If one wants to test the maximum of possible emissions, the network switch should be connected with cables of resonating length (so 1/4 lambda) for each available RF frequency on-board and be tested accordingly. Emission testing (unless for large EUT) is all about cables -antennas- !!!! Measuring without wires will obscure 99% of the emission properties of your network switch.
Most emission is due to common mode current created by ground voltages that appear between connectors on different places. A quarter lambda of cable has the maximum emissions property. This makes emission testing unpractically long to carry out, (cut cut cut) and requires immense measuring set ups for low frequencies. So "we" decided some time ago to start measuring radiated E-field emissions (for civil apps) at 30 MHz, and in the FAR field to overcome the measurement problems with magnetic field emissions, and to standardize the cable layout into a measurement volume (latest CISPR16), allowing for a maximum (small EUT) of 2 x 80 cm H/V at each side ( or one side) of the EUT. Now at the end of this part of cable a typical termination is required for those cables that carry SIGNALS with frequencies within the measurement range (in band). Why terminate ? Well, a badly terminated cable will transform differential mode current ( the signal current) into common mode current. ( if you do not know the difference between DM and CM current, take your study book NOW, or you will never understand EMC)!!! And common mode current radiates. It makes no sense to typically terminate a 600 OHM PSTN cable from the view point of emission testing, as it carries no RF signals. Network cabling is divided into several categories (CAT 3,4,5) These categories do correspond to high frequency signal integrity (read symmetry) of the wire pairs. Low category cables will -due to bad symmetry and impedance fluctuations- transform a part of DM current into CM current. The measurement quantity that goes with these is called LCL - value (Longitudinal Conversion Loss). This will create radiation, independent of the quality of the screen, or connection to the enclosure. Another topic that rises here is where to place a auxiliary equipment that might me necessary to initiate and maintain any network traffic. If the cable set up requires 80H/80V cm and a termination, the AE needs to be placed on the ground plane of the measurement volume (if it fits), and its radiation will thus be included in the measurement results. So we need a way to "get out" of the test volume, preferably outside the screened room without this cable part contributing to the emission profile. For this reason a "duct" or metal conduit is used to guide the cable outside the room, and on the room's screen a high quality (and complex) filter is required to filter off any CM current from outside. The duct starts at the centre of any rotating platform, and the cabling from the edge of the test volume to the duct may be used for terminating the cabling using ferrites (this is not yet in the standards, I know) SO EUT inside, within measurement volume, and AE outside measurement volume filtered on the room's screen. On an OATS this is somewhat confused. I usually extend cabling to such an amount that the AE is definitely "out of sight" behind the EUT , and do some experiments with cable lay out left and/or right of EUT. I hope this has been of inspiration to you.... Any comments or additions ???? Anyone ??? Regards, Gert Gremmen Approvals manager ====================================================== ce-test, qualified testing Member of EMC committee CENELEC/IEC + Independent Consultancy Services + Compliance Testing and Design for CE + Improvement of product quality and reliability + Testing services according to: Electro magnetic Compatibility 89/336/EC Electrical Safety 73/23/EC Medical Devices 93/42/EC Radio & Telecommunication Terminal equipment 99/5/EC Website: www.ce-test.nl Phone : +31 10 415 24 26 Fax : +31 10 415 49 53 ====================================================== FW>-----Original Message----- FW>From: [email protected] [mailto:owner-emc- FW>[email protected]] On Behalf Of [email protected] FW>Sent: vrijdag 28 januari 2005 19:46 FW>To: [email protected]; [email protected] FW>Subject: RE: OATS testing of network switch FW> FW>Dave, FW> FW>If the cables are shielded and the shielded are intimately connected to FW>the Network Switch shielded enclosure (you have a shielded enclosure, I FW>assume?), AND the loads do not excite common mode current, then these FW>cables will not radiate. No common mode current- no radiation. FW> FW> Dave FW> FW>-----Original Message----- FW>From: [email protected] FW>[mailto:[email protected]] On Behalf Of Garnier, David S FW>(GE Healthcare) FW>Sent: Friday, January 28, 2005 11:39 AM FW>To: [email protected] FW>Subject: OATS testing of network switch FW> FW>Hello, FW> FW>We don't do routine OATS evaluation of stand-alone ITE products FW>(other than items integrated in to our products,) so pardon my FW>ignorance or poorly thought out questions. FW> FW>I have a couple of 8 port gigabit network switches I would like to FW>evaluate on the OATS. ANSI C63.4-2003 gives me some guidance FW>in this area, such as "each I/O port being terminated in typical load FW>of actual usage." FW> FW>What kind of a circuit would I used for IEEE 802.3AB load? I am FW>thinking transformer (?) passive, but placed where??? Directly FW>plugged into the 8 port switch or placed on the end of short (length?) FW>Ethernet cable? If I use short cables, how do I not create 8 small FW>horizontal antennas 1 meter high? Comments? FW> FW>Regards, FW> FW>Dave Garnier FW> FW>---------------------------------------------------------------- FW>This message is from the IEEE Product Safety Engineering Society FW>emc-pstc discussion list. Website: http://www.ieee-pses.org/ FW> FW>To post a message to the list, send your e-mail to [email protected] FW> FW>Instructions: http://listserv.ieee.org/listserv/request/user-guide.html FW> FW>List rules: http://www.ieee-pses.org/listrules.html FW> FW>For help, send mail to the list administrators: FW> FW> Scott Douglas [email protected] FW> Mike Cantwell [email protected] FW> FW>For policy questions, send mail to: FW> FW> Richard Nute: [email protected] FW> Jim Bacher: [email protected] FW> FW>All emc-pstc postings are archived and searchable on the web at: FW> FW> http://www.ieeecommunities.org/emc-pstc FW> FW>---------------------------------------------------------------- FW>This message is from the IEEE Product Safety Engineering Society FW>emc-pstc discussion list. Website: http://www.ieee-pses.org/ FW> FW>To post a message to the list, send your e-mail to [email protected] FW> FW>Instructions: http://listserv.ieee.org/listserv/request/user-guide.html FW> FW>List rules: http://www.ieee-pses.org/listrules.html FW> FW>For help, send mail to the list administrators: FW> FW> Scott Douglas [email protected] FW> Mike Cantwell [email protected] FW> FW>For policy questions, send mail to: FW> FW> Richard Nute: [email protected] FW> Jim Bacher: [email protected] FW> FW>All emc-pstc postings are archived and searchable on the web at: FW> FW> http://www.ieeecommunities.org/emc-pstc 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
