[time-nuts] Thermal impact on OCXO

Scott Stobbe scott.j.stobbe at gmail.com
Sun Nov 6 00:01:29 EDT 2016


Well bob's comments & caution are accurate, everything drifts. In your
case, if the OCXO is rock solid then you would see a 160 PPM change on the
EFC line over 7 days which is a 1mV change on your 6 V full scale, which is
fairly easy to measure if you have a 6 1/2 digit DMM.

On Sat, 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.  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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>
> ------------------------------
> *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.
>  ----------------------------- ------------------------------ ------
> AE6RV.com
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