Re: CDC and FDC characterization

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

Hi Gerard,

Thanks for your feedback.

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.
>     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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