A Study on Coplanar Structures for High Speed Transmission

摘要

Demand for higher speed digital signal processing is notable today in the electronics industry. To meet this demand, circuit designs that employ coplanar structure, either for both single-end and differential transmission, are increasing. The purpose of coplanar structure is mostly cross-talk (noise) reduction. It is mostly the case that guard-earth lines are added to microstrip line or strip line structure to make coplanar structure. However, electro-magnetic field tends to be very complicated for such structures, and so how the structure is related to characteristic impedance or cross-talk is not well understood yet. For example, there are some cases where characteristic impedance would be 30% or more different by adding coplanar lines to simple micro-strip line structure. In these cases, conventional methods will not work well to predict characteristic impedance with sufficient accuracy. What is making it so difficult is the complicating relation between the signal line references to the ground layer and to the coplanar lines, and so understanding the electro-magnetic reference mechanism with coplanar structure is essential to achieve good enough control of characteristic impedance, noise, and cross -talks. We made a detailed analysis on a coplanar structure and worked out an explanation as to the mechanism with a simple, practical coplanar structure. In this paper, details of our study on coplanar structures shall be reported, with some proposals as to the design of coplanar pattern in terms of cross-talk reduction.