Hi If your resolution plus sawtooth is ~ 250 ns, then you have 250 ppb at 1 second. At 300 seconds you would have just under 1 ppb. To get close to the full range temp stability of the OCXO you would have to run at least 10X that long. Put another way, the GPS can only stop hurting things past about 60 minutes. The OCXO is likely stable at a constant ambient to < 10 ppt for moderate amounts of time. The GPS could only start helping the OCXO past about 600 minutes.
That all assumes the loop settles instantly with no error. If you have a more normal loop, multiply all those times by some number between 3 and 20. If you use 10 then the GPS would start helping at about 6,000 minutes ( 100 hours or 4 days). That's not a "moderate amount of time" so the real cross over should be a bit sooner than that. Bob On Aug 16, 2013, at 1:57 PM, Bob Stewart <b...@evoria.net> wrote: > Hi Bob, > > So, assuming your math is right at .15 ppb, that seems to be within the > figures you gave for the 34310-T. > > I'm not attempting sawtooth correction for a number of reasons, the biggest > of which is the uncertainty factor in the PIC. I think I read in the > datasheet that the CCP1 input where I'm getting the 1PPS pulse has a delay of > 3-4 oscillator ticks, but I can't find that at the moment. Assuming that's > correct, with an 8MHz oscillator, that works out to an uncertainty of 125ns. > I don't know what the uncertainty is on the T1CKI input (10MHz) but let's say > that it's the same 125ns. That's 250ns, to which we have to add an > uncertainty of +/- 52ns for the UT+ sawtooth. Those figures seem to be borne > out by the second to second variance I'm seeing in the clock count. It can > go as high as 5 ticks when the oscillator starts moving. > > So, I've set the hysteresis for the PLL at +/- 2 ticks of the 10MHz > oscillator. Anything above that and I count it as a phase error and move the > DAC by ~0.0003V (not a typo). If I see a phase error above 8 ticks, I count > it as a error, move the DAC by ~0.0048V, and zero the cumulative phase error > so it doesn't go into a tailspin. Generally this doesn't happen except > during the power up/oven warming period, though I have seen it once. > > So, bottom line: is .15 ppb over 5 minutes a reasonable accuracy for what I > have to work with? > > Bob > > > > > >> ________________________________ >> From: Bob Camp <li...@rtty.us> >> To: Discussion of precise time and frequency measurement >> <time-nuts@febo.com> >> Sent: Friday, August 16, 2013 10:47 AM >> Subject: Re: [time-nuts] My GPSDO project: OCXO Thermal Oscillation? >> >> >> Hi >> >> Ok, let's try some math and see if I can do it without blinking this time…. >> >> +/-4 Hz for 6 volts is 0.66 Hz / V >> output is 10 MHz so 1 Hz is 0.1 ppm >> your OCXO is running at 0.066 ppm / V >> That's also 66 ppb / V >> >> 0.02 V at 66 ppb / V is 0.0132 ppb or 13.2 ppt >> >> The UT+ has a sawtooth output that's about 45 ns >> That's 45 ppb at one second >> >> 5 minutes is 300 seconds >> >> so 45 / 300 = 0.15 ppb or 150 ppt >> >> If it's the later clone version it might be about 1/2 of that. >> >> Are you doing sawtooth correction? >> >> Bob >> >> On Aug 16, 2013, at 11:09 AM, Bob Stewart <b...@evoria.net> wrote: >> >>> I'm converting the code for the VE2ZAZ FLL to a PLL. I'm seeing the phase >>> correction change the EFC up and down about .02V to .03V over a period of 5 >>> minutes or so (it varies). The full range on the OCXO is about +/- 4Hz >>> varied by 0 to +6V, so at least this is a tiny value. I feel pretty >>> confident with my code at this point. I'm using a Trimble 34310-T OCXO for >>> which I've been able to find almost no information. Could this oscillating >>> phase correction be some sort of thermal oscillation? I've tried two >>> separate 34310s and both act more or less the same. My GPS device is >>> normally a UT+, but I just now swapped in an "Adafruit Ultimate GPS >>> Breakout" to the same effect. Is this good, bad, or indifferent for a >>> GPSDO? I started this project not knowing what to expect, and I still >>> don't. Experienced help, speculation, or even just kind words at this >>> point would be appreciated! =) I don't have a known good/stable reference >>> to compare > this >>> to. >>> >>> Bob - AE6RV >>> _______________________________________________ >>> 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. _______________________________________________ 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.
