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%3Adocuments%3Alofar_documents&cache=cache&media=public:documents:19_detailed_description_of_clock_sync.pdf _______________________________________________ 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.
