Evaluation of the Shielding Characteristics of a Commercial 19-Inch Rack- Based Cabinet

摘要

From an electromagnetic compatibility perspective,a commercial 19-inch rack-based cabinet of 40U height is comprised of different functional modules housed in well-shielded enclosures. A main source of high-frequency radiated EMI from the cabinet is common-mode current on the cables. This common-mode current is caused by connector systems,printed circuit boards,and cabling systems used in the equipment. Two methodologies,in situ measurements on a functioning cabinet using a spectrum analyzer,and three-port mixed-mode S-parameter measurements with a swept frequency range of 100MHz to 2GHz using a vector network analyzer,are applied to investigate the overall shielding performance of various cabinet features,including the effects on radiated emissions from cable egress through the top of the cabinet,as opposed to the bottom of the cabinet. The results of the in situ measurements on a functioning cabinet show that,by bringing the cables out the top of the cabinet,the resulted radiated levels likely to exceed the EMI regulatory limit at a clock frequency of 627 MHz. The frequency response of the cabinet was further studied using the second methodology. Both approaches show that the cabinet,while not specifically designed to be a high-performance EMI shielded enclosure,does,however,provide about 5 ~ 10 dBμV/m of overall shielding performance that is important in meeting EMI regulatory requirements with the current system. The swept frequency approach,as proved,can be an effective method in the evaluation of shielding performance of similar equipments.