Yes, all were made with GPS antenna connected (and running from the same antenna via an 8-port splitter).
What I was mainly commenting on is the TBolt's pronounced "hump" that shows up in the measurements against the FTS-1050A but largely disappears in the pairwise measurement. The longer tau readings are interesting mainly to see how the GPS loop kicks in; tau below about 1000 seconds seem related to OCXO performance on the TBolts, while the Z3801A seems to have a longer time constant, and somewhat more gradual attack. (By the way -- the TBolts were running with the loop parameters they were set to when I got them). John ---- John Miles said the following on 08/09/2008 05:41 PM: > These tests were made with the GPS antenna connected? At t >> 100 seconds, > they should all look about the same, because that's where GPS disciplining > comes in, no? They should not be uncorrelated in the long run. > > To the extent one Z3801 looks worse than the other at large values of tau, > I'd expect there to be a good reason, like better GPS reception on one of > them, or a much-worse OCXO. > > Measuring phase noise by comparing the two against each other should be > fine, though, since their short-term drift isn't being corrected at that > timescale. Your noise floor is a couple dB worse than the one I tested, but > they're otherwise about the same. > > - john, KE5FX > > >> -----Original Message----- >> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] >> Behalf Of John Ackermann N8UR >> Sent: Saturday, August 09, 2008 1:28 PM >> To: Discussion of precise time and frequency measurement >> Subject: [time-nuts] TBolt and Z3801A performance data >> >> >> I just put the results of some tests of 2 "Time-Nuts Special" >> Thunderbolts, as well as 2 Z3801As, at >> http://www.febo.com/pages/gpsdo_comparison/ >> >> I learned an interesting (and important) lesson in doing these >> measurement. I initially measured the pairs of GPSDO against each other >> (e.g., one TBolt as "reference" and the other TBolt as "DUT"). In >> theory, if the two oscillators are identical, and if their noise is >> uncorrelated, the results of the pair can be used to deduce the results >> of the individual units. >> >> However, in this case doing so gave a very optimistic view -- the TBolts >> were better than the pair of Z3801As! Another set of measurements >> comparing the GPSDOs against an independent reference revealed that the >> first measurement was a lie. >> >> I guess you can think of it like this. Picture two OCXOs that both age >> at the same rate and in the same direction. Because they drift >> together, measuring their relative phase hides their actual drift and >> makes them look better than they are. On the other hand, if they were >> drifting in opposite directions, they would look worse than they are. >> An identical aging trend is a "correlation" even though it's external to >> the things we usually think of. >> >> John > > > _______________________________________________ > 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. > > _______________________________________________ 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.
