Theoretical characterization of coplanar waveguide transmission lines and discontinuities.

Abstract

The widespread use of microwave integrated circuits (MIC's) in recent years has caused rapid progress in its theory and technology. Recently, with the push to high frequencies and monolithic technology, coplanar waveguides (CPW) have experienced a growing demand due to their appealing properties. With the increasing use of coplanar waveguides, accurate high frequency models are needed which can result only from rigorous analytical methods. This dissertation presents a general full-wave analysis, based on the Space Domain Integral Equation (SDIE) approach, which can be implemented to theoretically characterize shielded CPW discontinuities. In this approach, a full-wave dyadic Green's function that accounts for multiple dielectric layers is used. This SDIE approach is applied to evaluate the performance of various one-port and two-port CPW discontinuities and has proven to be very accurate, computationally efficient and versatile in terms of the geometries it can solve. It is shown that the parasitic effects of an open-end CPW can be reduced by mitering the center conductor. In addition, it is demonstrated that the open-end-coupled CPW discontinuity may exhibit a resonance effect depending on the separation distance and the CPW dimensions. Moreover, it is shown that the behavior of a CPW stub depends largely on its shape and position. A hybrid technique is also developed in this thesis to analyze symmetric and asymmetric CPW discontinuities with air bridges. In this technique, the effect of the air bridges is taken into account qausi-statically. It is found that CPW shunt stubs do not behave as expected in the absence of the air-bridges due to the presence of the slotline mode in the study itself. On the other hand, an asymmetric CPW shunt stub without longitudinal air-bridge still behaves as a shunt stub. Moreover, a preliminary theoretical study of a new monolithic planar transmission line, the microshield line, is presented. A closed form expression for the evaluation of its characteristic impedance is derived using conformal mapping techniques. Several three-dimensional discontinuities are analyzed and their response is compared to the same discontinuity geometries in conventional CPW form. In all cases, the microshield line discontinuities show much lower radiation loss than the CPW ones.