[Halld-tagger] first results on fiber light yields
Fernando J Barbosa
barbosa at jlab.org
Thu Feb 13 11:11:17 EST 2014
Hi Richard,
You may also want to check your software and get a raw data dump to confirm.
Best regards,
Fernando
On 2/13/2014 10:22 AM, Fernando J Barbosa wrote:
> Hi Richard,
>
> The fADC250 has a linearity of ~1% full scale and also a ch-to-ch gain
> variation of 1%. We measured these before and during production with
> pulses and levels. There is no processing at all in raw data mode.
> Your signals are so close in amplitude and so far away from saturation
> that I doubt there is any non-linearity effects, other the edge speed
> and sampling as I mentioned earlier. You can easily check this with a
> ramp signal or a precision variable attenuator.
>
> I would check with another scope channel and another ADC channel to be
> sure there are no inconsistencies.
>
> Best regards,
> Fernando
>
>
> On 2/13/2014 1:01 AM, Richard Jones wrote:
>> 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 <mailto: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 <mailto:somov at jlab.org>>
>> To: Richard Jones <richard.t.jones at uconn.edu
>> <mailto:richard.t.jones at uconn.edu>>
>> Cc: Hall D beam working group <halld-tagger at jlab.org
>> <mailto: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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