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Hall D FDC prototype
Hall D folks,
Since I sent out my original questions on the FDC prototype design
drawings provided by Randy Wojcik (on 5/19/03), several questions
have been answered or addressed. In this email I sum up the
responses to the questions and identify the remaining issues. For
those who have been following this line of emails, I separate the
answered/solved issues from the open questions/problems. I will
begin by listing the issues that are still open. Let me know if you
have any questions. I will arrange with Elton upon his return to
have a meeting focussed on these issues.
My comments are based on the latest drawings from Randy (version 4;
circulated on 5/21/03). This is my complete list. If you have
any questions or concerns, please pass them on to me.
Daniel
Open questions:
---------------
I). Figure: randyV4.pdf (wire frame)
1). The definition of the ground planes needs to be revisited. In the
past I have had the highest noise immunity when I surround the signal
traces with a sizeable ground plane. In this design the signal
traces are ``out in the open". Noise immunity is an important
consideration for this prototype so that we can make the
most informed decisions on the electrode layout.
Randy's response: Currently no ground plane is planned to be used. Please
contact Elliot and Fernando about that. I did add a top and bottom
ground trace around the edge of the boards.
Fernando's response: I agree we should have a ground plane surrounding
the traces for noise immunity. For that matter, we should have it
everywhere, except the active area of the detector and away from
any HV sections.
I also talked with the IUCF wire chamber guru (Keith Solberg). He also
agreed with my statements. He also indicated that he likes to include
ground plane definition between the signal traces in order to minimize
channel to channel cross talk.
2). Need to include external sighting mark to locate chamber relative
to an external reference.
Randy's response: Please tell be what sighting marks you want and where
you want them.
I would like a target included on the PC board for at least the wire
plane in a place that can be seen after the chamber is put together.
I guess this means on the HV `ear'. We can then connect the
measured wire positions to this external target for alignment within
the test stand.
II). Figure: cathodeV4.pdf (cathode board)
1). The cathode board design still has a number of open questions. When
last we talked, we decided to make the cathode plane (the copper-cladded
kapton) larger in size so that we could include dowel pin holes to
improve the alignment and positioning. I assume that we are mounting
this on top of a G10 frame. The question is how we will attach this
to the G10. In my mind it seems that we will have to epoxy the
cathode plane to the G10 to ensure that it cannot move and also to
ensure that our chamber remains gas tight. Once the attachment is
made, then we will have a mechanically sturdy cathode board that we
can mount Fernando's signal on.
Randy's response: I am not sure how it is planned to be mounted to the
G10. I have asked that as well and not received and answer yet. The
connection to Fernando's readout is at the signal connector pads on
the left and right side of the drawings. I would guess we would make
a G10 frame with a step down to .062" where the connectors are. The
G-10 gives the mechanical strength and Kapton gives the electrical
connection to solder to.
You have not received an answer yet probably because I am supposed to
provide the answer. However I want to discuss this with the group.
Perhaps Howard Fenker can provide some input to this question based
on his experience. Also because it seems that we have to permanently
attach the cathode sheets to the G10, and because we are testing
sets of cathode sheets with different cathode strip spacings, we will
have to order enough G10 frames for each.
2). Why are the cathode strips uneven at the ends?
Randy's response: Does it matter? If so, how close do they have to be?
My feeling is that if they do not line up evenly then we are asking
for trouble. The unevenness at the ends of the strips will give
rise to a non-uniform electric field configuration near the ends of
the strips. I think this can lead to resolution smearing.
3). The present design has an o-ring laying across the top of the cathode
strips. Is this an issue. Usually we spread a thin layer of o-ring
grease around the o-ring to maintain elasticity. But this can give
rise to a high-resistance short across the strips.
III). Figure: mechv4.pdf (mechanical drawings)
1). The gas inlet and outlet ports on the aluminum frames should be
placed in opposite corners for the best gas flow through the
chamber.
2). Is it better to put the o-ring in the aluminum frame than on the
corresponding G10 frame?
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Addressed questions:
--------------------
I). Figure: randyV4.pdf (wire frame)
1). We need provisions for HV connectors on the PC board. These
should be PC Board mounted connectors. Also, what are the
holes in the PC board near the HV ``ears".
Randy's response: The holes on the PC board are for mounting standard
SHV connectors Kings type 1704-1. From them we will just wire to
the board with silicon wire.
2). Will the PCB manufacturer drill the through holes on the board?
If so, how are the dowel pin holes specified?
Randy's response: The boards are made by CNC means. All locations of
the holes are in the image and downloaded to the CNC machine during
manufacture of the PCB.
3). The width of the solder pads seems to be too narrow. I prefer to
have them 1.5 to 2.0 mm wide.
Randy's response: I narrowed the solder pads to allow voltage difference
of up to 1500V which was asked for. In general the spacing should be
.1" per 1000V. If we choose 1.5 mm, the spacing will support a
difference of 1380V. If that is ok, I can easily increase them.
Currently they are .03" wide.
I think that 0.03" will be ok after some further reflection. It is
just a little narrower than I have used in the past.
4). Will there by a solder mask included?
Randy's response: No solder mask is planned just to reduce the possibility
of gas poisoning materials in the chamber.
5). The signal traces running from the solder pads to the connector
have sharp corners. Is this just an artifact of the drawing
program? I prefer smooth curves to reduce the possibility of HV
breakdown and signal reflection.
Randy's response: The image you see is after conversion to PDF which does
not do a good job with curves since it is basically a compression
routine. All traces are curved.
6). What is the difference between the blue HV bus and the red HV
bus? Are they meant to be on opposite sides of the board?
Randy's response: Red is one side, blue is the other side. Otherwise it
would all be shorted together.
Obviously I recognized that if the busses were on the same side they
would short. My confusion stemmed from the fact that I thought that
the entire back side of the wire plane was copper. That is not the
case.
II). Figure: cathodeV4.pdf (cathode board)
1). What about holes for gas flow through the kapton?
Randy's response: Yes, I forgot about those originally. I added them
about a week ago but did not think it was important enough of a
change to send out a new version.
2). Is there going to be any ground plane definitions on the cathode
board?
Randy's response: Currently no ground plane is planned to be used.
Please contact Elliot and Fernando about that. I did add a top and
bottom ground trace around the edge of the boards.
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* Dr. Daniel S. Carman e-mail : carman@jlab.org *
* Ohio University office : (740)-593-2964 *
* Department of Physics fax : (740)-593-1436 *
* *
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