RE: [SI-LIST] : Coplanar Transmission Line

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From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Thu Feb 03 2000 - 13:02:08 PST


Alfredo Moncayo wrote
> ...
> Impedance control can
> generally be quite good for two reasons: 1) there is negligible
> effect due to dielectric spacing to a reference plane, 2) to
> achieve 50ish ohm impedance, the trace width will be somewhat
> wider than most folks are used to, making the variation of
> impedance due to etching less severe.

I'd disagree with this statement. We've found impedance control of coplanar
structures to be somewhat difficult when geometries cannot be tightly
controlled (i.e., in PCB processing) as the impedance is quite sensitive to
1) conductor-conductor gap spacing, which typically must be fairly small for
a 50 Ohm impedance (as Pat Zabinski pointed out in an earlier post), 2)
conductor thickness, and 3) conductor surface roughness. True, sensitivity
to dielectric thickness and conductor width is reduced, but we've found that
the increased sensitivity to these other three factors makes controlled
impedance coplanar geometries challenging in a PCB environment.

- Erik

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Erik Daniel, Ph.D. Mayo Foundation
Voice: (507) 284-1634 Guggenheim 1011B
Fax: (507) 284-9171 200 First Street SW
E-mail: daniel.erik@mayo.edu Rochester, MN 55905
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