Chris You are thinking even-mode, not common mode. The differential signal is carried on most microstrip traces as 90% even-mode and 10% odd-mode. However, it is still a differential excitation, and will "know" to become fully odd-mode as it dereferences at a pad with plane relief. This can be further tuned and optimized in a 3D full-wave solver. With common mode excitation, both signal traces carry the same signal with the same polarity. This is propagated as two even-mode waves. When the common-mode hits a plane relief on both traces, there can be a huge discontinuity, if the return path is not managed well. This can cause EMI. Scott Scott McMorrow Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 (401) 284-1827 Business (401) 284-1840 Fax http://www.teraspeed.com Teraspeed® is the registered service mark of Teraspeed Consulting Group LLC Cotton, Chris wrote: > > Thanks for the clarifications. But as far as the <10% of the energy > typically being common mode, wouldn't this depend how tightly or > loosely coupled the differential routing is? Although I haven't > quantified it for loosely coupled routing I would expect more common > mode energy than 10%. > > > > ------------------------------------------------------------------------ > > *From:* Scott McMorrow [mailto:scott@xxxxxxxxxxxxx] > *Sent:* Wednesday, November 12, 2008 10:35 AM > *To:* Cotton, Chris > *Cc:* David Instone; Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx > *Subject:* Re: [!! SPAM] [SI-LIST] Re: High Speed traces and connectors > > > > Chris, > > If the plane relief under the pads is done correctly, the differential > mode can be tuned to nearly 100 ohms, and the common mode can be > well-contained. In general, we don't much care whether the common > mode impedance is well-matched, since it represents <10% of the signal > energy typically. However, it is possible to provide a reasonable > impedance match and control of the return path. These sorts of > optimizations are strongly dependent upon the location of the plane > underneath the traces and pads. > > regards, > > Scott > > > Scott McMorrow > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > http://www.teraspeed.com > > Teraspeed® is the registered service mark of > Teraspeed Consulting Group LLC > > > > Cotton, Chris wrote: > > Thanks David. Yes. I agree. I think we are all directly or indirectly talking > about impedance here. My point is that differential signals are not purely > differential mode. > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx> on > behalf of David Instone > Sent: Wed 11/12/2008 5:56 AM > To: Cotton, Chris > Cc: Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx > <mailto:si-list@xxxxxxxxxxxxx> > Subject: [SI-LIST] Re: High Speed traces and connectors > > The width of the fingers is set by the requirement to mate with the PCI > connector. The required width is such that if you have any plane > anywhere under them their differential impedance will be much lower than > the 100 Ohms required, thus in the area of the PCB which is under those > fingers there must not be any planes, or traces either. (That's a plane > anywhere in the stackup under those fingers, the board has a thickness > defined by the need to mate with that connector.) You can easily see > the effect on a TDR. Getting the differential impedance right is much > more important than any effect on the common mode. > > Regards > Dave Instone > +44 (0)1235 824963 > > OXFORD SEMICONDUCTOR LIMITED > 25 MILTON PARK > ABINGDON > OXFORDSHIRE > OX14 4SH > Registered in England no 2733820 > Registered Address: As above > > > > Cotton, Chris wrote: > >> Correction - The differential mode current should not be >> affected. >> >> -----Original Message----- >> From: si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx> >> [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On Behalf Of Cotton, Chris >> Sent: Tuesday, November 11, 2008 1:31 PM >> To: Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx >> <mailto:si-list@xxxxxxxxxxxxx> >> Subject: [SI-LIST] Re: High Speed traces and connectors >> >> This is not something that I generally do. That having been said, I >> assume they are only asking you to remove the immediate or adjacent >> pwr/gnd plane. If this is the case, the rationale is that the connector >> pads are much larger than the trace and will look like a capacitive >> load. Removing the immediate plane will lower the capacitance. >> >> The CM return current has to find a GND pin anyway so I don't think it >> will be severely impacted. The differential mode current will be >> affected but difficult to quantify to what degree. I would not make >> these changes without them providing a technical rationale and >> assessment of risk. >> >> Chris >> >> -----Original Message----- >> From: si-list-bounce@xxxxxxxxxxxxx <mailto:si-list-bounce@xxxxxxxxxxxxx> >> [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On Behalf Of Lakshmi N. Sundararajan - PTU >> Sent: Tuesday, November 11, 2008 12:58 PM >> To: si-list@xxxxxxxxxxxxx <mailto:si-list@xxxxxxxxxxxxx> >> Subject: [SI-LIST] High Speed traces and connectors >> >> Hi Team, >> I had a curious review mentioned on my board for high speed traces >> for which I could not get a satisfactory explanation. >> >> For all high speed traces from a connector, the review mentioned >> to remove GND/PWR plane from under the *signal pads* under connector. >> Even for the PCIE connector, for all the signal pads, the GND/ >> PWR plane was requested to be removed. >> Now, these are 6Gig and 5Gig high speed signals. If there isnt GND/PWR >> plane then definetely I would assume there isnt any reference plane for >> return current. >> >> So, can you experts suggest, whether this is a right thing to do >> and if so why? >> >> Thanks >> -LN >> ------------------------------------------------------------------ >> To unsubscribe from si-list: >> si-list-request@xxxxxxxxxxxxx <mailto: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 <mailto:si-list-request@xxxxxxxxxxxxx> with >> 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> 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 <mailto: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 <mailto:si-list-request@xxxxxxxxxxxxx> with >> 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> 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 <mailto: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 <mailto:si-list-request@xxxxxxxxxxxxx> with >> 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> 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 <mailto: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 <mailto:si-list-request@xxxxxxxxxxxxx> with > 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > 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 <mailto: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 <mailto:si-list-request@xxxxxxxxxxxxx> with > 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > 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 technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu