Hi Charles, I've had a night to digest what I'm seeing, and this is what I've come up with:
There were only 2 updates to the DAC during the 24 hours tested. So, long term doesn't system drift dominate? That would include thermal drift and the stability of the OCXO. Also, there was a phase crossing for both of those updates. Short term there's the fact that this an Adafruit GPS Nav receiver (MTK-3339) with a spec of "10ns jitter". Looking at the GPS track, it is moving in a diamond shaped area about 41 ft from point to opposite point. Sometimes it's not a smooth movement, with a jerk of perhaps 6 feet. And there's the fact that the 1PPS has lots of ugly quantization errors, but I have no way of sensing them, much less correcting them. (The 5335B is not part of the loop. The phase error value is only an external measurement fed to my PC.) So, short term, it seems to be a measurement of the 1PPS from the MTK-3339, and long term a measurement of the drift of the Tekelec DOC-1903 OCXO, including the impact of the phase crossings that my code can't prevent. There is also the thermal drift in there somewhere, but it was very well behaved during this test run. OK, I've just reread your post, and it seems that I've just reworded it. Well, at least that means I'm beginning to understand what I have. I don't have any more accurate standards than this, so I don't see any way of improving my measurements. But, all in all, I'm satisfied that it's as accurate as I'll ever need. But, I'll probably keep fiddling with the code to see if I can eliminate that dependency on phase crossings to update the DAC. Bob >________________________________ > From: Charles Steinmetz <[email protected]> >To: Discussion of precise time and frequency measurement <[email protected]> >Sent: Wednesday, February 5, 2014 5:47 AM >Subject: Re: [time-nuts] ADEV computed, now what? > > >Bob wrote: > >> here's the result for 1PPS vs 10MHz for my GPSDO, as measured by a 5334B >> clocked by the same 10MHz. I don't know how to read these, but >> 6,3,1,6,3,1 etc. doesn't look normal. > >The adev results you obtained look very much like the "adev" results reported >by Lady Heather, very likely for the same reason -- you do not have any >independent standard by which to measure. > >From what you say above, it appears that you measured the time interval >between one edge of the PPS pulse and the next zero-cross of the GPSDO 10 MHz, >using a 5334B clocked by the GPSDO 10 MHz. Is that correct? I take it the >GPSDO is disciplined by the PPS? > >Bear in mind (i) that you are comparing one noisy source to another, and (ii) >that the errors are correlated more and more strongly as tau increases beyond >the point where the discipline loop begins controlling the 10 MHz oscillator. >At small tau, the GPS PPS is very noisy (much noisier than the 10 MHz >oscillator), and it gets better and better as you average for longer and >longer periods until at tau above 10k it's reasonably decent. Your GPSDO >should leave the oscillator more or less on its own at low tau (where the >jitter in the oscillator is lower than the jitter in the PPS), and correct it >beginning at some longer tau where the jitters are comparable (continuing to >even longer tau where the jitter in the PPS is lower than the jitter in the >oscillator). > >So, measuring as you appear to be doing, at low tau you are essentially >measuring the improvement of the PPS with averaging -- 10x per decade -- using >the essentially undisciplined 10 MHz oscillator as a standard. At some point, >you would expect to reach a floor where you would essentially be measuring the >residual jitter in the disciplined oscillator and the PPS. In fact, you can >see this in your results starting around tau = 500, but your series does not >go far enough to show the floor clearly. > >Bob suggested that you are measuring the trigger error of the 5334B, and that >may be contributing to your results as well. With good measurement >techniques, the outer bound of the 5334B trigger error should be less than >1nS, probably more like 200-300pS. Your actual error is probably >significantly lower than this outer bound unless something is wrong with your >setup. The trigger contribution to your computed adev should also fall 10x >per decade with increasing tau. > >Best regards, > >Charles > > > >_______________________________________________ >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.
