The way DC is coupled to the pairs, using center taps on the cable-side, potentially allows the board and power supply noise to circumvent the CM chokes in the magnetics, and end up being radiated or conducted by the cable - as if there is no EMI suppression there. It is very much dependent on the particular design. With prudent design - e.g. extra EMI suppression in the POE-feed area (plus other general techniques), POE can work just fine wrt conducted and radiated emission. Neven
-------------- Original message -------------- From: "Sterner, David [S&FS]" <[email protected]> > Has anyone tested RF-conducted emissions on Power-Over-Ethernet? It > uses 8 lines, but 4 lines are DC. > > What were the results? > > > _ Honeywell > David W. Sterner > Quality Assurance Engineering > Security & Custom Electronics > 165 Eileen Way > P.O. Box 9035 > Syosset, NY 11791 > Phone: (516) 921-6704 x6970 > Fax # (516) 364-6953 > [email protected] > > [email protected] > > Electromagnetic Compatibility > IEEE 05888136 > > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of >! ; neven [email protected] > Sent: Monday, May 21, 2007 8:26 PM > To: [email protected] > Subject: Re: EN 55022:1998+A!+A2 vs EN 55022:2006 > > Their recommendation puzzles me as well. I believe it comes from not > understanding how Ethernet front-end is designed. Ghery is actually the > first person in ALL my discussions on this subject who points out what I > think is correct. > > I still do not think ISN/CDN is appropriate and I do not get why V-clamp > and I-clamp method would only be alternative and why ISN would prevail > in case of dispute, especially since using the clamps can only overtest > and never undertest, although you can often experience higher > ***apparent*** emission with ISN than with the clamps - which does not > make sense unless something is wrong - and I believe it is. > > Neven > > > -------------- Original message --------! ------ > From: [email protected] > > Hi Neven, > > I have talked to a manufacturer of the TLISNs and they > recommended using a two balanced pair for 10/100 Ethernet and four > balanced pair for Gigabit. The two balanced pair will accept 8 wires > but only 4 of them go through the network while the others are passing > straight through. It makes sense to me to use the 4 balanced pair for > all Ethernet. Ghery pointed out that the TLISN is decided by how many > wires the cable has and not the number of wires used. Why would they > suggest a two balanced pair unit for 10/100 when all the Ethernet cables > have 8 wires? > > Thanks, > Tim Pierce > > > > > ________________________________ > > See what's free at AOL.com > . > - > ----------------------------------! ------- ----------------------- This > message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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