[time-nuts] Thermal impact on OCXO

[Bob Camp]

Hi

I would suggest you read the papers on the subject. They are out there. Even over a few days
OCXO aging is likely to be non-linear ( => curved, not straight line). On an OCXO that is 
warming up, your are dealing with retrace rather than aging. This can have an impact for a 
few months after turn on. I have one unit in the basement that took 9 months to come out
of retrace after being in storage for “a while”. It was an eBay item so no idea just how long 
the power off time was. 

Bob


> On Nov 7, 2016, at 12:47 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> 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 <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> Cc: Scott Stobbe <scott.j.stobbe at gmail.com>
> 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 <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  ------------------------------ ------------------------------ -----
> > AE6RV.com
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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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