[time-nuts] Thermal impact on OCXO

Scott Stobbe scott.j.stobbe at gmail.com
Mon Nov 7 16:47:28 EST 2016


Deeper into the rabbit hole :)

I'm not sure what specific pic you are you using but most of them have at
least one timer that will run fully asynchronously and its timer input is
usually shared with one of the 32k osc pins. Which you can then use to wake
the prossesor from one of its sleep states. Do you end up hitting a clock
sync with the pwm block?

Good old 74 series to the rescue.

On Monday, 7 November 2016, Bob Stewart <bob at evoria.net> wrote:

> Hi Scott,
>
> I wish I had some long term data, but I don't.  I had initially set out to
> build an accurate GPS frequency reference type of GPSDO.  So, aging wasn't
> an issue.  It's either on and locked or it's not.  So, I didn't worry about
> leaving a unit running for months and collecting aging data.
>
> But then I had this crackpot idea of using the latches in a 7474 to
> stabilize the 1PPS from the receiver to the OCXO.  Tom had a bit of a
> misunderstanding about what I was doing and we had a bit of a discussion
> until he caught on to what I was doing and its limitations.
>
> But, somewhere along the line, I realized that the idea was sound but my
> implementation was poor.  The basic problem with using a timer in the
> dsPIC33 is that they use a PLL to generate the internal clock - even if you
> supply a clock.  That gives you a 1-count jitter in any output pulse you
> try to create.  So, I realized that I could use the latches in a 7474 to
> latch the OCXO to the output of a timer on the PIC.  With the PIC running
> at 40MHz, I have plenty of room for the jitter without the worry of a phase
> slip.  I think I've proved that that works, so now I have the possibility
> of using my system as a time server.  And that means I now have to deal
> with such arcane matters as holdover, aging, and generating the time from
> the OCXO.  The learning curve has been a bit steep.
>
> Bob
>
> -----------------------------------------------------------------
> AE6RV.com
>
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>
>
> ------------------------------
> *From:* Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:_e(%7B%7D,'cvml','scott.j.stobbe at gmail.com');>>
> *To:* Bob Camp <kb8tq at n1k.org
> <javascript:_e(%7B%7D,'cvml','kb8tq at n1k.org');>>
> *Cc:* Bob Stewart <bob at evoria.net
> <javascript:_e(%7B%7D,'cvml','bob at evoria.net');>>; Discussion of precise
> time and frequency measurement <time-nuts at febo.com
> <javascript:_e(%7B%7D,'cvml','time-nuts at febo.com');>>
> *Sent:* Monday, November 7, 2016 9:34 AM
> *Subject:* Re: [time-nuts] Thermal impact on OCXO
>
> 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
> <javascript:_e(%7B%7D,'cvml','kb8tq at n1k.org');>> wrote:
>
> Hi
>
>
> > On Nov 5, 2016, at 10:43 PM, Bob Stewart <bob at evoria.net
> <javascript:_e(%7B%7D,'cvml','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
> <http://groups.yahoo.com/neo/groups/GFS-GPSDOs/info?soc_src=mail&soc_trk=ma>
> >
> >      From: Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:_e(%7B%7D,'cvml','scott.j.stobbe at gmail.com');>>
> > To: Bob Stewart <bob at evoria.net
> <javascript:_e(%7B%7D,'cvml','bob at evoria.net');>>
> > Cc: Discussion of Precise Time and Frequency Measurement <
> time-nuts at febo.com <javascript:_e(%7B%7D,'cvml','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
> <javascript:_e(%7B%7D,'cvml','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/
> <http://groups.yahoo.com/neo/groups/?soc_src=mail&soc_trk=ma>
> GFS-GPSDOs/info
> >
> >
> >
> > From: Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:_e(%7B%7D,'cvml','scott.j.stobbe at gmail.com');>>
> > To: Bob Stewart <bob at evoria.net
> <javascript:_e(%7B%7D,'cvml','bob at evoria.net');>>; Discussion of precise
> time and frequency measurement <time-nuts at febo.com
> <javascript:_e(%7B%7D,'cvml','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
> <javascript:_e(%7B%7D,'cvml','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/
> <http://groups.yahoo.com/neo/groups/?soc_src=mail&soc_trk=ma>
> GFS-GPSDOs/info
> >
> >
> >
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> >
> >
> >
> >
> >
> >
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