[Halld-tagger] active collimator signal vs translation stage displacement

[Richard Jones]

Hovanes,

  *

    How are the error bars calculated? Are these the spec uncertainties on
    the ADCs and amplifiers propagated into this signal ratio?


This is a profile histogram using hbook.  The error bars are simply the spread in the values observed within a bin divided by sqrt(n).  The step size in this scan was 0.002 inches, so there are many samples within a single bin.  These error bars show the uncertainty in the mean output at that displacement, calculated the usual way.  In case you would like to see it, I have created a new profile histogram of the same data showing each individual step, this time no error bar, just the data points.  See plot below.  That's a lot of points!

  *

    What is the origin of the vertical shift of the curve, and apparently
    a scale change as well? The limits are not -1 and +1, which means that the
    zero-crossing does not indicate the beam position.  I assume this can be calibrated out. But then if these signals are not well calibrated,  can the 60Hz, 120Hz etc oscillation components that you see in the FFT be interpreted as fast beam current fluctuations resulting in the asymmetry variations
    rather than fast position motion?


I have not subtracted any DC offsets that are present in these preamplifiers.  The zero points should be calibrated.  I wanted to show you very raw results, to show that the output is very simple to interpret and does not require much processing.  Of course, I could have centered it vertically.

  *

    Is this the finest step scan that was done?


The step size was 0.002 inches.  That was the smallest step size we took.  It is only 50 microns, which seems very small to me.  It is smaller than the size of the beam motion that we see, maybe the smallest step that the translation stage could do, not sure.

  *

    My understanding of the sensitivity of this device to the beam
    position was the uncertainty on the location of the "zero-crossing" of a linear fit in the central region (inversely related to the uncertainty of the gain in the central region).  Do you have an estimate for that number from a simple fit of this curve in the central region?


Yes, that is what I reported.  The slope is only part of the story.  It converts an uncertainty in the difference/sum (dimensionless) to an uncertainty in position (cm).  The rest of the story is the uncertainty on the difference/sum itself.  Extracting that from real data requires one to understand what part of the spread in the difference/sum comes from real beam motion and what part comes from detector noise.  That separation can only be done with a FFT and the knowledge that the detector plus preamp response curve is flat up to the bandwidth cutoff.  That was the point of showing the FFT plot.

-Richard Jones



On 10/31/2011 3:47 PM, Hovanes Egiyan wrote:
> Hi Richard,
>
> thanks for the plot. Indeed, this is the plot I asked about. I have a
> couple questions about it:
>
> o How are the error bars calculated? Are these the spec uncertainties on
> the ADCs and amplifiers propagated into this signal ratio?
>
> o What is the origin of the vertical shift of the curve, and apparently
> a scale
> change as well? The limits are not -1 and +1, which means that the
> zero-crossing does not
> indicate the beam position.  I assume this can be calibrated out. But then
> if these signals are not well calibrated,  can the 60Hz, 120Hz etc
> oscillation components that you see
> in the FFT be interpreted as fast beam current fluctuations resulting in
> the asymmetry variations
> rather than fast position motion?
>
> o Is this the finest step scan that was done?
>
> o My understanding of the sensitivity of this device to the beam
> position was the
> uncertainty on the location of the "zero-crossing" of a linear fit  in
> the central region
> (inversely related to the uncertainty of the gain in the central region).
> Do you have an estimate for that number from a simple fit of this curve
> in the
> central region?
>
> Hovanes.
>
>
>
> On 10/31/2011 02:58 PM, Richard Jones wrote:
>> Hovanes and all,
>>
>> I have attached a plot of what I think you wanted to see: the
>> difference over the sum of currents in the two inner wedges as a
>> function of displacement of the translation stage carrying the active
>> collimator across the beam.  Notice that in the central linear region
>> of high sensitivity, the error bars are much larger than outside this
>> region, where the sensitivity to position decreases.  The error bars
>> in the low slope regions on either side of the plot are a good
>> indication of the intrinsic resolution of the device.  The gain of the
>> central linear region is about 1.8/cm.
>>
>> In this plot you can see slow drifts in the beam position as bumps and
>> wiggles in the otherwise smooth transition from the region of -1 to
>> +1.  I estimate the intrinsic resolution at 15nA electron beam current
>> on a 10^-4 radiation length target to be less than 50 microns at 600
>> Hz bandwidth.
>>
>> -Richard Jones
>>
>>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://mailman.jlab.org/pipermail/halld-tagger/attachments/20111031/1998dc24/attachment-0001.html 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: diffoversumhires.gif
Type: image/gif
Size: 4687 bytes
Desc: not available
Url : https://mailman.jlab.org/pipermail/halld-tagger/attachments/20111031/1998dc24/attachment-0001.gif 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 2757 bytes
Desc: S/MIME Cryptographic Signature
Url : https://mailman.jlab.org/pipermail/halld-tagger/attachments/20111031/1998dc24/attachment-0001.bin