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What's My Impedance?

Daniel G. Swanson, Jr.
Bartley R.F. Systems Inc., Amesbury, MA 01913

Microwave circuit simulators have models for all the traditional transmission line cross-sections such as microstrip, stripline and coplanar-waveguide (CPW). However, there are cases that the standard models do not cover. This is particularly true in multilayer environments such as MMIC's, packages and printed circuit boards (PCB's).

In all of these environments, we have the freedom to design transmission lines that may not fit any of the conventional definitions. We may need microstrip or CPW with a dielectric overlay for an MMIC or package. In a multilayer PCB, we may have a buried stripline conductor with grounded isolations strips in close proximity and upper/lower ground planes that are not symmetrically located.

This configuration is some kind of stripline/CPW hybrid depending on the strip widths and the various ground plane spacings. For any strip configuration, the effects of strip thickness and the actual strip cross-section due to etching or other processing are also of interest.

Fortunately, some inexpensive electromagnetic fieldsolvers can help us compute the impedance and effective dielectric constant of arbitrary transmission line crosssections. For good engineering results, our goal is 1% to 2% accuracy. These tools also give us the freedom to ask many interesting "what if" questions regarding the configuration we have chosen.