[Halld-tagger] first results on fiber light yields

Thu Feb 13 23:36:49 EST 2014

Hi Richard,

I took another look at your scope pictures. The scale is 10ns/div or 2ns 
per sub-division. Take 2 of these small divisions (4ns) and slide them 
by 2ns: I easily see a difference of 30% in the peak value that the ADC 
would register.

You can also change the scope settings so that the sampling on the scope 
is 250MSPS, adjust the trigger level and set to envelope. You should see 
the 30% effect.

Best regards,
Fernando

On 2/13/2014 11:11 AM, Fernando J Barbosa wrote:
> 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
>>>     >
>>>     _______________________________________________
>>>     Halld-tagger mailing list
>>>     Halld-tagger at jlab.org <mailto:Halld-tagger at jlab.org>
>>>     https://mailman.jlab.org/mailman/listinfo/halld-tagger
>>>
>>>
>>
>

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