Prakash, I am not sure. Whether you want to or nead to go through connectors or socket boundaries may have something to do with it. Maybe list members on those committees could comment on the reason. Regards, Istvan From: "Chauhan, Prakash" <pchauhan@xxxxxxxxxxx> Date: Thu May 25 08:37:01 CDT 2006 To: istvan.novak@xxxxxxxxxxx, jain.nitin@xxxxxxxxx, si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: AC coupling Capacitor Hi Istvan, I have seen design guides that suggest that for Ethernet (Gig-E and Xaui) the caps should be close to the receiving end while for PCI express these should be closer to the transmit side (4.75 inch max suggested in both cases). Both these are 8B/10B encoded, 100 ohm differential, serial links and it is not clear to me why this should be different for these two serial interfaces (given that the PCB loss and impedance discontinuity due to the caps should affect both types of links in very similar manner). Perhaps it has something to do with the preemphasis and receive equalization characteristics of the driver/receiver?! =20 --Prakash. =20 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of istvan.novak@xxxxxxxxxxx Sent: Thursday, May 25, 2006 8:10 AM To: jain.nitin@xxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: AC coupling Capacitor Nitin, Regular surface-mount DC-blocking capacitors will create discontinuities in the differential lines, because most of our traces today are much narrower than the capacitor pad and body. The typically strongest discontinuity is the parallel body capacitance to ground. If you use two capacitors along eachline, you get two discontinuities, so do it only if there is a strong reason. =20 Regarding their location along the lines: as a first-cut approach, their location should not (and does not) matter, you just want to place the capacitors on both lines at the same location symmetrically so that mode conversion is minimized. If you (have to) take a closer look, the location of the discontinuity along the line may matters somewhat as it interacts with the losses and non-ideal impedances of the lines and transmit/receive impedances. If this matters, you need a simulator to find the answer. Regards, Istvan Novak SUN Microsystems From: jain.nitin@xxxxxxxxx Date: Thu May 25 06:03:58 CDT 2006 To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] AC coupling Capacitor Hello, For a gigabit differential pair AC coupling capacitor is used. But what exactly is the use of AC coupling capacitors? I believe that they are used when the bias voltage of driver & receiver are different.=3D0D Also where exactly AC coupling capacitors should be placed when the differential trace is running on a backplane from one daughter card to other (at driver end or at receiver end). What will happen if two AC coupling caps are used. One at driver side and other at receiver side. With regards, Nitin The information contained in this electronic message and any attachments to=3D this message are intended for the exclusive use of the = addressee(s) and=3D may contain proprietary, confidential or privileged information. If you=3D are not the intended recipient, you should not disseminate, distribute or=3D copy this e-mail. Please notify the sender immediately and destroy all=3D copies of this message and any attachments.=3D0D WARNING: Computer viruses can be transmitted via email. The recipient=3D should check this email and any attachments for the presence of viruses.=3D The company accepts no liability for any damage caused by = any virus=3D transmitted by this email. =3D0D www.wipro.com ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //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 =20 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //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 =20 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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