Coplanar waveguide

Cross section of a conductor-backed coplanar waveguide transmission line
A 517 μm tall copper coplanar waveguide created using LIGA technique.[1]

Coplanar waveguide is a type of electrical planar transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. On a smaller scale, coplanar waveguide transmission lines are also built into monolithic microwave integrated circuits.

Conventional coplanar waveguide (CPW) consists of a single conducting track printed onto a dielectric substrate, together with a pair of return conductors, one to either side of the track. All three conductors are on the same side of the substrate, and hence are coplanar. The return conductors are separated from the central track by a small gap, which has an unvarying width along the length of the line. Away from the central conductor, the return conductors usually extend to an indefinite but large distance, so that each is notionally a semi-infinite plane.

Conductor-backed coplanar waveguide (CBCPW), also known as coplanar waveguide with ground (CPWG), is a common variant which has a ground plane covering the entire back-face of the substrate.[2][3] The ground-plane serves as a third return conductor.

Coplanar waveguide was invented in 1969 by Cheng P. Wen, primarily as a means by which non-reciprocal components such as gyrators and isolators could be incorporated in planar transmission line circuits.[4]

The electromagnetic wave carried by a coplanar waveguide exists partly in the dielectric substrate, and partly in the air above it. In general, the dielectric constant of the substrate will be different (and greater) than that of the air, so that the wave is travelling in an inhomogeneous medium. In consequence CPW will not support a true TEM wave; at non-zero frequencies, both the E and H fields will have longitudinal components (a hybrid mode). However, these longitudinal components are usually small and the mode is better described as quasi-TEM.[5]

  1. ^ Forman, Michael A. (2006). "Low-loss LIGA-fabricated coplanar waveguide and filter". 2006 Asia-Pacific Microwave Conference. pp. 1905–1907. doi:10.1109/APMC.2006.4429780. ISBN 978-4-902339-08-6. S2CID 44220821.
  2. ^ Cite error: The named reference Gevorgian1995 was invoked but never defined (see the help page).
  3. ^ Kuang, Ken; Kim, Franklin; Cahill, Sean S. (2009-12-01). RF and Microwave Microelectronics Packaging. Springer Science & Business Media. p. 8. ISBN 978-1-4419-0984-8.
  4. ^ Cite error: The named reference wen1969 was invoked but never defined (see the help page).
  5. ^ Rainee N. Simons, Coplanar Waveguide Circuits, Components, and Systems, pp. 1–2, Wiley, 2004 ISBN 9780471463931.