Most of the time the problem is that the ground point centertap of the Ethernet transformer is insufficiently grounded.
If that is not the case, it is the chipset, that has not been designed with EMI in mind. A CM coil may help, but from 64 dB to 30 dB down at 125 MHz is too much for just a CM-coil in my opinion. Assuming the PCB is 4+ layers and 1 layer is fully ground, (when I say fully, it means NO interruption whatsoever) I guess that your Ethernet port is placed as a plug-in board on a kind of motherboard construction. Most of these OEM modules lack sufficient ground connections on their edge connectors, creating a huge ground EMI voltage that smoothly couples out through the transformer (via centertap) as CM current on the Ethernet wire. Gert Gremmen ce-test, qualified testing Van: [email protected] [mailto:[email protected]] Namens deepak Engineer Verzonden: woensdag 29 september 2010 17:15 Aan: [email protected] Onderwerp: EMI issue with Ethernet Hello Expert, I am facing some problem with Emmision from Ethernt port. case is that chassis groung and Signal ground are seperated by 0ohms but still EMI radiation above the class A limit (40bd), and getting a big spike (at 125MHz) upto 64bd.. 1 )for chassis ground.. I have four holes on the board for assembly to chassis.. I am planning to use three holes to connect to signal ground while the last hole(forth) connect to chasis ground of Ethernet jack. do u think it imbalance the system..on PCB there is only one link(0ohms resistor) between the signal groung and chassis ground.. 2) somebody suggested to use external CM coil with RJ45 jack(which already have internal magnetic with CM coil). and pull up on RX/TX -PN line with 50 Ohms to maintain the control impedence. CM coil part #:acm2012e http://www.tdk.co.jp/etvcl/spara/cmf.htm#ACM2012 <https://webmail.bolton.ac.uk/exchweb/b n/redir.asp?URL=http://www.tdk.co.jp/etvcl/spara/cmf.htm%23ACM2012> with this coil they are claiming that it resolved the EMI issue.. but I am bit suspicious about this device due to following reasons: 1. only device supports are USB and firewire only 2. USB differential Vpp = 0.400 while Ethernet supports Vpp=1v If they remove the pullups from RX/TX signals with CM coil near the RJ jack..then they cannot see any signal on the Ethernet port.. is it good practise to use two cM coils? is this a right device to support the Ethernet std.? thanks in advance, Deepak - ---------------------------------------------------------------- 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]>
