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.
They also state that the ionosphere contribution to ADEV is about 8E-12 @10s. Bruce Markus Kern wrote: > Thanks everyone for all the input. > > I actually want to work at frequencies below 50Mhz where a 50 degree > phase error corresponds to about 3ns. So for 100 seconds integration > time an ADEV of 1E-11 or better at tau = 1s should be fine. I want to > keep the system as simple and cheap as possible which is why I want to > give the Thunderbolt a try. > > Also at these low frequencies the required accuracy approaches that > claimed by some GPS receivers (4ns for the M12+ ?) so might it be > possible to build a system which is accurate for arbitrary integration > times? > > > Markus > > > On 22.02.2009, 02:20 Bruce Griffiths <[email protected]> wrote: > > >> Tom Van Baak wrote: >> >>>> For observations at 1420MHz, and short integration times (~ 100sec) the >>>> phase error needs to be 100ps or less. >>>> For longer integration times (10,000 sec is desirable) and higher >>>> frequencies the timing constraints are far more stringent. >>>> Whilst clock modeling can be used to improve performance it can only >>>> correct systematic effects. >>>> >>>> >>> Bruce, >>> >>> Approximately what range of tau and ADEV(tau) would this >>> VLBI requirement correspond to? A couple of the TBolts I've >>> tested are at or just under 1e-12 at 1 second and stay almost >>> flat out to 100 or even 1000 seconds. >>> >>> /tvb >>> >>> >>> >>> >> Tom >> > > >> Really should use something like TDEV but roughly (within a factor of 2 >> or so at 1420MHz): >> > > >> ADEV < 1E-10 or better @ tau = 1sec for 1 second integration time. >> (integration time to short for anything but very bright objects) >> ADEV < 1E-11 or better @ tau = 1sec for 10 second integration time. >> (integration time to short for anything but very bright objects) >> ADEV < 1E-12 or better @ tau = 1sec for 100 second integration time. >> (integration time somewhat marginal for most objects) >> ADEV < 1E-13 or better @ tau = 1sec for 1000 second integration time. >> ADEV < 1E-14 or better @ tau = 1sec for 10,000 second integration time. >> > > > > >> Also need to take into account that a similar LO would presumably be >> used at each antenna. >> > > >> For integration times of up to a few hundred seconds VLBI has been done >> with a hydrogen maser at one antenna and a PRS10A at the other. >> However the performance isnt Earth shattering and there are some issues >> with long term drift and transport induced frequency shifts of the PRS10A. >> > > >> For very short integration periods a good OCXO is adequate (at 1420MHz). >> When an integration time of 1000sec or more is required the LO >> requirents are more stringent. >> > > >> 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. > > _______________________________________________ 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.
