Vinu, Can you show me a capacitor that works at 3.125 Gb/S for decoupling? Lee ----- Original Message ----- From: Vinu Arumugham To: Scott McMorrow Cc: Lee Ritchey; Michael Rose; si-list Sent: 9/15/2008 12:24:44 PM Subject: Re: [SI-LIST] Re: plane-to-plane decoupling Scott, I was not suggesting that capacitors connecting split planes were a "clean" solution. I just wanted to point out that Lee's statement, "There are no capacitors that work at 3.125 Gb/S for decoupling.", is not entirely true. Thanks, Vinu Scott McMorrow wrote: Vinu Not quite. As long as there is a ground plane underneath, and close to, the capacitor, some return path energy will get across. But, there is a mismatch in impedance between the capacitor and plane, and here is still an inductive loop for the return energy to get to the capacitor. Because of this, quite a bit of the common mode energy will be reflected back into the near end power/ground plane cavity. 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 Vinu Arumugham wrote: Lee, The capacitor used for AC coupling on the signal path, should be able work just as good if it were placed on the return path as a decoupling capacitor for that signal. Each signal trace will of course need a dedicated capacitor to avoid additional crosstalk. Thanks, Vinu Lee Ritchey wrote: Michael, There are no capacitors that work at 3.125 Gb/S for decoupling. The way to provide this path is by placing the planes close to each other. I use 3 mils all of the time for this purpose. Works greast! Lee Ritchey Speeding Edge [Original Message] From: Michael Rose <mrose@xxxxxxxxxxxx> To: SI-List <si-list@xxxxxxxxxxxxx> Date: 9/15/2008 10:01:17 AM Subject: [SI-LIST] plane-to-plane decoupling I looking for some suggestions regarding decoupling between co-planar plane splits. I'm working on a backplane with a number of 3.125Gbps diff pairs. I've specified a dual stripline stackup assigned as follows: 1 - P 2 - G 3 - S 4 - S 5 - P 6 - G and so on Some diff pairs on L4 will cross power plane splits (actual different power sources and loads) and I wanted to provide an effective AC path for any common-mode return currents. I was thinking about placing some nearby decoupling caps from plane-to-plane across the split. Do you think it would be better to decouple from plane-to-ground on both sides to steer the current through the L6 ground layer? L5 and L6 are already coupled through the inter-plane capacitance (they're about 4mils apart). Which will provide a lower inductance path? Mike ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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