Here are ADEV plots and interesting results from a recent experiment on varying the time-constant of a GPSDO:
http://www.leapsecond.com/pages/tbolt-tc/ There was a thread recently where Warren suggested that the loop time constant (TC) of GPSDO was less than ideal. He is correct. There are a couple of reasons for this, if I may guess. 1) Some GPSDO, like the surplus SmartClock designs from HP, were designed to meet spec even when S/A was still in effect. With the much greater wander in civilian GPS timing during those years, the TC needed to be less than what you can get away with today. 2) If you are a manufacturer and have a GPSDO spec to meet, you need to make sure the TC is valid for all OCXO that you ship, not just the average one. The way to do this is to be conservative and to ship the units with a TC that is short enough that even the parts on the lower end of the bell curve still meet your spec. The other alternative is to individually measure (days, weeks?) every OCXO and individually burn a default TC into each unit shipped. 3) Most commercial GPSDO need to work over a fairly wide temperature range. This might require a tighter TC. If the user has a more controlled environment they can probably tolerate a longer TC that what the manufacturer dare ship as a default. 4) A GPSDO should still work reliably in the face of phase or frequency jumps in the OCXO. Although the timing of these is not predictable, their typical magnitude is probably something that the designer can learn. The TC needs to be short enough so that the GPSDO gracefully handles these jumps. If one is too aggressive the GPSDO will wander out of spec instead of more closely tracking GPS. 5) I'm not sure it's possible to optimize for both time stability and frequency stability at the same time. A long TC will help avoid too sudden frequency changes in the GPSDO; a short TC will help the 1pps stay close to UTC. I suppose a GPSDO might be optimized more for one application than the other; this would affect the choice of default TC. 6) Most OCXO demonstrate much better drift rates after they have been in operation for weeks or months. The drift rate has a some impact on the choice of TC. It's probably not a good business model to ship a GPSDO with a TC optimized for how the unit might eventually run a year from now. It has to work out of the box. So this cause the default TC to be set shorter than ideal. 7) There may also be a SV, sky-view, or latitude dependence. Someone enjoying all 32 SV today, with a clear 360 degree view of the sky at mid-latitude will probably enjoy slightly better performance than someone a few years ago when there were less operational SV in orbit, or with mountain, forest, building obstructions, or at extreme latitudes. If you have much better than average reception you could probably move the TC out further. So for these reasons (more like guesses), it would not surprise me if most GPSDO have the TC set on the low side. Let me know if you have additional info on this topic. The good news is that if you, the time-nut, have the gear and the time to measure the stability of the OCXO in your GPSDO, and know your environment well, then you can probably safely lengthen the TC and achieve much better mid-term stability out of your GPSDO as shown in the plot above. /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.
