DesignCon 2008 EMI Shielding of Cable Assemblies

Shielded cables are a common component in today’s standard electronic systems that transmit high-speed video and data. However, industry standard cables don’t necessarily exhibit standard shielding performance. This paper is a survey of the shielding performance actually measured on commercially available cable assemblies. When significant differences in performance were discovered, autopsies were performed in order to determine, where possible, the cause of the difference. Author(s) Biography Dana J. Bergey earned his BS degree in Physics from the Pennsylvania State University, and his MSEE from the Air Force Institute of Technology (AFIT). While in the United States Air Force, he served as the Senior Engineer of the AFIT Advanced Development Laboratories, where he supervised the microwave, IR, laser, and radar cross-section labs. He spent several years working on stealth technology and teaching microwave and electromagnetics classes at the school. Bergey has spent the last 18 years working as a signal integrity and EMC engineer at AMP Incorporated, W.L. Gore & Associates, and FCI. He is presently manager of FCI’s US signal integrity team, which is involved in the development of high-speed interconnects. Nathan E. Altland has worked as a signal integrity engineer at FCI for the past 8 years. He has been involved in all aspects of signal integrity and EMC test and measurement, including the support of next-generation, high-speed connectors and cable assemblies, development of test systems and methodologies, product qualification, failure analysis, and customer application support. He works closely with SI simulation engineers to validate connector models. His current responsibilities include managing the day-to-day operations of FCI’s Signal Integrity Laboratory, including supervising SI co-ops and interns. Introduction As high-speed serial data transmission technologies have proliferated, numerous industry standards have been published. These standards document specific electrical performance requirements for high-speed cable assemblies. However, while signal integrity parameters, such as characteristic impedance, insertion loss, crosstalk, propagation delay, and skew are thoroughly addressed, the EMI shielding performance of these standard cable assemblies is rarely specified. This paper reports on several studies that were conducted to determine the variability of shielding performance that can be found in commercially available cable assemblies. Sample groups of Fibre Channel, HDMI, USB, FireWire, and Infiniband cable assemblies were tested. In each case, when “outliers” were discovered (i.e. samples that exhibited much better or much worse shielding performance than the average of the group), an exploratory investigation was performed to diagnose the cause of the performance difference. Shielding Effectiveness Test Procedures Absorbing Clamp Test Method The Fibre Channel cable assemblies were tested using an absorbing clamp procedure (See Fig. 1) similar to the Common Mode Power Transfer (CMPT) method described in the SFF-8410 Specification for High Speed Serial Copper Testing and Performance Requirements [1]. Modifications to the documented procedure included use of shielded boxes with receptacles installed, rather than the suggested “stovepipe” enclosure and direct measurement of the input signal, which enables a more accurate calibration than using a “T” connector. Shielded Box with receptacle fixture Shielded Box with receptacle fixture Termination inside box VNA