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Evaluation of the Shielding Characteristics of a Commercial 19-Inch Rack-Based Cabinet
Keywords: commercial 19-inch rack-based cabinet, common-mode radiation, rack shielding performance
A commercial-grade 19-inch rack-based cabinet is comprised of different computer technology modules housed in commercial-grade chassis enclosures. A main source of high-frequency radiated EMI from the cabinet is common-mode current on serial data cables. Two methodologies are applied to investigate the overall shielding performance of various cabinet features, including key effects on radiated emissions when cable egress is moved from the bottom to the top of the cabinet. Firstly, in-situ measurements are performed on a functioning cabinet in a shielded anechoic enclosure using an antenna and a spectrum analyzer. Secondly, three-port mixed-mode s-parameter measurements are made over a swept frequency range of 100MHz to 2GHz using a vector network analyzer. These swept frequency measurements are performed on a cabinet mockup in order to investigate the overall shielding performance of the rack. The in-situ measurements on a functioning cabinet show that, bringing the cables out the top of the cabinet results in a rise in radiated levels that risk exceeding the EMI regulatory limit at some frequencies, particularly the critical serial data clock frequency of 627 MHz. The swept frequency measurements were performed at several antenna heights and at both polarities. This method provides clear evidence of the shielding benefit of specific features, such as the rear door and cable egress method. In particular, this method shows that the effect of changing cable egress from the bottom of the cabinet to the top increases radiated levels substantially, i.e., by more than 15 dB at some frequencies. Both measurement approaches show that the cabinet, while not specifically designed to be a high-performance EMI shielded enclosure, does exhibit an overall shielding performance on the order of 10 dB. The swept frequency approach is shown to be an effective method for the evaluation of the shielding performance of similar equipments.
Jue Chen, Student
University of Missouri-Rolla
Rolla, MO

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