[Halld-tagger] first results on fiber light yields

[Richard Jones]

Fernando,

We cannot use the scope for these tests because we can only see a few
channels at a time.  We would very much like to understand the DAQ so we
can move on and start fiber QA studies.  The decay time of these signals is
so slow that the rise time will not affect the max pulse height by more
than about 10%.  Running the signals shown on the scope through a 125MHz
low-pass filter reduces the pulse height somewhat, but does not affect the
ratios.  There are variations based on the exact delay through each channel
and the FADC clock, but these effects are +/- 10% because of the slow decay
time.  As you state, we are comparing ratios of 40% on the scope and 15% on
the ADC.

We do not show the pictures on the wiki, but if we swap the inputs on the
FADC, the results do not change by more than a couple percent.  This shows
that it cannot be gain variations between the different channels on the
FADC, but must be some kind of signal processing that is doing very
different things to the peak heights depending on what preamp output
generated them.  Is there possibly some nonlinear processing of the raw
FADC values that is happening in the module before we see them?  As Alex
points out, the pulse shapes of channels 1 and 3 are virtually identical,
but the ratio of their amplitudes disagrees: 6% on the FADC vs 20% on the
scope.

-Richard J.


On Thu, Feb 13, 2014 at 12:16 AM, Fernando J. Barbosa <barbosa at jlab.org>wrote:

> Hi Richard,
>
> From the pictures, I get for each set the following peak amplitudes:
> 1) 64mV, 278 channels=4.3 ch/mV
> 2) 46mV, 248 ch=5.4 ch/mV
> 3) 76 mV, 295 ch=3.9 ch/mV
>
> Clearly not linear where a 40% change on the scope corresponds to 16% on
> the ADC. This is because we are looking at peak values and the pulse rise
> time is less than 4ns (ADC sampling). If you are triggering the ADC from
> the laser pulser, try adding cables in 1ns increments to see how the ADC
> amplitude changes. The pulse risetime is just too fast for the ADC to
> sample properly (must be > 4ns). Because the pulse duration is much longer
> than the pulse risetime, charge should be OK as it is to be used in the
> experiment. The scope is the better tool for your tests of the fibers
> unless you put a low pass filter to increase the pulse risetime.
>
> Best regards,
> Fernando
>
>
>
>
> ----- Original Message -----
> From: Alexander Somov <somov at jlab.org>
> To: Richard Jones <richard.t.jones at uconn.edu>
> Cc: Hall D beam working group <halld-tagger at jlab.org>
> Sent: Wed, 12 Feb 2014 22:02:13 -0500 (EST)
> Subject: Re: [Halld-tagger] first results on fiber light yields
>
>
> Hello Richard,
>
> Actually fadc spectra for 1) and 3) agree reasonably well
>
> Why 2) is about 20% wider (was it a different readout channel)?
>
> (there could be some sampling effects due to the different pulse
> shape around the peak. I would also compare amps without the
> splitter).
>
> Cheers,
>  Alex
>
>
>
>
>
> On Wed, 12 Feb 2014, Richard Jones wrote:
>
> > Dear colleagues,
> >
> > We are working to obtain light yields for the first article fiber bundle
> > that was produced for the tagger microscope some time back. We have run
> > into a problem trying to interpret what we see coming from the FADC250
> > module that is installed in the data acquisition crate we are using for
> > fiber QA tests. Please see the plots on the wiki page linked below that
> > illustrate the problem we are seeing.
> >
> >
> https://halldweb1.jlab.org/wiki/index.php/Tagger_Microscope#Light_yield_tests
> >
> > When we look at the signals on the scope, we see one set of amplitude
> > ratios that clearly reveal the differences in light yield resulting from
> > different lengths of the fibers that are covered in reflective paint. The
> > results from the FADC are much less distinguished, and disagree in the
> > ratios of pulse heights. Comments or suggestions as to what we are doing
> > wrong would be appreciated.
> >
> > -Richard Jones
> >
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>
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