Vinum, You are forgetting the series inductance of the capacitor. It doesn't hurt much when used as an AC coupling capacitor becasuse it is part of a transmission line that has substantial inductance along its length. When you are trying to build a PDS that same inductance renders the capacitors unusable at frequencies much avove 150 Mhz. > [Original Message] > From: Vinu Arumugham <vinu@xxxxxxxxx> > To: <leeritchey@xxxxxxxxxxxxx> > Cc: Scott McMorrow <scott@xxxxxxxxxxxxx>; Michael Rose <mrose@xxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx> > Date: 9/15/2008 4:02:18 PM > Subject: [SI-LIST] Re: plane-to-plane decoupling > > Lee, Scott, > I don't see the difference whether you want to look at the capacitor as > a series or shunt element. A decoupling capacitor may look like a shunt > element when it is part of a PDN but that PDN could serve as a series > element in the signal return path. So, if a capacitor is acceptable in > an AC coupling role in the signal path, the same capacitor should be > acceptable as part of a PDN that is a return path for that signal. In > other words, think of it as an AC coupling capacitor for the return path > instead of the signal path (US Patent 7262974). > > For this application, the capacitor only needs to support ~10mA of > switching current at 1.5GHz, and a few tens of mV drop across its > impedance would be acceptable. > > Thanks, > Vinu > > Lee Ritchey wrote: > > Scott, > > > > I suspect you are right, but the thread was about decoupling power planes. > > > > > > > >> [Original Message] > >> From: Scott McMorrow <scott@xxxxxxxxxxxxx> > >> To: <leeritchey@xxxxxxxxxxxxx> > >> Cc: Vinu Arumugham <vinu@xxxxxxxxx>; Michael Rose <mrose@xxxxxxxxxxxx>; > >> > > si-list <si-list@xxxxxxxxxxxxx> > > > >> Date: 9/15/2008 12:53:27 PM > >> Subject: [SI-LIST] Re: plane-to-plane decoupling > >> > >> Lee > >> I believe that Vinu is speaking of using a capacitor as a series pass > >> element, rather than as a shunt element. > >> > >> 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 > >> > >> > >> > >> Lee Ritchey wrote: > >> > >>> Vinu, > >>> > >>> Can you show me a capacitor that works at 3.125 Gb/S for decoupling? > >>> > >>> Lee > >>> > >>> > >>> > >>> ----- Original Message ----- > >>> *From:* Vinu Arumugham <mailto:vinu@xxxxxxxxx> > >>> *To: *Scott McMorrow <mailto:scott@xxxxxxxxxxxxx> > >>> *Cc: *Lee Ritchey <mailto:leeritchey@xxxxxxxxxxxxx>; Michael Rose > >>> <mailto:mrose@xxxxxxxxxxxx>; si-list <mailto:si-list@xxxxxxxxxxxxx> > >>> *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 > >> > >> > > > > > > > > > > > > ------------------------------------------------------------------ > 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