baak <t...@leapsecond.com> wrote: > Hi Nick, > > Nice project. Thanks for sharing. > > I was hoping someone would someday use a cheap Sparkfun / Parallax / > Adafruit GPS to make a low-cost GPSDO. Mostly what people on this list do > is go to the extreme of using serious GPS timing receivers (such as Oncore > M12+T, or ublox-5T or 6T or 8T) but those require significant amounts of > configuration, tuning, survey, etc. to meet ultimate performance levels. > > The good thing about using a cheap hobbyist-grade 3D GPS/1PPS receiver is > that they work anywhere, without fiddling or survey, within seconds of > power-on. Ok, you lose a few ns of precision compared to serious receivers > -- but for a TCXO that doesn't matter. > > Anyway, to answer your question -- to measure its true performance you > only need two things. 1) a phase meter (or time interval counter) that's > good to 1 ns or better, and 2) a local reference standard that's maybe 10x > better than the TCXO and the Adafruit GPS. Usually that means a cesium > standard, or supremely qualified GPSDO, or equivalent. > > A number of us here on the time-nuts list have such equipment at home. And > unlike professional labs, we will do it for free/fun if you loan the GPSDO > for a week. > > If you want to play with raw timing data from an Adafruit GPS board see > file gps-mtk3339.txt.gz under http://www.leapsecond.com/pages/gpsdo-sim/ > and then use TimeLab for phase, frequency, and Allan deviation analysis ( > http://www.ke5fx.com/timelab/readme.htm). > > /tvb > > ----- Original Message ----- > From: "Nick Sayer via time-nuts" <time-nuts@febo.com> > To: <time-nuts@febo.com> > Sent: Sunday, August 16, 2015 11:47 AM > Subject: [time-nuts] I've designed a GPSDO, but how "good" is it? > > > 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 -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.