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 -----------------------------------------------------------------
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Hal Murray <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Cc: [email protected]
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.
PlotCal.sh
Description: application/shellscript
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