Ed Price wrote: >> I have a twisted-pair Ethernet receive and a twisted-pair Ethernet transmit set of 4 wires penetrating what would otherwise be a shielded box. Internal box noise, as low as 35 MHz, is getting out along this path. I put a 4700 pF feedthrough capacitor on each of the four lines. That worked pretty well, so well that the Ethernet shut down. Does anyone have a guideline about how much capacitance, L-L & L-G, that is typically OK? <<
Hi Ed, Yeah, that bypasses all right. It bypasses everything. You need, as was already suggested, to look for the source, and failing that, to look for common mode filtering. Shielded twisted pair can help; we used it to meet Class B where I worked years ago, but IIRC we were violating the Ethernet specification of the time by grounding the shield to the chassis. I don't know if that requirement is still in place but if it is, you can connect the shield to chassis with a suitable capacitance. You may find that you have an Ethernet filtered connector that requires a chassis connection of its own to work (center tap bypassing) and that the PWB needs to be pushed towards the chassis wall for it to make contact. Have fun! Cortland Richmond, KA5S "I" am the difference between "compliant" and "complaint!" - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
