[Esa-t474] Today's T474 phone conference minutes
Bino Maiheu
bino at hep.ucl.ac.uk
Thu Jun 15 19:03:52 BST 2006
Hi All,
You can find the minutes for today's phone conference in attachment. These
were also posted to the web :
http://www.hep.ucl.ac.uk/~bino/T474/20060615.html
Cheers,
bino
--
!!!!!
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Minutes of the 15 June 2006 T474 Phone conference
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- Bino reported on the progress that was made in the BPM analysis and
the DDC optimisation. The key was finding that the algorithm in the
lastest version of the code had a bug. The IQ plots had a lot of
phase jitter. The reason for this was incorrect treatment of the
filtering algoritm in timedomain, where the filter reference time
(tfilter) was put to the starttime (startFit, picked up from
calibration file) and the end time, tstop, was put to that value
+dt, where dt is calculated from the filter bandwidth. This
technically speaking reduces the bandwidth of the filter as well by
a factor of 2.
This issue is extensively supported by plots in the electronic
logbooks at :
https://www.hep.ucl.ac.uk/elog/Bino's+logBook/5
https://www.hep.ucl.ac.uk/elog/Bino's+logBook/4
https://www.hep.ucl.ac.uk/elog/T-474/16
A new version of the code (NanoModules version yury-13jun06a) was
sent around by Yury, with the following changes :
* the value t0Ref picked up from the calibration files now refers
to the filter time (tfilter)
* tstart = tfilter - dt, tstop = tfilter + dt
This code produced good IQ plots again so optimisation could
continue.
- For external clock data, the optimal bandwidth and t0 Bino and Yury
agree upon to be
t0 = 68.0
filtBW = 0.20
For t0Ref Yury finds an optimal value of 86, which produces after
some fudging resolutions on BPMs 3-5 of 500nm in y and about 800 nm
in x, Bino finds 89 as optimal value with slightly worse resolutions
in y at 800 nm and x ~ 1 um.
Having the tfilter value a little further (89) filters the waveform
only at it's end, thus conveniently bypassing any saturation effects
but having a slightly worse resolution. Having tfilter at 86 takes
more of the beginning of the waveform into account, thus having
better resolution, but more sensitive to saturation effects. So this
is also the reason why Yury had to apply some 'fudging' to account
for saturation effects that might slip into the calibration.
So basically this boils down to the fact that we need to find a way
to treat saturation better in the DDC algorithm.
- The idea there is to have a look to the downconverted waveform,
try to fit the gamma's and use those then. However problem here is
that for some strange reason the phase has a slope across the
pulse. This slope can be changed by tweaking the frequency assuming
that this dependence is linear with time. This is however not the
case, so the phase slope changes with amplitude (eg. in calibration
runs)
The reason for this is either maybe influence from a monopole
component or some other pathology (possibly hardware related). Note
that there appear also 2 peaks for e.g. 417-419 in the FFT spectra,
which give possible indications for hardware problems...
So the play here is to use a macro called "plotDDC.C" in NanoMacros
(version yury-13jun06a) and tweak the frequencies manually in order
to make the phase plots flat across the pulse, next step is to fit
the gamma's in the downconverted waveforms and use those in the DDC
algoritm.
---> In future we will tabulate this variation of the phase across
the pulse and correct for it by means of a lookup table....
- Yury brought up the issue of runs 960, which have external clocking
and where the terminators were removed. This effectively changes the
frequency and the width of the cavity, so one should take care of
that when optimizing the data. This will be taken care of.
- Chit produced a new set of waveform info files for the entire
runrange having the correct settings depending on internal or
external clocking. Yury will send these around as a tarball.
Bino produced new values for the fixed t0 for external clock, he
will send these around as well.
- Yury will commit a new version of the t0 finding routine for
internal clocking, which essentially removes the code which 'goes
back a couple of samples' to do the linear fit of the rising edge of
the trigger pulse. This should hopefully improve t0 determination
for internal clock runs and resolution.
- Bino brought up an issue about calibrations where the IQ phase is
around pi/2. Yury suspects this is probably due to saturation and
not so much the actuall IQphase value. A quick check by slightly
changing t0 (which changes the IQ phase) should bring clarity on
this issue.
- Bino was wondering why the phase difference between q9Phase and q10Phase
is more or less Pi. This is purely by coincidence Yury informed
:). It brought up however the issue of looking to the Phase
resolution. Bino will put up a plot in the eLog so we have an idea
about it.
- The conversion of the calibration files to have the real frequencies
and widths of the cavities (so to make them uniform between external
and internal clocking) has been postponed as everyone seems to be
happy with the current version. Might implement it once the entire
optimisatio/understanding of the data is finished.
- Plans for future :
o split calibration / reprocessing according to internal & external
clocking...
o Implement a correction table for the phase change with time in
the DDC algoritm...
o Check whether the ADC values for the corrector magnets are in the
root files and write some code to automatically have this do the
calibrations, possibly having calibrations 'on the fly'...
- A mailing list was setup called 'esa-t474 at hep.ucl.ac.uk' for mass
mail concerning T474 analysis. To subscribe, people should send an
email to "esa-t474-request at hep.ucl.ac.uk" with an empty subject and
in the message body "subscribe address=<email address>"
- Time for phone conference next week will be decided via email
exchange as Yury is away.
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