Was kind of hoping Neven would speak up here. The digital and the I/O grounds should be separate. You are going to mote between the main pcb and the I/O connections. The common mode chokes span the mote. Neither the power or the digital ground planes should extend under the mote. You'll probably want to put a ferrite between + stuff and the isolation transformer center taps, and you can experiment with the filtier caps between the CT and digital ground for noise reduction. ON the output of the transformers you had to watch the capacitance between the center tap and the I/0 ground. You will violate the Ethernet specs pretty quickly if you put much more than 1 nfd, 2kV but you can play with the R values a bit. K. Make sure the Ethernet connector is tied to the I/O ground - don't break off the mounting tabs. You need 1500 Vdc between signal and ground, pin-pin and between I/O ground and PCB digitial ground. Adjust the mote spacing to get this right. In the past I've found it helpful for this split to be as small as possible but still meeting the 1500 Vdc dielectric gap.
The "broad band" noise you describe suggests to me that you do indeed have some sort of ground problem going on. Since this is EMI you'll do all the right things and still find something hosing you up - plenty agrevating but good luck. Gary From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Thursday, April 27, 2006 10:47 AM To: [email protected] Subject: Re: 100 Base-T chassis ground In message <[email protected]>, dated Thu, 27 Apr 2006, Alex Horvath <[email protected]> writes >In my experience it's sort of a no-brainer to have a separate chassis >ground. My question is would you expect that the problems we are having >could be caused by the common ground implementation? It's very likely. The digital 'ground' is sharing its disturbing energy with the output cable in common-mode. Just modify a sample to have a 'clean' ground on the output side of the port and see if it helps. In EMC, a gram of test can outweigh a tonne of theory. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - 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 - 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
