Not being an expert on this subject, I am writing only in the interest of
providing a fast answer, pointing, I hope, in the right direction.

At 10:38 AM 4/6/00 -0700, Doug wrote:

>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?

In a word, no. That impedance was determined by infinite plane conditions;
the situation described will have a higher impedance. My offhand guess is
that impedance would roughly double. I'd be interested to see a calculation.

50 ohm traces for stripline construction and 5 mil prepreg/core, Er 4.5,
half ounce copper, would be something like 3.2 mils. That's pretty thin!!!

In fact, cutting up the plane into tracks running parallel underneath the
traces would allow a fatter trace to be used. Doing the calculations is
beyond the time I could devote to this at the moment, though!

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Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433