Re: TOF constant fraction

[Beni Zihlmann <zihlmann@jlab.org>]

Hall D PID Mail List:

Hi Paul
sorry me again. I made a mistake the bandwidth of the scope is good 
because the signal output of the CFD
labeled TDC-stop in the slide is very sharp. but the signal labeled 
TDC-start which I gather from the slide is the trigger
output of the function generator has a very limited band width. that 
signal should be like the TDC-stop signal.
If this function generator has an external trigger option I would 
recommend to use this option and generate
a clean trigger signal externally that can then also be used for the 
"TDC-start".

cheers,
Beni

> 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
>>
>