In the same vein if it takes 1000 seconds to measure the relative phase of a pair of clocks to within 500ps then the relative ADEV of the clock pair at 1000 sec needs to be somewhat less than 5E-13. For 100 s averaging the relative ADEV of a clock pair needs to be better than 5E-12 @ 100sec. For 10s averaging the relative ADEV of the clock pair needs to be better than 5E-11 @ 10s. Thus if the measurement takes too long the cost of the local clocks becomes unaffordable. Comparison techniques that don't require more than 10-100 sec of averaging are preferable to keep the cost of the local clocks sufficiently low.
Bruce On Wednesday, May 04, 2016 03:03:59 PM Tom Van Baak wrote: > Hal, > > > How close could you get if you brought two of them together, compared > > phase, drove them to the site for a nights work, drove them back to the > > same location and compared the phase again. > > That's essentially asking what the ADEV (or, TDEV) is for tau 1 day. Rb > isn't near good enough. Neither is Cs, for that matter. > > See www.leapsecond.com/tmp/5071a-12-run8-5d-10d.gif for a plot of a bunch of > 5071A Cs clocks. They are compared together for 5 days to determine their > relative phase and frequency offsets and then go on a 5-day trip. You can > see how the phase drifts as random walk does its thing. It's way more than > 500 ps per day. > > That's why the OP cannot use free-running clocks. He needs some method to > actively keep them in tight phase lock or passively compare them to within > 500 ps in order to adjust the timestamps in post-facto. > > /tvb > > ----- Original Message ----- > From: "Hal Murray" <hmur...@megapathdsl.net> > To: "Tom Van Baak" <t...@leapsecond.com>; "Discussion of precise time and > frequency measurement" <time-nuts@febo.com> Cc: <hmur...@megapathdsl.net> > Sent: Wednesday, May 04, 2016 10:30 AM > Subject: Re: [time-nuts] Fw: Optical transfer of time and frequency > > > t...@leapsecond.com said: > >> Any of these methods is going to be a challenge, given their 500 ps > >> requirement and their $2k budget. > > > > How stable are surplus rubidium oscillators? > > > > How close could you get if you brought two of them together, compared > > phase, drove them to the site for a nights work, drove them back to the > > same location and compared the phase again. > > _______________________________________________ > 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.