[time-nuts] Thermal impact on OCXO

Bob Stewart bob at evoria.net
Sat Nov 5 14:36:09 EDT 2016

Hi Hal,
I don't think I understand your question.  So, I've attached a plot and you can tell me if that gives you anything to work with.  This uses my standard plotting script, so there are things you aren't interested in.  But, this is a plot of one unit from startup on the night of 10/29 through right now.

The blue band is the plot of the TIC in my GPSDO.  The TIC is affected by noise, which is filtered by the PID software, as well as an LPF in the EFC line.  So, the OCXO output is much more stable than the blue band would indicate.  The red trace (the DAC voltage in hex as read on the left) is what's important for this discussion.  Notice the steady decrease in the DAC voltage over time.  The orange trace is the temperature.  For scaling, use the right side numbers divided by 10 to get delta degrees F.  IOW, from 0 to 10 would be a 1 degree Fahrenheit temperature change.  Other items of interest on the plot are TDOP, number of sats seen, and number of sats used.

So, looking at the plot, it seems clear that time dominates the change in the DAC voltage.  But, there is a noticeable impact from temperature change.  That impact is not linear, except that small changes do seem to affect it in a linear manner.
As to how I'm calibrating this:  I've got several GPSDOs running.  One is being used as the 10MHz reference for the 5370A.  Another is being used as the 1PPS reference for the one I'm testing.  Since these are essentially identical units, though with different firmware, the impact of ionospheric change on the results is muted.  So, what the testing boils down to is bringing up a unit with the firmware to be tested, and allowing it to be well locked before disconnecting the antenna.  Generally I leave it overnight and reconnect the antenna some time in the morning.  So, this gives me two figures:  One is how far the unit drifts over some time period, as well as the rate of recovery once the antenna is restored.  

I'd include a Timelab plot except that I don't have the two units skewed enough in time to allow for 1uS of drift.  So, the time reported on Timelab would be misleading due to the 5370 slipping into "one sample every two seconds" mode as the phase difference exceeds the time skew.  For the next test I think I'll skew the DUT by 2uS so that I can get a clean plot.

Anyway, does any of this answer your question?  If not, let me know what's missing.

Bob -----------------------------------------------------------------


      From: Hal Murray <hmurray at megapathdsl.net>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Cc: hmurray at megapathdsl.net
 Sent: Saturday, November 5, 2016 12:55 PM
 Subject: Re: [time-nuts] Thermal impact on OCXO

> At 12 hours of holdover...
> I think I'll need a lot more understanding of the impact of aging vs temperature

At that timescale, I'd expect aging to be lost in the noise.

How are you calibrating things?

These are my opinions.  I hate spam.

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