[time-nuts] Different Thunderbolt versions

[Bruce Griffiths]

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.

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.

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.

Bruce