[time-nuts] Thermal impact on OCXO

Bob Camp kb8tq at n1k.org
Mon Nov 7 14:01:01 EST 2016


Hi

Thermocouples in test leads are nasty because they operate on the temperature
delta rather than the absolute temperature. It’s the gradient you have to worry about.
Until you have run out a few dozen sensors around your bench (or test chamber) and
monitored them for a few days … you won’t believe just how little “point A” has in common
with “point B”.

Bob

> On Nov 7, 2016, at 1:44 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> -accidentally omitted timenuts from my reply to bob-
> Hi Bob,
> The data is at the 10uV level.  As to whether I've plotted "data" or data, I can only report what I have.  It seems to be consistent over the long run.  I would expect to see something else if it were just noise.  If the 4 attachments were strongly thermocoupled, wouldn't the data have a stronger correlation to temperature, rather than just to temperature transients?
> 
> Bob -----------------------------------------------------------------
> AE6RV.com
> 
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>      From: Bob Camp <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net> 
> Cc: Discussion of Precise Time and Frequency Measurement <time-nuts at febo.com>
> Sent: Monday, November 7, 2016 11:54 AM
> Subject: Re: [time-nuts] Thermal impact on OCXO
> 
> 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
>>   
>> -----------------------------------------------------------------
>> AE6RV.com
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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.
>>>   ----------------------------- ------------------------------ ------
>>> AE6RV.com
>>> 
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>>> 
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