>> Here I put the two runs in color on the same plot: >> http://www.leapsecond.com/museum/z3801a/z3801a-free-lock.gif > > That's an interesting graph. > > I find it strange that the free-running system is better than the locked > system in the range from 50 to 5000 seconds.
Hi Hal, All GPSDO, by definition, will have a range of tau where the free-running LO is better than the locked system. That is the compromise you accept for a hybrid system. And even when perfectly tuned, any GPSDO is worse than sqrt(2) of GPS and LO performance at or near the cross-over point. If you use a GPSDO as your house reference a case could be made, depending on your needs, to let the GPSDO free-run during the day, and then catch-up overnight. You pay a medium performance penalty to track GPS short-term. Of course, it all depends on the quality of the LO; but it's not uncommon for a good GPSDO to use a loop filter on the order of many hundreds or a few thousand seconds. So that's why those free vs. locked plots look they way they do. Note also for long-term, high -accuracy UTC measurements (against Cs or maser) I don't use a GPSDO. Instead I use a plain, non-disciplined OEM GPS board or something like a CNS II (http://www.cnssys.com/cnsclock/CNSClockII.html). High-end, short-term, or low phase noise, you always use quartz; long-term you always used GPS. The cool thing about a GPSDO is that you get (almost) the best of both in one box. > Does that mean the filter needs a longer time constant? Or is there > something fundamental about systems like this that makes graphs with that > sort of pattern? Yes, in that example, with that particular S/N of Z3801A, in my particular hands-off, temperature controlled lab, and my well-placed GPS antenna, a longer time constant would have been more ideal. But that's a lot of if's. When HP ships a product they have to make sure it works to spec; into the wild cell-tower near Fairbanks or in my home lab. In addition, some surplus GPSDO randomly have better LO than others so the disparity between free-run and locked will appear to be greater. > How much did they understand about things like this graph when the Z3801A was > shipped? Based on conversations with some of the SmartClock engineers years ago, and from reading the published papers, I would judge it was all perfectly calculated, in the usual HP way. /tvb _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
