[time-nuts] Thermal impact on OCXO
Bob Camp
kb8tq at n1k.org
Mon Nov 7 07:08:46 EST 2016
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
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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 ------------------------------ ------------------------------ -----
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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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