Ulrich Another clue as to the likely disciplining loop time constant can be found on page 6 of the FS725 manual: http://www.thinksrs.com/downloads/PDFs/Manuals/FS725m.pdf Where it states that the available range for the loop time constant is 480 sec to 64800 sec
Bruce Ulrich Bangert wrote: > Bruce, > > >> If you look at the Allan deviation plot on the PRS10 page: >> http://www.thinksrs.com/products/PRS10.htm >> >> This indicates that the likely disciplining loop time >> constant will be several thousand seconds. >> > > It should be noted that this plot shows a VERY PESSIMISTIC (almost two > decades) behaviour of the GPS 1 pps, perhaps a remainder from SA and/or a > not so well timing receiver. > > A M12+ would have its sawtooth corrected 1 pps at 2-4E-12 @ 1000 s, giving a > crosspoint to the AD of the rubidium alone anywhere between 1000s and 10000 > s. > > Best regards > Ulrich > > > >> -----Ursprungliche Nachricht----- >> Von: [email protected] >> [mailto:[email protected]] Im Auftrag von Bruce Griffiths >> Gesendet: Dienstag, 24. Februar 2009 21:38 >> An: Markus Kern; Discussion of precise time and frequency measurement >> Betreff: Re: [time-nuts] Different Thunderbolt versions >> >> >> >> Markus Kern wrote: >> >>> On 22.02.2009, 21:12 Bruce Griffiths <[email protected]> >>> wrote: >>> >>> >>> >>>> Markus >>>> >>>> >>> >>> >>>> Even with sawtooth correction the performance of the M12+T >>>> >> was found >> >>>> inadequate for the LOFAR >>>> >> <http://www.lofar.org/p/systems.htm> array. >> >>>> They use SRS FS725 rubidium sources disciplined by M12+T >>>> >> GPS timing >> >>>> receivers. >>>> >>>> >>> I didn't mean using the M12 by itself, obviously a clock >>> >> stable enough >> >>> over the time the M12 pps must be integrated has to be used. >>> >>> If we are using the ADEV limits you proposed then at 50 MHz (= 3ns >>> acceptable error) the timing requirement is an ADEV of >>> >> 3*1E-(8+x) at >> >>> tau = x seconds. From the measurements at >>> http://www.leapsecond.com/pages/gpsdo/ it seems the >>> >> Thunderbolt gets >> >>> pretty close to that. >>> >>> The LOFAR clock system is described at [1]. In section 3.1.3.3 they >>> say: >>> >>> "Some Crystal Oscillators have the advantage that they have >>> >> a better >> >>> Allan variance for periods of up to 10s and therefore it can be >>> claimed that they have a better performance than the SRS-FS725 >>> Rb-reference standard. The performance for time periods >>> >> above 10s, the >> >>> SRS-FS725 performs better. Therefore choosing an OCXO would >>> >> require a >> >>> maximum calibration interval of 10s and it would require a >>> significantly better GPS (or GALILEO) receiver because de >>> >> Rb-reference >> >>> is used to average the PPS signal from the GPS receiver >>> >> thereby making >> >>> it possible to identify the time difference between stations at >>> receive frequencies above 10MHz." >>> >>> I think this means that they are using pps integration >>> >> times above 10 >> >>> seconds. I couldn't find any reference to the actual value though. >>> >>> LOFAR is also working at frequencies up to 240MHz so the timing >>> requirements are definitely higher. They say that a station time >>> offset of 200ps does not affect performance as long as it remains >>> stable over time. >>> >>> >>> >>>> They also state that the ionosphere contribution to ADEV is about >>>> 8E-12 @10s. >>>> >>>> >>> Yes, from which they infer that "the reference clock shall have an >>> Allan variance of 1e-11 or less over 10s." I am not sure if >>> >> this has >> >>> to do with the propagation of the GPS signal or if they >>> >> mean that they >> >>> need a clock stable enough to later compensate for the different >>> delays of the observed signal through the ionosphere. >>> >>> Markus >>> >>> [1] >>> >>> >> http://www.lofar.org/operations/lib/exe/fetch.php?id=public%3Adocument >> >> s%3Alofar_documents&cache=cache&media=public:documents:19_detailed_des >> >>> cription_of_clock_sync.pdf >>> >>> >>> >>> >> Markus >> >> The ionosphere contribution to the Allan deviation at GPS >> frequencies is much smaller (by a factor of 10-100 or so) >> than that, as is evident from carrier phase measurements. At >> 50MHz the ionospheric phase shift, dispersion and instability >> will be much greater than at GPS frequencies. They are merely >> ensuring that the LO contribution to Allan deviation is much >> smaller than that of the ionosphere. >> >> If you look at the Allan deviation plot on the PRS10 page: >> http://www.thinksrs.com/products/PRS10.htm >> >> This indicates that the likely disciplining loop time >> constant will be several thousand seconds. >> >> Close isn't good enough: the phase differences between pairs >> of stations is significant, the Allan deviation needs to be >> at least 30% lower per station. If the errors at station >> pairs have significant correlation the requirement can be >> relaxed somewhat. >> >> Bruce >> >> _______________________________________________ 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.
