For a typical logic design, there is a short period during the switching interval when both differential outputs are off. This prevents high current spikes through the transformer that overlap would cause, but since this makes both outputs high at the same time it is also a common mode component. Current steering designs don't have this limitation, but not all ethernet chips use current steering output drivers. More often you have a pair of open drain FETs tied to ground, hence the anti-overlap feature and associated CM noise.
Orin Laney On Mon, 4 Oct 2010 18:06:12 +0100 John Woodgate <[email protected]> writes: > 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]>
