FDC questions (fwd)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Thu, 27 Feb 2003 10:24:52 -0500
From: Fernando J. Barbosa <barbosa@jlab.org>
To: Elton Smith <elton@jlab.org>
Subject: FDC questions

Hi Elton,

    Sorry about the delay but I only now checked my voice mail. I am also
including some comments with respect to some email from Elliott and
Mehmet.

    The gap on the cathode strips will be dictated by manufacturing
variables which influence cost. Presently, the best reliable width for
traces and spaces (gaps) in circuit boards is 4 mil and expensive; 8 mil
is the norm.

    Each sense wire needs a bias resistor of 10K (or so but this is not
critical) connected to +HV. The size of the resistor will be dictated by
the potential across it. Each sense wire will also need its own HV
decoupling capacitor (~220-2200 pF, again not critical) to connect to the
preamp.

    The field wires can be ganged together, maybe in groups, and 
connected
to a second HV source through a bias resistor (~10K not critical). A more
detailed implementation would try to balance the resistive and capacitive
loads on sense wires and shared field wires but this is not critical.

    All the HV inputs should be decoupled to ground to filter out noise.
The capacitor can be like the HV decoupling capacitor.

    The cathodes are connected directly to preamps as they are at near
ground potential. The cathode strips could be Gold plated, as Elliott
suggests (with a Nickel layer under it). Copper oxides that form on Copper
surfaces are conductive and should pose no problems with electrostatics or
reliable operation of the chamber, though.

    Another variable concerns the length of the strips and sense wires.
One should carefully consider using termination techniques if the
propagation delay is comparable to the rise time of the pulses. For a 5 nS
rise time, termination would perhaps be required at the far end (opposite
preamp side) for lengths greater than 3 to 5 ft. This adds complexity,
especially to the sense wires which would require additional HV capacitors
in series with the termination resistor. Maybe one can tolerate some level
of reflections or noise if timing information is not critical from this
detector.

    Please, let me know if you need additional information.

    Fernando