[time-nuts] Different Thunderbolt versions

[Bruce Griffiths]

Markus

Article discussing modelling effects of the ionosphere at low frequencies:
http://ens.ewi.tudelft.nl/pubs/tol07isscs.pdf

I presume you intend to make the VLBI observations at night when the
ionosphere has the least effect on the propagation delay at 50MHz.

Bruce


Markus Kern wrote:
> On 24.02.2009, 21:37 Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>
>   
>> Markus Kern wrote:
>>     
>>> On 22.02.2009, 21:12 Bruce Griffiths
>>> <bruce.griffiths at xtra.co.nz> 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
>>>
>>>
>>>   
>>>       
>
>   
>> Markus
>>     
>
>   
>> The ionosphere contribution to the Allan deviation at GPS frequencies is
>> much smaller (by a factor of 10-100 or so) than that, as is evident from
>> carrier phase measurements.
>> At 50MHz the ionospheric phase shift, dispersion and instability will be
>> much greater than at GPS frequencies.
>> They are merely ensuring that the LO contribution to Allan deviation is
>> much smaller than that of the ionosphere.
>>     
>
> Yes, that's what I thought.
>
>   
>> If you look at the Allan deviation plot on the PRS10 page:
>> http://www.thinksrs.com/products/PRS10.htm
>>     
>
>   
>> This indicates that the likely disciplining loop time constant will be
>> several thousand seconds.
>>     
>
> Ok, so it may indeed be necessary to use a rubidium oscillator which
> has the required stability over that time frame.
>
> I suppose a GPS disciplined Rb-clock will be much more expensive than
> a Thunderbolt. However there are relatively cheap rubidium oscillators
> like the LPRO 101 out there. Are they suitable and has anyone tried to
> slave them to GPS? From reading the mailing list archives it seems
> Brooks Shera's circuit won't be suitable for this.
>
>   
>> Close isn't good enough: the phase differences between pairs of stations
>> is significant, the Allan deviation needs to be at least 30% lower per
>> station.
>> If the errors at  station pairs have significant correlation the
>> requirement can be relaxed somewhat.
>>     
>
> I realize that the phase difference is important and that things won't
> work if it's not low enough :)
>
>   
>> Bruce
>>     
>
>
>
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