Re: CDC and FDC characterization

[Gerard Visser <gvisser@indiana.edu>]

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... :)
	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").
	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.


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