Criteria for single mode operation of packaged coplanar waveguide circuits

For some of its advantages, the coplanar waveguide (CPW) has been widely used in MICs and MMICs in the past few years. The two important modes of this waveguiding system are the CPW mode and the slot mode. The latter mode is parasitic and is often eliminated using airbridges or bond wires. Like other MICs/MMICs, a CPW circuit is packaged in practice. If the effect of this package is not accounted for properly, it can lead to multi-mode wave propagation and hence to the degradation of the performance of the CPW circuit. This paper examines the wave propagation in a packaged CPW and addresses the effect of the package on the characteristics of the CPW and slot modes. Specially, it elaborates on the suppression of the slot mode by shifting it out of a desired band of frequencies through a proper packaging. The results obtained are compared with those of a dielectric filled rectangular waveguide of the same cross-section as that of the CPW substrate. The comparison sheds lights on criteria for the design of the package for single mode operation of conductor-backed coplanar waveguides without airbridges