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.
