Joel, 802.3ap only specifies that AC coupling is required between transmitter and receiver. If there is no high common potential that could be induced onto the Giga Ethernet HS pair, a cap coupling would suffice. For active channel usually external termination is not required - they are taken care by the ASIC. For idle pairs on transform (if you do have reason to use them), personally I think a 100 Ohm across the pair with one lead of the pair connected to ground via a hi potential cap would be good. I also wonder how the Bob-Smith termination was verified in the first place, and whether the article "Bob_Smith_Termination_vs_Proper_Termination ", which is against this termination method, is correct. Anyone has done such measurement on CAT6E or other cables? I might spend some time making a VNA measurement on the CM impd. of the pair... Best Regards, Sherman Chen Signal Integrity EMC Global Hardware Engineering Tel: +86 21 60951100-3329 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Eric Tollefson Sent: Wednesday, September 17, 2014 7:52 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: si-list Digest V14 #214 Joel, In addition to Steve's reference to 802.3ap, I would also refer you to ANSI VITA 46.6 (if you have access), which discusses Gigabit Ethernet over VPX backplanes. There are two standard variants, one which uses the "standard" 4-pair 1000BASE-T format and the other which uses the 2-pair 1000BASE-KX discussed in 802.3ap (basically, SERDES Ethernet). VITA 46.6 calls for magnetics on BASE-T ports, which I presume is to ensure drop-in compatibility of standard Ethernet parts and interfaces, and 100-ohm differential traces on the backplane with no coupling caps. Unfortunately, I don't have any particular expertise in designing termination for Ethernet ports, so I can't offer much there. Hope that helps, Eric Msg: #7 in digest > Date: Tue, 16 Sep 2014 15:24:32 -0700 > Subject: [SI-LIST] Question about common mode termination of Ethernet > transform > From: Joel Brown <joel@xxxxxxxxxx> > > I am working on a design that has gigibit Ethernet running between two > cards connected by a backplane using 100 ohm differential traces > approximately 12 inches in length. My understanding is that actual > data rate per pair is 125 Mb/S. My first thought was just to use AC > coupling capacitors between the two parts (Ethernet controller and Ethernet > switch). > Since Ethernet parts are usually connected through transformers to > provide isolation and filtering I decided I should include the option > for either the AC coupling capacitors or a transformer. > Most of the time the cable side of Ethernet transformer center taps > have a common mode termination of 75 ohms connected to ground through > a 1000pF capacitor (aka as the Bob Smith termination). I have read > some things that claim 75 ohms isn't the optimum value and that the > common mode termination may not be needed at all (see > people.eecs.ku.edu/~callen/713/Bohnert.pptx) > I am sure that 100 differential traces are more like 25 ohms common > mode impedance, so I am wondering if I should use 25 ohms instead of > 75 ohms or just not have any common mode termination. > Thanks - Joel > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu