[time-nuts] Thermal impact on OCXO

Scott Stobbe scott.j.stobbe at gmail.com
Mon Nov 7 10:47:35 EST 2016


Typo, "Aging [PPM]" should read "Aging [PPB]".

On Mon, Nov 7, 2016 at 10:34 AM, Scott Stobbe <scott.j.stobbe at gmail.com>
wrote:

> Here is a sample data point taken from http://tycho.usno.navy.mil/
> ptti/1987papers/Vol%2019_16.pdf; the first that showed up on a google
> search.
>
>          Year   Aging [PPM]  dF/dt [PPT/Day]
>             1       180.51       63.884
>             2       196.65        31.93
>             5          218       12.769
>             9       231.69       7.0934
>            10       234.15        6.384
>            25        255.5       2.5535
>
> If you have a set of coefficients you believe to be representative of your
> OCXO, we can give those a go.
>
> On Mon, Nov 7, 2016 at 7:08 AM, Bob Camp <kb8tq at n1k.org> wrote:
>
>> 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
>> >  -----------------------------------------------------------------
>> > AE6RV.com
>> >
>> > GFS GPSDO list:
>> > groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>> >
>> >      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
>> >
>> > GFS GPSDO list:
>> > groups.yahoo.com/neo/groups/ GFS-GPSDOs/info
>> >
>> >
>> >
>> > 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
>> >
>> > GFS GPSDO list:
>> > groups.yahoo.com/neo/groups/ GFS-GPSDOs/info
>> >
>> >
>> >
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>> >
>> >
>> >
>> >
>> >
>> >
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