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CPW (Coplanar Waveguide) Basics

This page covers CPW (Coplanar Waveguide) basics and provide advantages and disadvantages of Coplanar Waveguide.

The Coplanar Waveguide is a planar transmission line. It is widely used for microwave Integrated Circuit design.

CPW, Coplanar Waveguide

As shown in the figure, Coplanar Waveguide consists of a conductor strip at the middle and two ground planes are located on either sides of centre conductor. All these lie in the same plane.

In coplanar waveguide, EM energy is concentrated within the dielectric . The leakage of the Electromagnetic energy in the air can be controlled by having substrate height (h) twice that of the width (S). The coplanar waveguide supports quasi TEM mode at low frequencies while it supports TE mode at high frequencies.

The effective dielectric constant of CPW is same as that of slotline. The characteristic impedance of a coplanar waveguide is not affected by thickness and depends on width(W) and space(S). The lowest characteristic impedance of 20 Ohm can be achieved by maximum strip width(W) and minimum slot space(S). It typically ranges from 200 to 250 Ohm.

Advantages of Coplanar Waveguide

•  Low dispersion
•  simple realisation due to etching on one side.
•  broadband performance, as it does not need via holes for shunt and series elements

Disadvantages of Coplanar Waveguide(CPW)

•  Fabrication of coplanar waveguide is costlier. As gold ribbons are needed to suppress higher order modes at every quarter wavelengths
•  Relative thick substrates are needed.

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