Experimental and Numerical Investigation of Crosstalk Effect in Coupled Coplanar Waveguides—Part II: Multimode Coupled Line Representation

An approach for the crosstalk effect description in coupled coplanar waveguides with ideally grounded central strip conductor (C-CPWGS) that, for the first time, takes into account nonideal grounding by means of multimode propagation is presented in this paper. The frequency-domain full-wave computations of modal eigenvectors and propagation delays combined with multiconductor transmission line equations have been employed to obtain near- and far-end crosstalk waveforms. Developed theory has been verified experimentally by comparison with results of the time-domain crosstalk measurement of selected test structures. A very good degree of coherence between numerically computed and experimentally measured data has been achieved

I. Introduction

Recently, coplanar-type lines and structures have become more often preferred over more traditional microstrip-like lines and structures for creating the interconnections of digital circuits and the realization of integrated microwave components [1], [2]. Unfortunately, so far, behavior of the coplanar waveguides carrying periodic square waves has not been the subject of extensive studies. The technique of using additional grounded conductors [3] at the stage of the printed circuit board (PCB) design used to more tightly bind signals running along active paths with structural ground and, thus, minimize parasitic electromagnetic compatibility (EMC)-related phenomena, can also be used in designs implementing coplanar transmission line structures. In such cases, especially in a system employing the so-called coplanar waveguides (CPWs) [4], serious practical problems may arise due to imperfect grounding, especially in wide-band RF analog and high-speed digital applications. Due to a widely expanding range of digital circuits used in electronics nowadays, the latter case is taken into consideration in this paper. An earlier approach [5] taking into account the combined time–frequency domain algorithm of the crosstalk effect description in the coupled coplanar waveguides (C-CPWs) has relied on bi-mode approximation of the wave propagation along symmetrical coupled lines. However, with increasing logic speed (rise time and fall time of square-wave pulses being a fraction of nanosecond), assumption of an ideal electrical connection between the grounded strip conductor and lateral ground planes may no longer be valid. Such a situation may occur in the case of a nonideal connection resulting, for instance, from improper via holes distribution on the PCB. According to the general coupled line theory, multimode (more than two guided modes in the system) propagation is, then, inevitable. Though the effect of the nonideal grounding can be approximated by different means, it will not always take into account the multimode nature of the coupled lines of a complex geometrical cross section.