[time-nuts] quartz drift rates, linear or log
djl
djl at montana.com
Sat Nov 12 17:16:32 EST 2016
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
>
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--
Dr. Don Latham
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304
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