See: http://www.leapsecond.com/pages/tbolt-temp/
Hi Mark, This is very interesting work that you're doing with Thunderbolt DS1620 temperature sensors. I hope you stick with it. I agree with Said about the double bind idea. I worry too that your TBolts are remembering something of the past in spite of the hardware changes you make for each new run. Do you do a full factory reset each time? And then let the unit "re-learn" for several hours, or maybe several days? Do we really know how or what it is learning and how that affects the response to temperature? Having tried tempco measurements in the past I have several concerns about mythology. It's really hard to get this right and even harder when you don't know what "smart" algorithms are inside the box you're trying to test. In this situation, it seems to me the main thing about temperature is not temperature at all, but the *rate* at which the temperature changes. In that case, even careful cycling of room temperature every day or cycling temperature inside a special chamber every hour will not give you the real story -- because in both cases the focus is on varying the temperature; not the rate at which the temperature is changing. Compounding the problem is that different components in the system will react to temperature at different rates. The DS1620 is plastic and may react quickly. The OCXO is metal and will react slowly. Who knows what additional component's tempco are relevant to the final 10 MHz output. Some may overreact at first and then settle down. I guess in the ideal world you'd want to do a "sweep" where you go through several cycles of temperature extreme at rates varying from, say, one cycle per minute all the way up one cycle per day. It seems to me you'd end up with some kind of spectrum, in which tempco is a function of temp-cycle-rate. Has anyone seen analysis like this? For example, I'd guess that most GPSDO have low sensitivity to wild temperature cycles every second -- because of its own thermal mass. And I bet most GPSDO have low sensitivity to wild temperature swings every few hours -- because the OCXO easily handles slow changes like this well. It's for time scales in between those two that you either hit sweet spots or get very confused and react just opposite of what you should. I'm thinking another testing approach is to varying the temperature somewhat randomly; with random temperature *amplitude* along with random temperature *rate*. Using this temperature input, and measured GPSDO phase or frequency output, you might be able to do some fancy math and come up with a transfer function that tells the whole story; correlation; gain and lag as a function of rate, or something like that. I'll do some reading on this, or perhaps someone on the list can fill in the details? I say all of this -- because of an accident in my lab today. Have a careful look at these preliminary plots and tell me what you think. It shows anything but a nice one-to-one positive linear relationship between ambient temperature and GPSDO output. http://www.leapsecond.com/pages/tbolt-temp/ /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.
