Full-wave, TwinAx, differential cable modeling

Abstract

In modern data centers, rack mount equipment provides ease of planning, installation, and management due to the standardized form-factors that multiple vendors follow. Within a single server or a blade environment, the predominant form factor is the backplane where multiple cards communicate through connectors. Within a rack, one or multiple servers may be installed and communication between racks and within a rack is accomplished through cables. In order to guarantee system performance, designers need accurate models of the cable's electrical behavior. Shielded TwinAx differential pair based cables are commonly used for short to medium reach (less than 10-20 meters) in standards such as SAS, InfiniBand, SATA, PCI-Express. In TwinAx shielded differential pair construction where the shield is wrapped around the TwinAx pair, it often has a band stop filter, or 'suckout' characteristic that limits its performance. This paper models and characterizes this effect using a full-wave finite element method (FEM) solver implemented in Ansoft HFSS. In this paper, the wrap effect is captured using full 3D electromagnetic model for the shielded TwinAx construction. The physics behind the suckout effect is explained using both 2D and 3D models. Finally, this paper describes the methods and techniques that can mitigate the suckout effect as well as a novel cable construction with a helical shield wrap without any suckout effects.