Degradation factors for eye diagrams using FDTD/SPICE

Abstract

This paper examines several contributors to eye diagram degradation in high-speed digital systems. The eye diagrams This paper deals with the effect of geometric details on digital signals propagated through transmission line structures used in today's printed circuit hoard (PCB) technologies. Sevare generated through a combination of full wave simulation and circuit analysis. The interconnect part of the analyzed structure is first characterized in terms of S parameters ohtained with the finite difference time domain (FDTD) method. The resulting S parameters are used together with the convolution approach to perform time domain simulation of the complete circuit in SPICE. 1. Introduction With the continued increase in speeds of digital systems, designers and researchers are faced with problems containing an increased number of unknowns. Analysis and modeling issues that can be neglected at lower speeds become significant at higher speeds and must be considered in the design of the system. Today's digital systems increasingly rely on passive interconnect components to guide and shape the propagated digital signals. These passive components consist of various transmission line structures containing complex geometric details. Signals propagating through the interconnect shctures are subjected to distortion, losses, and dispersion. These signal degradations can be considered as unwanted noise in the desired signal. The total effect of the interconnect on the received signal is related to all of the structure details and to the exact waveshape of the generated signal. For the purpose of interconnecl design, it is useful to view the interconnect as a. cascade of mutually connected components and to separate the contribution of each component to the total noise in the desired signal. Understanding the amount of noise disturbance to expect from a particular type of interconnect component can he very useful for the interconnect design. The stochastic noise that is present in the desired signal can be viewed in different ways. A commonly used visualization method used for digital systems is the eye diagram. The eye diagram can be obtained by oscilloscope measurements or by simulations. It provides information about the amplitude and time separation of the data hits. The eye diagram merges together a number of signal charactcristics in one graph, including timing jitter, overshoot, ringing, crosstalk, and intersymbol interference. A number of industry standards are based on properties orthe eye diagram. Eye diagram simulations are typically performed on long uniform structures using transmission line modeling methods (e.g., [I]). The shapes of the eye diagrams on these long structures is determincd primarily by losses, resulting in dispersive attenuation of the signals. Degradations to eye diagrams caused by structure discontinuities have not been studied extensively.