Interesting, Tom. I don't think I see any of those pesky grain boundary
shifts or readjustments in the lattice structure? If I remember, these
can cause instant shifts in frequency that do not heal?
Don
On 2016-11-12 14:54, Tom Van Baak wrote:
There were postings recently about OCXO ageing, or drift rates.
I've been testing a batch of TBolts for a couple of months and it
provides an interesting set of data from which to make visual answers
to recent questions. Here are three plots.
1) attached plot: TBolt-10day-fit0-e09.gif (
http://leapsecond.com/pages/tbolt/TBolt-10day-fit0-e09.gif )
A bunch of oscillators are measured with a 20-channel system. Each
frequency plot is a free-running TBolt (no GPS, no disciplining). The
X-scale is 10 days and the Y-scale is 1 ppb, or 1e-9 per Y-division.
What you see at this scale is that all the OCXO are quite stable.
Also, some of them show drift.
For example, the OCXO frequency in channel 14 changes by 2e-9 in 10
days for a drift rate of 2e-10/day. It looks large in this plot but
its well under the typical spec, such as 5e-10/day for a 10811A. We
see a variety of drift rates, including some that appear to be zero:
flat line. At this scale, CH13, for example, seems to have no drift.
But the drift, when present, appears quite linear. So there are two
things to do. Zoom in and zoom out.
2) attached plot: TBolt-10day-fit0-e10.gif (
http://leapsecond.com/pages/tbolt/TBolt-10day-fit0-e10.gif )
Here we zoom in by changing the Y-scale to 1e-10 per division. The
X-scale is still 10 days. Now we can see the drift much better. Also
at this level we can see instability of each OCXO (or the lab
environment). At this scale, channels CH10 and CH14 are "off the
chart". An OCXO like the one in CH01 climbs by 2e-10 over 10 days for
a drift rate of 2e-11/day. This is 25x better than the 10811A spec.
CH13, mentioned above, is not zero drift after all, but its drift rate
is even lower, close to 1e-11/day.
For some oscillators the wiggles in the data (frequency instability)
are large enough that the drift rate is not clearly measurable.
The 10-day plots suggests you would not want to try to measure drift
rate based on just one day of data.
The plots also suggest that drift rate is not a hard constant. Look at
any of the 20 10-day plots. Your eye will tell you that the daily
drift rate can change significantly from day to day to day.
The plots show that an OCXO doesn't necessarily follow strict rules.
In a sense they each have their own personality. So one needs to be
very careful about algorithms that assume any sort of constant or
consistent behavior.
3) attached plot: TBolt-100day-fit0-e08.gif (
http://leapsecond.com/pages/tbolt/TBolt-100day-fit0-e08.gif )
Here we look at 100 days of data instead of just 10 days. To fit, the
Y-scale is now 1e-8 per division. Once a month I created a temporary
thermal event in the lab (the little "speed bumps") which we will
ignore for now.
At this long-term scale, OCXO in CH09 has textbook logarithmic drift.
Also CH14 and CH16. In fact over 100 days most of them are logarithmic
but the coefficients vary considerably so it's hard to see this at a
common scale. Note also the logarithmic curve is vastly more apparent
in the first few days or weeks of operation, but I don't have that
data.
In general, any exponential or log or parabolic or circular curve
looks linear if you're looking close enough. A straight highway may
look linear but the equator is circular. So most OCXO drift (age) with
a logarithmic curve and this is visible over long enough measurements.
But for shorter time spans it will appear linear. Or, more likely,
internal and external stability issues will dominate and this spoils
any linear vs. log discussion.
So is it linear or log? The answer is it depends. Now I sound like Bob
;-)
/tvb
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Dr. Don Latham
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
