[time-nuts] Different Thunderbolt versions
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Feb 25 20:29:22 UTC 2009
Ulrich
Another clue as to the likely disciplining loop time constant can be
found on page 6 of the FS725 manual:
http://www.thinksrs.com/downloads/PDFs/Manuals/FS725m.pdf
Where it states that the available range for the loop time constant is
480 sec to 64800 sec
Bruce
Ulrich Bangert wrote:
> Bruce,
>
>
>> 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.
>>
>
> It should be noted that this plot shows a VERY PESSIMISTIC (almost two
> decades) behaviour of the GPS 1 pps, perhaps a remainder from SA and/or a
> not so well timing receiver.
>
> A M12+ would have its sawtooth corrected 1 pps at 2-4E-12 @ 1000 s, giving a
> crosspoint to the AD of the rubidium alone anywhere between 1000s and 10000
> s.
>
> Best regards
> Ulrich
>
>
>
>> -----Ursprungliche Nachricht-----
>> Von: time-nuts-bounces at febo.com
>> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Bruce Griffiths
>> Gesendet: Dienstag, 24. Februar 2009 21:38
>> An: Markus Kern; Discussion of precise time and frequency measurement
>> Betreff: Re: [time-nuts] Different Thunderbolt versions
>>
>>
>>
>> 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%3Adocument
>>
>> s%3Alofar_documents&cache=cache&media=public:documents:19_detailed_des
>>
>>> cription_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
>>
>>
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