Doug From What I understand, you may have the reference plane at a width no
less than 3 times the trace width in question or the height of trace to
reference plane which ever is greater.

ATI Technologies Inc.,
Patrick Jabbaz
Sr. Circuit Board Design Eng.
33 Commerce Valley Drive East,
Thornhill, Ontario, Canada, L3T 7N6
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-----Original Message-----
From: Doug [mailto:[log in to unmask]]
Sent: Thursday, April 06, 2000 1:38 PM
To: [log in to unmask]
Subject: [DC] Long Question Needs Short Answer ...


Hello All,

I might as well jump right into the question.

Suppose there's a trace construction as follows.
It's 50 ohm controlled impedance with a separation
"d" and a width of 5 mils between two ground planes
in a symmetrical construction.  I'm assuming the
controlled impedance is calculated by assuming the
ground planes go off to infinity.

CASE #1:
         ground plane
  ---------------------------
            5 mils  d
            -----
                    d
  ---------------------------
         ground plane

Now, for some reason, someone wants to start cutting
back the ground planes to a width W without changing
separation "d" of the trace to the ground planes.

Shown as below.

CASE #2:
         ground plane
       | <---- W ----> |
       -----------------
            5 mils  d
            -----
                    d
       -----------------
       | <---- W ----> |
         ground plane

Question:  Ignore fab tolerances for the moment.

 At what point does the controlled impedance of the
 trace begin to be compromised by 1%, 10%, 20% ... ?

 Could I in fact reduce the width of the ground planes
 down to 5 mils, equal to the width of the trace,
 without compromising impedance?

Regards, Doug McKean