[esa-t474] All corrections to T474 paper complete

[Yury Kolomensky]

	Hi Mark,

a few comments on the answers to the referee:


On Mar 9, 2008, at 4:50 AM, Mark Slater wrote:
> ********************************
> 2) In several places (line 178 on p. 12, and later, in lines 671 to  
> 674 on
> p. 42, line 587, p. 35), the paper talks
> about unwanted coupling with other modes
> in the cavity. BPM 1 is said to have rectangular cavities, and I am
> guessing that the mechanical damage brought the
> detuned orthogonal mode closer to the
> filter acceptance window.  Is the artifact in the BPM 4x signal due  
> to the
> same issue? In that case, analysis of the
> residual vs. motion in the y-direction
> would give a much clearer indication of the source of the cross- 
> coupling
> (if data is available).
>
> MWS:  The artifact seen in 4x (assuming you mean fig. 22a) is just an
> example seen in other BPMs - notably y and 9-11. If it was leaking  
> from
> the y direction, I would expect it to show an overall worsening of
> resolution, not a correlation (unless the y movement of the beam was
> correlated with the x movement, which it didn't seem to be).
>

I think what the reviewer means is that if you plot the X residual vs  
y position, and there is a coupling of Y signal into X, you will see  
a correlation. This may require a fairly large range of Y motion  
though, something we only see during calibration runs. Those may set  
the limit on the amount of cross-coupling. For BPMs 9-11, this  
coupling is about -30-40 dB, and it's mainly the coupling between  
adjacent X and Y cavities, not X and Y dipole modes within a single  
cavity (the rule of thumb is that the adjacent cavities couple at  
about 30 dB per diameter of Z separation; the diameter is 0.8", and  
the cavities are about 1" apart). For the A-line style cavities (BPMs  
1-2), the coupling is smaller (i.e. more dBs :) for the same reason.  
I do not recall what the coupling is for BPMs 3-5, but these are the  
only cavities where the coupling is between modes of the same cavity  
(by construction). I am sure Chris et al have measured it. Probably  
in 30-40 dB range as well.

So the question is reasonable, but if we look at X residual vs Y  
position (e.g. for a mover or corrector calibration), I do not know  
if we'd be able to discriminate between the EM coupling and the  
geometric misalignment (e.g. the mover rotated relative to the BPM  
axes). The only way to measure the coupling, I think, is to do an FFT  
of the waveforms at different offsets in Y, and look for any evidence  
of the second peak. Have you or Bino tried to do that ?

With regard to the paragraph in on p.42 (lines 671-674 in the old  
draft), are you talking about correlation of X residual to X or Y ?  
The former can be caused by small non-linearities in the BPM response.

>
> 11) In general, the paper would benefit from a statement of the BPM
> sensitivity (signal voltage for a given bunch
> charge and displacement),  typical signal
> levels seen at various points in the processing electronics, a more
> detailed description of the electronics
> components used (mixers, amplifier), and a
> discussion of the likely sources of electronic noise.
>
> MWS:  Added more specifications on the equipment,

There is one correction to the specs. The IF amps are Minicurcuits  
ZHL-2010: 20 dB gain, and very linear:

http://www.minicircuits.com/pdfs/ZHL-2010.pdf

We also use BIF-70 IF filters:

http://www.minicircuits.com/pdfs/BIF-70+.pdf


> added the BPM
> sensitivity and improved the discussion on dominant noise sources.
> Unfortunately, we don't have the data available for the signal levels
> through the electronics chain, but a quick analysis showed that the
> thermal noise level was significantly below the digitiser noise  
> level and
> so this was assumed to be domoinant.
>

We actually did some of these measurements, although I can't find the  
numbers for every channel (did a quick search through the logbook).  
See for example

https://www.hep.ucl.ac.uk/elog//T-474/13
https://www.hep.ucl.ac.uk/elog//Hardware+and+Electronics/13

and the calculation of the kick factors:

https://www.hep.ucl.ac.uk/elog//T474/9

Not sure if it's worth adding to the text though, but it does not  
sound good when you say we did not measure the signal levels ! :)

Yury