The center tap attenuates CM coming from the transceiver. The assumption that the ethernet signal is "true-differential" as coming from the source is not a good assumption.
You are right, it would not help if the issue were transformer imbalance. Conencting the center tap through a cap to GND would make it worse. BTW, you need a cap in any way, you can't connect the center tpa directly to GND. EVen for the voltage-mode Tx, the pairs coming out of the transceiver are DC-biased. N ----- Original Message ----- From: "John Woodgate" <[email protected]> To: [email protected] Sent: Monday, October 4, 2010 10:06:12 AM Subject: Re: [PSES] EMI issue with Ethernet In message <271069724.1576253.1286204923170.javamail.r...@sz0149a.emeryville.ca.mail .comcast.net>, dated Mon, 4 Oct 2010, [email protected] writes: >The cap at the center tap does indeed provide considerable CM rejection >in both cases, up to somewhere in the 50MHz-100MHz range. At these >frequencies it is very much needed, and a recommendation to remove the >center tap caps was not such a good recommendation. The (im)balance of >these transformers is typically not terrible, i.e. the center-tap cap >helps and does not increase the emission due to the transformer imbalance. Where does the CM signal that is attenuated by the capacitor come from? If it were due to transformer imbalance, the centre-tap would probably not be in the true electrical centre, so grounding it might make the emission lower ... or higher. -- 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]>
