Those are wonderful plots :) I vaguely recall that a 1ppm frequency shift is approximately equivalent to the mass transfer of one molecular layer of a crystal. So at some point your counting atoms if there was no noise, thermal disturbance, mechanical disturbance...
On Sat, Nov 12, 2016 at 5:00 PM Tom Van Baak <[email protected]> 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. _______________________________________________ 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.
