[Fwd: Re: Ground plane near FDC readout strip]

[Elliott Wolin <wolin@jlab.org>]

-------- Original Message --------
Subject: Re: Ground plane near FDC readout strip
Date: Thu, 8 May 2003 10:01:48 -0400
From: "Fernando J. Barbosa" <barbosa@jlab.org>
To: "Elliott Wolin" <wolin@jlab.org>
References: <3EB975C7.7020805@jlab.org>

Hi Elliott,

     You are correct. The ground plane near the cathode strip would cause the
signal to be coupled to ground and therefore cause the signal to noise ratio
to decrease, quite considerably. As ionization takes place somewhere in the
cell, the ion cloud will move towards the cathode strip and any capacitance
to ground will cause part of this charge to bleed out into the ground. With
a ground plane close to the cathode strip, there would be three capacitances
(disregarding adjacent cells): the cell capacitance established by the
geometry of the anode and cathode; the 'readout' capacitance determined by
the connections to the preamp and the preamp input capacitance; and the
cathode to ground capacitance.To get the best performance, the 'readout'
capacitance should be matched to that of the cell capacitance for maximum
signal to noise ratio and the cathode to ground capacitance should be
minimized.

     The above describes only what's pertinent to the detector. Depending on
the readout scheme it may be necessary to provide some sort of pole/zero
compensation to further optimize performance. Another area of importance is
to minimize any external noise coupling into the system which is usually
accomplished by shielding the detector and every connection to it, by using
low noise instrumentation, by using good power supplies, etc. I believe the
design of the FDC calls for aluminized Mylar windows to be grounded.
Therefore, you don't need an additional ground plane under the cathodes.

     I hope this helps. Best regards,

         Fernando

----- Original Message -----
From: "Elliott Wolin" <wolin@jlab.org>
To: "Hall D at JLAB" <halld-jlab@jlab.org>; <wojcik@jlab.org>
Sent: Wednesday, May 07, 2003 5:08 PM
Subject: Ground plane near FDC readout strip


 > Hi,
 >
 > I talked with Fernando, and as many of us suspected it is not a good idea
to have a ground
 > plane near the cathode readout strip.  It is important to shield the
entire detector from
 > external noise, so the outside frame and windows should be made of
conducting materials
 > (and be hermetic...even small gaps can let lots of noise in).  But the
capacitance of the
 > readout strips to ground should be minimized, so putting a ground plane
close to the
 > strips is not a good idea.
 >
 > I believe one can think of the ground plane as causing the preamp input to
see the signal
 > as being dropped across a capacitive divider with the preamp connected to
the middle, so
 > you want the capacitor to ground to be small so that the signal isn't lost
(Fernando...is
 > this correct?).
 >
 >
 > Sincerely,
 > Elliott
 >