I'm not sure if I fully understand your situation, you might want to go to
http://www.ultracad.com/calc.htm and use their transmission line calculator
to play with the numbers and see what you get.

        ----------
        From:  Doug [SMTP:[log in to unmask]]
        Sent:  Thursday, April 06, 2000 1:38 PM
        To:  [log in to unmask]
        Subject:  [TN] 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

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