Hi In *general* the crystal in an OCXO should drift positive. The reason often mentioned is fairly simple:
You can only get the blank + base plate + calibration just so clean. You can go crazy getting the enclosure clean. The result is a long term mass transfer from the blank (it’s “dirty”, looses mass, frequency goes up) to the enclosure. Are there a whole lot of other things that may be happening? Of course. The telling point is still that negative aging is a bit unusual on a well made crystal. As Rick pointed out a few decades ago, OCXO performance may or may not be limited by the crystal …. Bob > On Nov 12, 2016, at 6:25 PM, Joseph Gray <[email protected]> wrote: > > TCXO, not OCXO, but related. Sorry, but I have no graphs. > > I work for a municipal radio shop. We service radios that span 20 > years (through acquisitions, it was GE, Ericsson, Com-Net, M/A-COM, > Tyco, now Harris). There are several different model handhelds and > mobiles, with different designs and TCXO's. Some are adjusted manualy, > most via software. I have found that every single TCXO in the various > model radios drift downward in frequency over time. > > One interesting case was a set of radios that sat on the shelf, unused > for several years. They were issued to some custodians about a year > ago. I checked all of them on the service monitor beforehand and they > were well within spec. All of these radios came back to the shop > recently. They were 1-3 KHz low in transmit frequency. That is an > unusual amount of drift in one year. Perhaps it has something to do > with how long they sat on the shelf. > > I don't have enough history on our newest radios, so I don't know if > this downward trend will hold true for them. > > Joe Gray > W5JG > > > On Sat, Nov 12, 2016 at 2:54 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. _______________________________________________ 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.
