[time-nuts] Thermal impact on OCXO
Bob Stewart
bob at evoria.net
Mon Nov 7 12:38:02 EST 2016
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:
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From: Scott Stobbe <scott.j.stobbe at gmail.com>
To: Bob Camp <kb8tq at n1k.org>
Cc: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at 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 <kb8tq at n1k.org> wrote:
Hi
> On Nov 5, 2016, at 10:43 PM, Bob Stewart <bob at 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
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> GFS GPSDO list:
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> From: Scott Stobbe <scott.j.stobbe at gmail.com>
> To: Bob Stewart <bob at evoria.net>
> Cc: Discussion of Precise Time and Frequency Measurement <time-nuts at 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 <bob at 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:
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>
>
>
> From: Scott Stobbe <scott.j.stobbe at gmail.com>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at 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 <bob at evoria.net> wrote:
>
> Oh dear. I attached the wrong file. Here's the correct one.
> ----------------------------- ------------------------------ ------
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