The conductive bench is used largely for reason (1) below. In addition to what you state, the presence of a ground plane radically affects the efficiency of radiated emissions and radiated susceptibility. Both are lessened by the presence of the ground plane - especially as regards horizontal polarization.
The reason for the low impedance of ground plane and test chamber connection is again related to RE & RS testing. The typical test article is not a stand-alone device, it connects to support equipment which is situated outside the test chamber. Any interconnecting cable must penetrate the shield room wall. Any cable shields must ground at the penetration point. That ground points must be a low impedance connection to the ground plane, so that the cable common mode currents have a low impedance connection back to the ground plane. If you don't have this, RE will be higher than they should, and RS fields will couple more strongly than they should. Reasons 2a & b below are incorrect. If it were 2a alone, the low rf impedance requirement would be totally superfluous. And it is certainly not 2b! The very existence of that ground plane is what differentiates the whole of military, aerospace and automotive EMI testing, what I lump together as "platform-installed" equipment from the treatment of "commercial equipment." The difference is not "military vs. commercial," It is metallic platform vs. the lack thereof. This was developed in some detail in a paper in the 1997 EMC Symposium record, "On Field-To-Wire Coupling Versus Conducted Injection Techniques." That paper explains the difference between CS and RS drives in platform-based testing (1.5 mA per Volt per meter), vs. in non-platform testing (1 Volt open-circuit >from a 150 Ohm source vs 1 Volt per meter). From: "Luke Turnbull" <[email protected]> List-Post: [email protected] Date: Thu, 5 Jul 2007 08:14:23 +0100 To: [email protected] Subject: Question about Conducting Test Benches Hi all, I have two questions about conducting test benches as used for automotive, aerospace and military testing. I wonder whether anyone has a definitive answer. 1. Why is the bench used? 2. Why does it have a low inductance connection to the test chamber? Here's my guesses: 1. To replicate the chassis of a car / aircraft / tank / ship and hence provide a return path for any common mode RF current on a cable harness. 2. a) To provide an electrical safety earth for the bench. b) because we've always done it that way. Thanks, Luke Turnbull ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
