In message <[email protected]>, dated Wed, 29 Sep 2010, "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> writes:
>Most of the time the problem is that the ground point centertap > >of the Ethernet transformer is insufficiently grounded. This may well be a practical finding, but ground really has no function for a balanced transmission line, which is what Ethernet (and several other techniques) is based on. Ground at one end is never at the same potential as ground at the other end, especially at RF. The suppression of EMI by balanced transmission depends on: A) impedance balance - the impedances between each terminal of the line at the sending end to ANY reference point must be very closely equal, and the same at the receiving end. The four impedances then form a balanced bridge, with the wanted signal across one diagonal and any unwanted signal (CM interference) across the other, so it has no effect on the wanted signal. This is often quite unrecognised. B) signal voltage (or current) balance - the signals on the two conductors must be equal in magnitude and exactly opposite in polarity (so zero-crossings occur at the same instant). The emissions from each conductor are opposing and, since the two conductors are very close together, at a reasonable distance (e.g. >30 cm) they interfere completely destructively. This affects EM emission only and NOT the integrity of the link, which does NOT require equal and opposite signal voltages. > >If that is not the case, it is the chipset, that has not been designed >with EMI in mind. If the chipset generates, from a source that does not comply with A) above, a signal that does not comply with B) above, or if just ONE condition is violated, the emission problem is probably insoluble. Change to another chipset. -- 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]>
