[time-nuts] Different Thunderbolt versions

[Ulrich Bangert]

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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