Re: CDC and FDC characterization

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Yves,


Yves Van Haarlem wrote:
> Hi Fernando,
>
> On Wed, 15 Oct 2008, Fernando J. Barbosa wrote:
>
>> Hi Yves,
>>
>>
>> Yves Van Haarlem wrote:
>>> Hi All,
>>>
>>>> Gerard Visser wrote:
>>>>> Hi Fernando,
>>>>>     Thanks, this is a very interesting document! In particular I 
>>>>> hope that people are encouraged more to do the gain studies in 
>>>>> terms of pulse integrals rather than peak height... :)
>>>> Indeed.  I am not sure if Simon and Yves have done this 
>>>> systematically but they could re-check their analysis.
>>> For the CDC the gas gain is determined by tuning GARFIELD to Fe-55 
>>> data with the gas gain as only free parameter. For the shaper gain 
>>> both CDC/FDC use the signal amplitude to measure the shaper gain. 
>>> This gain is signal shape dependent.
>> Ok, but when you analyze the data from the fADCs (amplitude 
>> distributions), you need to sum all the samples within a time window 
>> (drift time?) for each trigger, instead of just peak values.
>
> In the GlueX-doc-1079 you'll see we always do both, integrating and 
> taking the peak value. We notice that there is just a conversion 
> factor between them (around 4) for signals with the same shape (this 
> is not a suprise I think). For dE/dx measurements we probably want to 
> integrate the signal as signal shapes may vary due to different track 
> lengths. The integration does not depend on signal shape that much 
> while the amplitude might. To see how the amplitude scales with 
> tracklength we tilted the chamber. What we saw was a x2 amplitude for 
> x2 tracklength. We emphasize more on amplitude because of the dynamic 
> range of the fADC (which might overflow). At least this is how I 
> understand why we did these studies.
Conversion factors between amplitude and charge are only applicable for 
the same conditions, i.e. same position, track length, etc. I agree you 
need to look at the amplitude, but only to check the dynamic range on 
the ADC for no overflows. Charge, i.e. integrating the data over a time 
window, is the only reliable way to understand the behavior of the 
tracking detectors, as shown in GlueX-doc-1131.

Regards,
Fernando



>
> Cheers,
>     Yves
>
>
>
>
>
>>
>> Regards,
>> Fernando
>>
>>
>>>
>>> Cheers,
>>>     Yves
>>>
>>>
>>>
>>>>>     FYI the capacitance (in free space, far from any ground plane) 
>>>>> of one strip to it's neighbors, I think you are saying it is about 
>>>>> 38.8 pF/m, is indeed in excellent agreement with the analytical 
>>>>> formula I mentioned a long time ago for the capacitance of one 
>>>>> strip to all other strips. (The neighbors being expected to 
>>>>> dominate of course).
>>>>>     The capacitance of so-called "coplanar waveguide" which 
>>>>> consists of a strip (of width w), with gaps (of width s) on each 
>>>>> side, with a half-infinite ground plane after that, should be a 
>>>>> very good approximation to the total capacitance from one of the 
>>>>> cathode strips to all others.
>>>>>     Anyway, (infinitely thin) coplanar waveguide capacitance is 
>>>>> 41.4 pF/m for w=4mm, s=1mm.
>>>>>     The formula for coplanar waveguide capacitance is
>>>>>
>>>>> C/length =
>>>>> 4 pi epsilon0 / ( -ln( q( w/(w+2s) ) ) ),
>>>>>
>>>>> where q(k) is the theta-function nome function of the elliptic 
>>>>> modulus k (conveniently this is a built-in function in 
>>>>> Mathematica, "EllipticNomeQ").
>>>> The 38.8 pF/m capacitance is to the ground foil for the present 
>>>> configuration of the FDC; 12 pf/m to the adjacent strips. In free 
>>>> space, the inter-strip capacitance will increase, as you suggest. 
>>>> This is in very good agreement with your earlier estimates and 
>>>> regarding the input load to the preamp.
>>>>>     Sincerely,
>>>>>
>>>>>     Gerard
>>>>>
>>>>> p.s. ASIC design simulations (for the FDC case) should probably be 
>>>>> done with 12-15pF between adjacent channels and 39-50pF to ground, 
>>>>> according to your results. I believe this is not something to 
>>>>> ignore - lab tests with GAS-1 confirmed that capacitance between 
>>>>> channels has a different impact on the noise frequency spectrum 
>>>>> than capacitance to ground. I don't think that was a surprise or a 
>>>>> problem, it is just a fact. Now we have real numbers (actually you 
>>>>> may be able to inflate not by wild guesses as I did but by 
>>>>> measurement of bare preamp boards and signal routing boards) for 
>>>>> the capacitance they should be used for ASIC design.
>>>> I have additional simulations and tests left to perform and noise 
>>>> is one of the most important.
>>>>
>>>> Thanks,
>>>> Fernando
>>>>
>>>>
>>>>>
>>>>>
>>>>> Fernando J. Barbosa wrote:
>>>>>> Hello everyone,
>>>>>>
>>>>>> I have performed electrical simulations on the CDC and FDC 
>>>>>> detectors, both anodes and cathode strips. I have placed 
>>>>>> GlueX-doc-1031 on the portal. Please review and provide feedback.
>>>>>>
>>>>>> Thanks,
>>>>>> Fernando
>>>>
>>
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