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Re: TOF constant fraction



Hall D PID Mail List:

Hi Paul,
thank you. I had a look at the talk and in particular page 8. You used a 
function generator to make a rectangular logic pulse.
on the scope the rise time of that signal pulse which you label "signal" 
is about 10ns. However that rise time is probably an artefact
caused by the limited bandwidth of the scope. In reality the rise time 
is probably faster.
The plot on page 9 looks similar to what one would get when using just a 
regular discriminator. However it is not clear
from the plots to what value the threshold was set to.
I really recommend that you check what the delay settings of the CFD are 
and make sure it is set to a low value like 4ns to
accommodate for the fast scintillator signals.
If this CFD is really so bad then you have to resort to one of the 
following two options to improve on your
timing resolution.
1) record the ADC value of the signal as well, plot the TDC vs. ADC and 
fit this 2-dim spectrum
    to get a functional form. Use this functional form to correct the 
TDC value according to its ADC value.
2) the other possibility is to use two discriminator channels. One 
channel set to high threshold and one set
    to low threshold. Then build a coincidence between the two disc. 
signals where the signal from the low threshold
    is arriving later to define the time. use the output of this 
coincidence as input to the TDC.

cheers,
Beni

> Beni,
>
> See gluex doc GlueX Experiment Document 1211-v1.    This is the talk 
> at the Jan. collaboration meeting.  The time walk was measured from 
> -30mV to -1V.  The spread was 5.1 TDC counts which equals 286 ps.
>
>
> Paul
>
>
> http://argus.phys.uregina.ca/cgi-bin/private/DocDB/ShowDocument?docid=1211 
>
>
>
>
> On Jul 16, 2009, at 7:59 AM, Beni Zihlmann wrote:
>
>> Hall D PID Mail List:
>>
>> Hi Sasha,
>> thank you very much. While reading the manual I am not sure I like 
>> this CFD ;-)
>> First can you check what the delay settings are? According to the 
>> manual the
>> jumpers are set to 20ns by the factory. This would be far too much 
>> for fast scintillator
>> signals. It should be more like 4ns. This is the lowest possible 
>> setting for this CFD
>> unless your module is custom modified. (maybe 4ns is still too much I 
>> do not know
>> what the exact rise time of your signals are.)
>> Also at some point it might be useful to check how good this CAEN CFD 
>> really is.
>> The manual has this sentence:
>> "The maximum time walk is ±400 ps (for input signals in the range 
>> from -50 mV to -
>> 5 V with 25 ns rise time). "
>> The number 400 really scares me. If this is really true this CFD is 
>> useless.
>> To check this you could use an ORTEC CFD model 935 or the older 
>> module at lower
>> bandwidth. They are very good CFD, they are NIM modules and have the 
>> additional
>> feature to adjust the zero cross over. With such a CFD and good 
>> adjusted zero cross over
>> the time walk is very very small, like for example 10ps or so (that 
>> is a guesstimate).
>>
>> cheers,
>> Beni
>>
>>
>>> Hall D PID Mail List:
>>>
>>> On Wednesday 15 July 2009, Beni Zihlmann wrote:
>>>
>>>> Hall D PID Mail List:
>>>>
>>>> Hi Sasha,
>>>> I could not find the document where the constant fraction is
>>>> described. Can you again tell me what type of constant fraction you
>>>> are using?
>>>>
>>>> thanks ,
>>>> Beni
>>>>
>>>
>>> Beni,
>>>
>>> We are using CAEN V812 CFD.   Further info is here:
>>> http://www.caen.it/nuclear/product.php?mod=V812
>>>
>>> Sasha
>>>
>>
>>
>
> -- 
> Prof. Paul Eugenio
> Florida State University
> Department of Physics
> Tallahassee, Florida,  USA 32306
>
> (850) 644-2585
> eugenio@fsu.edu
>