FYI,
I see a lot of noise on this topic, so I wonder if I can help a little. I do not have the possibility to drill deep here. There are two types of Ethernet transceivers with regards to the Tx design. They are current-mode and voltage-mode. Current-mode transceivers feed their output from the center-tap of the transformer, and therefore need the center-tap connected to the power supply. They use current-steering logic, maybe this is too much detail for here. The filtering suggestion (ferrite bead in series between the voltage and the center tap) applies only to this type of the drive, and it is a good recommendation (for this type of tx). It protects the common-mode point of the transformer, i.e. the center tap, from the power supply noise. A recommendation to put a CM choke between the transformer and the Ethernet transceiver is a bad recommendation if this type of the transceiver is used, because it can distort the Tx signal. Also, in general, the magnetics packages that contain transformers and CM chokes should be sufficient, without externally added EMI-suppression devices. In vast majority of cases. The voltage mode transceivers do not need connection of the center tap to power supply, thus only a cap from the center tap to GND is sufficient. 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. After somewhere in the 50MHz-150 MHz range there are other more significant factors which affect the EMI-rejection and the center-tap bypassing (cap to GND) is not of a considerable importance at the higher frequencies. The most limiting factor is the total inductance in series between the center tap (measured fomr the body of the transformer core not from the pin of the package) and through the capacitor to GND. When this impedance gets too high the center tap does not bypass the CM energy any more, which happens in the aforementioned frequency range. Above 100MHz (+/-), factors such as construction of the magnetics including the location and type of the CM choke, Ethernet connector pins, layout and factors that allow coupling noise across the magnetics, play more role than the center tap. Even if all these factors are tuned to the best possible performance, the imbalance of the connectors and the cables are the final limiting factors, which one can hardly affect - at least practically, to the EMC performance of this type of interface. Going back to the OP, he has a major issue somewhere in his design that he needs to resolve first. Best, N ----- Original Message ----- From: "John Woodgate" <[email protected]> To: [email protected] Sent: Monday, October 4, 2010 2:49:01 AM Subject: Re: [PSES] EMI issue with Ethernet In message <[email protected]>, dated Mon, 4 Oct 2010, "Pawson, James" <[email protected]> writes: >If the magnetics you use for your Ethernet have a centretap on the >input side of the transformer (sometimes connected to a voltage rail, >sometimes to ground) then you should try filtering the supply to this >with an RC or bead-cap filter. You mean the sending end transformer, I suppose. Exactly how do you connect the filter? After all, if the CT is grounded, and you introduce a resistor or bead into the grounding wire, where do you connect the 'cold' end of the capacitor? It surely can't go back to ground! -- 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]>
