[esa-t474] Updates

[Yury Kolomensky]

	Hi Mark and Alex,

I tend to agree with Alex's suggestions. In terms of whether we  
understand why 1 and 2 behave so poorly... There is a potential  
smoking gun in the plots vs ESA temperature. There is obviously some  
correlation, but what could be the cause ? Cables is one obvious  
possibility, although the effect seems to be huge for just the cable  
explanation. Let's see... Assuming the offset at the time was around  
250 um, we are looking at a 5% drift (12/250) per measly 0.3 degree  
C. If it's due to an I/Q phase shift, we are talking about 18 degrees  
at S-band, or a 5 mm length change. if the cable were a bare copper  
waveguide, it would need the length of about 1 km to expand that  
much. RF cables usually have better thermal coefficients for both  
phase and attenuation, so this is doubtful.

However, there is a caveat. First off, what ESA temperature are you  
plotting, Mark ? It should be rack temperature, which is more  
representative of the environment. BPM temperatures are presumably  
more stable because the BPMs are on a dedicated water chiller. But  
even the rack temperature would not be representative of the  
temperature *outside* (although it would track it), or of the  
temperature of the west wall, along which the cables run. That  
presumably drifts a bit more.

Anyway, I think we have the conclusion that over the short distance  
scales (triplet) stability is OK, and we are limited by BPMs 1 and 2  
to test the long lever arms. What I am wondering though is what you  
would get if you just tried to extrapolate from BPMs 9,10 (which are  
pretty good) to 3,4,5 ? Or the other way, 3,4,5 (which would give you  
a little better handle on angle) to 9 or 10 ? This would have a  
disadvantage that any differential drift within a triplet is  
amplified, but the advantage is that the environment is the same, so  
perhaps we can set a limit on long-term stability of such configuration.

Yury

On Nov 7, 2007, at 9:22 AM, Alexey Lyapin wrote:

>
> Hi Mark et al,
>
> I suggest - for consistency - we show 1 vs 2 (fig23_12) as we do 4  
> vs 3,5 and 10 vs 9, 11 to show the intra-station stability. Then,  
> we have the linked system stability plots, which don't look as good  
> as we hoped.  We suspect BPMs 1 and 2, so we show 1 vs 3, 5, 9, 10  
> (fig23_10) and 2 vs 3, 5, 9, 10 (fig23_8) and say that both behave  
> the same way and the drift is huge, so the screwup is there, but I  
> don't think we can really explain it, can we? We could say though  
> that we decided to move all the BPM 1,2 electronics to the racks in  
> the ESA to avoid additional systematics due to longer cables and  
> different environment, to use 14 bit digitisers to improve the  
> resolution of 1 and 2, and to build the calibration system and  
> monitor drifts online.
>
> Cheers,
> Alex
>
>
> Mark Slater wrote:
>> Dear All,
>> I have tried to address some of the points raised at the meeting  
>> on Monday. To that end, I've put some additional analysis points  
>> on my elog:
>> http://pcfj.hep.phy.cam.ac.uk:8080/slater/35
>> http://pcfj.hep.phy.cam.ac.uk:8080/slater/36
>> These cover the problems with BPMs 1 and 2 and the position drifts  
>> against time. Essentially, it looks like the scale changes in BPM  
>> 1 are also present in BPM 2 and these are not based on the cross- 
>> talk in this BPM. This therefore explains the relatively poor  
>> stability of the linked system. I believe this answers most of the  
>> problems that were voiced but do please let me know if this is not  
>> the case.
>> I will hopefully add another entry on SVD coefficents, etc. on  
>> Friday.
>> Many Thanks for all your patience!
>> Mark
>> P.S. The plots shown in the ELOG also demonstrate some additional  
>> runs for the long stability plots.
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>
>
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