When the 3801A was designed, HP used a 300 or 400 second time constant for a SA filter. Its in one of the early app notes for HP GPSDOs.
Tom Van Baak wrote: >>> 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. > > _______________________________________________ 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.
