Hi At the most basic level, ADEV is a standard deviation. When people read an instrument they usually want a "max error" sort of number. ADEV operates on delta between readings so a simple "three sigma" mental math conversion is not all you need. It *is* the right measure. It can take a bit of work to convert it to useful units .
Bob > On Aug 20, 2016, at 7:38 AM, Charles Steinmetz <csteinm...@yandex.com> wrote: > > Bryan wrote: > >> With a GPSDO as a general rule is there much variation of the 0.005 to 0.02 >> ppb range. Would there me much if only 2 satellites were visible but the >> unit was no in holdover vs say 6 satellites. I assume there is probably a >> lot of variables such as quality of GPSDO and OCXO, multi path interference, >> antenna location, etc, but worse case what would a user expect assuming the >> unit isn't in holdover. > > There are lots of ADEV plots of GPSDOs on the web, if you search for them. > Tom has the attached plot posted on his web site -- it includes one decent > (but not stellar) GPSDO, the HP Z3801. That is about what you can expect > without heroic tuning efforts (on a GPSDO that supports user fiddling, such > as the Trimble Thunderbolt). > > Since many counters gate for one second (or gate faster, then average), the > ADEV at tau=1 second is a reasonable estimate of the real-world performance > of a good counter driven by a GPSDO. Note that, since the ADEV at 1 second > depends largely on the OCXO, the heroic tuning efforts mentioned above won't > change the ADEV at tau=1 second hardly at all. That is attributable strictly > to the OCXO (unless the GPSDO is very poorly designed). Building a GPSDO > using the best OCXO available can improve things. For example, look at the > "BVA" plot -- at 1 second, its ADEV is nearly a full decade better than the > Z3801, better even than the hydrogen masers. (While I referred to the 3801 > above as "decent but not stellar," that is based on its performance at tau > from 100-100k seconds. Its OCXO -- and, thus, its ADEV -- is better than > most others at 1 second.) > > Best regards, > > Charles > > > <ADEV_plots_of_various_oscillators_TVB.gif> > _______________________________________________ > 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.
