RE: [SI-LIST] : Coplanar Transmission Line

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Thu Feb 03 2000 - 11:43:47 PST


Roy,

Coplanar waveguide (CPW) is used quite frequently in RF/Microwave
circuits to avoid multimode, decrease dielectric loss effects,
and reduce discontinuities at bond wires/etc. However,
we've had difficulty with PCB
vendors being able to successfully obtain a desired
impedance.

From what I can tell, the PCB folks have played around with
microstrip/stripline enough that they have a sufficient
pool of experience to know what to adjust to obtain the
desired impedance (i.e., dielectric thickness or line
width). In CPW, the dielectric thickness is not a
first-order effect, which presents difficulty in processing.
In other words, from a practical standpoint, CPW in
PCB needs to be used with caution (at least until you've
worked out the details with a vendor).

If you can get the desired impedance, the electrical properties
of CPW are generally better that microstrip and stripline (at
the cost of reduced routing density).

For specific papers that might help out, try these:

TITLE Synthesis Formulas Simplify Coplanar-Waveguide Design
AUTHOR T. Q. Deng, M. S. Leong, P. S. Kooi, and T. S. Yeo
SOURCE Microwaves & RF
DATE March, 1997
PAGES 84-98

TITLE Finite Width Coplanar Waveguide for Microwave and Millimeter-Wave
Integrated Circuits
AUTHOR G. E. Ponchak, S. Robertson, F. Brauchler, J. East, and L. P. B.
Katchi
SOURCE Proc of 1996 Int'l Symposium on Microelectronics
LOC Minneapolis Convention Center, Minneapolis, MN
DATE 8-10, October, 1996
PAGES 517-521

TITLE Microwave Planar Passive Circuits and Filters
AUTHOR J. Helszajn
DATE 1994
PUBLISHER John Wiley & Sons, Ltd
ISBN 0-471-94056-9

In terms of more referenes, IEEE MTT (www.mtt.org) has a pile
of papers in their journals on the topic, and their
annual conference (http://www.ims2000.org/) generally
has several very good papers each year.

Pat

> Dear SI Experts,
>
> Who out there has worked with coplanar (waveguide-like)
> transmission lines as
> opposed to stripline/microstripline?
>
> What can you tell me about it other than what I can find in
> Collins or Bahl &
> Bhartia?
>

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