Analysis of coplanar waveguide-to-coplanar stripline transitions

The coplanar waveguide (CPW)-to-coplanar stripline (CPS) transition is anal yzed theoretically and experimentally in this paper. To characterize this t ransition in the lower frequency band, a simple equivalent-circuit model th at consists of uniform and nonuniform transmission lines is established. Th e elements of this model can all be obtained by the closed-form formulas; h ence, this model is suitable for computer-aided-design application. This mo del is then applied to design and analyze the CPW-to-CPS transitions with v arious structure parameters. In the higher frequency band, the partially pr izm-gridded finite-difference time-domain (FDTD) method is employed to take into account the bond-wire effect as well as the surface-wave leakage and space-wave radiation associated with the transition. In this study, results based on equivalent-circuit model, FDTD simulation, and measurement are co mpared. Good agreement among these results supports the usefulness of the p roposed equivalent-circuit model and also validates the FDTD method. By usi ng the equivalent-circuit model to optimize the transition configuration, t he CPW-to-CPS transition with broad bandwidth and low insertion loss may be achieved.