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

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