Hi Scott, I wish I had some long term data, but I don't. I had initially set out to build an accurate GPS frequency reference type of GPSDO. So, aging wasn't an issue. It's either on and locked or it's not. So, I didn't worry about leaving a unit running for months and collecting aging data.
But then I had this crackpot idea of using the latches in a 7474 to stabilize the 1PPS from the receiver to the OCXO. Tom had a bit of a misunderstanding about what I was doing and we had a bit of a discussion until he caught on to what I was doing and its limitations. But, somewhere along the line, I realized that the idea was sound but my implementation was poor. The basic problem with using a timer in the dsPIC33 is that they use a PLL to generate the internal clock - even if you supply a clock. That gives you a 1-count jitter in any output pulse you try to create. So, I realized that I could use the latches in a 7474 to latch the OCXO to the output of a timer on the PIC. With the PIC running at 40MHz, I have plenty of room for the jitter without the worry of a phase slip. I think I've proved that that works, so now I have the possibility of using my system as a time server. And that means I now have to deal with such arcane matters as holdover, aging, and generating the time from the OCXO. The learning curve has been a bit steep. Bob ----------------------------------------------------------------- AE6RV.com GFS GPSDO list: groups.yahoo.com/neo/groups/GFS-GPSDOs/info From: Scott Stobbe <scott.j.sto...@gmail.com> To: Bob Camp <kb...@n1k.org> Cc: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Monday, November 7, 2016 9:34 AM Subject: Re: [time-nuts] Thermal impact on OCXO Here is a sample data point taken from http://tycho.usno.navy.mil/ptti/1987papers/Vol%2019_16.pdf; the first that showed up on a google search. Year Aging [PPM] dF/dt [PPT/Day] 1 180.51 63.884 2 196.65 31.93 5 218 12.769 9 231.69 7.0934 10 234.15 6.384 25 255.5 2.5535 If you have a set of coefficients you believe to be representative of your OCXO, we can give those a go. On Mon, Nov 7, 2016 at 7:08 AM, Bob Camp <kb...@n1k.org> wrote: Hi > On Nov 5, 2016, at 10:43 PM, Bob Stewart <b...@evoria.net> wrote: > > Hi Scott, > D'oh. Thanks for the correction! Like I said, I don't do these calculations > often. > > If as Bob Camp implies, the aging isn't from the OXCO, then I'm a bit > stumped. I do have an op-amp in the EFC string with a voltage divider for > gain. The resistors are Panasonic ERA-6AEDxxxV resistors. Mouser says > they're temperature stable to 25PPM/C, but of course they don't mention an > aging rate. I don't really see anything else, other than the OCXO, that is > likely to be prone to a linear type of aging. OCXO’s don’t age in a linear fashion. At least 90% of them don’t. If you dig into the FCS papers there are various curves proposed as models. Mil-O-55310 has one of them as the “official” approach. All of them have the basic issue of mistakenly fitting to to short a time constraint. Bob > The aging rate appears to be stable from unit to unit, so naturally I >considered the OCXO first. > > There is one other bit in the EFC string that might be controversial, but I > don't see that it would be a candidate for the symptoms of aging. > > Bob > ------------------------------ ------------------------------ ----- > AE6RV.com > > GFS GPSDO list: > groups.yahoo.com/neo/groups/ GFS-GPSDOs/info > > From: Scott Stobbe <scott.j.sto...@gmail.com> > To: Bob Stewart <b...@evoria.net> > Cc: Discussion of Precise Time and Frequency Measurement <time-nuts@febo.com> > Sent: Saturday, November 5, 2016 9:19 PM > Subject: Re: [time-nuts] Thermal impact on OCXO > > If your DAC spans the full EFC range than 1LSB is 1/2^20 ~ 1 PPM of the EFC > range, and the EFC tuning range is 8/10E6 ~ 1 PPM full scale, so 1 LSB is > ~1PPT. So, if everything else is stable the DAC code reflects changes solely > due to the OCXO, which would be an aging of 24 PPT/day. > On Sat, Nov 5, 2016 at 9:57 PM, Bob Stewart <b...@evoria.net> wrote: > > Hi Scott, > The 20 bits span about 6 volts. The EFC range spans about 8Hz (+/-4Hz). I > don't do these calculations every day, but that's about 4.5PPT? > Bob ------------------------------ ------------------------------ ----- > AE6RV.com > > GFS GPSDO list: > groups.yahoo.com/neo/groups/ GFS-GPSDOs/info > > > > From: Scott Stobbe <scott.j.sto...@gmail.com> > To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency > measurement <time-nuts@febo.com> > Sent: Saturday, November 5, 2016 8:38 PM > Subject: Re: [time-nuts] Thermal impact on OCXO > > I think that's a nice plot, it looks like you have stepped 160 LSB over 7 > days or roughly 1 LSB per hour. With a 20bit dac you are trimming maybe 1 > ppt/LSB to 4 ppt/LSB? In allan devation terms, the case of 1ppt/LSB, solely > due to drift, you're at 1E-12 at 3600*sqrt(2) = 5000 s, in the case of > 4ppt/hour your at 1E-12 at 1280 s. Seems reasonable. > On Sat, Nov 5, 2016 at 2:47 PM, Bob Stewart <b...@evoria.net> wrote: > > Oh dear. I attached the wrong file. Here's the correct one. > ----------------------------- ------------------------------ ------ > AE6RV.com > > GFS GPSDO list: > groups.yahoo.com/neo/groups/ GFS-GPSDOs/info > > > > ______________________________ _________________ > 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.