[time-nuts] Different Thunderbolt versions
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Feb 22 20:12:14 UTC 2009
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 <bruce.griffiths at xtra.co.nz> 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
>>
>
>
>
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