[time-nuts] Thermal impact on OCXO

[Bob Stewart]

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
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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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