Hi Bob,
When you say that OCXOs don't age in a linear fashion, what does that mean?
IOW, is this a case where it's almost linear over a week, and the non-linearity
is only detectable when you consider the longer term? If that's the case, then
it seems reasonable to use the past 3 or so days of data collection to project
the near-term behavior; where near-term is less than 3 days into the future.
Bob -----------------------------------------------------------------
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From: Bob Camp <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Cc: Scott Stobbe <[email protected]>
Sent: Monday, November 7, 2016 6:08 AM
Subject: Re: [time-nuts] Thermal impact on OCXO
Hi
> On Nov 5, 2016, at 10:43 PM, Bob Stewart <[email protected]> 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 <[email protected]>
> To: Bob Stewart <[email protected]>
> Cc: Discussion of Precise Time and Frequency Measurement <[email protected]>
> 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 <[email protected]> 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 <[email protected]>
> To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
> measurement <[email protected]>
> 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 <[email protected]> wrote:
>
> Oh dear. I attached the wrong file. Here's the correct one.
> ----------------------------- ------------------------------ ------
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