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 <[email protected]> >To: Discussion of precise time and frequency measurement <[email protected]> >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 <[email protected]> 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 -- [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. > > > _______________________________________________ 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.
