Re: CDC and FDC characterization

[Yves Van Haarlem <yvhaarle@ernest.phys.cmu.edu>]

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.

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
>>> 
>