[time-nuts] Different Thunderbolt versions

Markus Kern mkern at fastmail.fm
Sun Feb 22 16:32:05 UTC 2009


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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