John, I am sorry to confuse you (and probably others), A bad ground can couple CM energy through the center tap of the Ethernet transformer right into both (in-phase) wires of transmit or receive pair.
Insufficiently grounded can IMHO also mean : connected to a bad ground instead of a good ground. The problems are often caused by segration of a motherboard PCB with analog signal processing, power supply supply and a plug-in "PC-board" with a Ethernet connection on it. If the ground of the plug-in board is not RF decently connected to the mother board the ground of the plug-in board is dirty and a "bad ground". And as the motherboard is connected to the enclosure and most of the other cabling too, that is a clean reference by definition. Most plug-in processors contain just a few ground pins were in fact a full row along the side of the board is needed so as to create a real low impedance path to ground. Not connecting the Ethernet transformer center-tap to ground of the processor would allow the CM current from the chipset Rx and Tx pair (and believe me they have) to the cable without attenuation. Regards, Ing. Gert Gremmen [email protected] www.cetest.nl Before printing, think about the environment. -----Oorspronkelijk bericht----- Van: [email protected] [mailto:[email protected]] Namens John Woodgate Verzonden: Thursday, September 30, 2010 9:44 AM Aan: [email protected] Onderwerp: Re: [PSES] EMI issue with Ethernet In message <[email protected]>, dated Thu, 30 Sep 2010, "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> writes: >No John, while what you state is correct, >the coupling from CM currents out of the ground >via the center tap of the transformer is one common way of creating >problems. Now I'm completely confused. Earlier in this thread, you wrote: Most of the time the problem is that the ground point center-tap of the Ethernet transformer is insufficiently grounded. Now you seem to be saying that the equipment 'ground' can be a source of CM energy, and I agree it can be. Therefore, it would be better NOT to ground the centre-tap. >And much more likely then balancing problems. Balancing problems will >immediately be detected by poor transfer rates and errors. CM current >will go unnoticed. I don't think so. The balance has to be a long way off before it noticeably affects the signal transfer, but only a little way off to significantly increase emission levels. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK If at first you don't succeed, delegate. But I support unbloated email http://www.asciiribbon.org/ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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]>
