Vinu, I think it is more mind set than anything else. Let's put some additional numbers to this: A really well-mounted 0402 capacitor is going to exhibit 0.5nH or more mounted inductance. At 1.5GHz that's 5Ohms. Ignoring resonances, a typical 3 mil cavity even 1" on a side is going to exhibit impedance in the 100's of milliOhms. So: 1) A single capacitor will still impose a substantial bump in the impedance for a single signal. 2) A typical plane cavity will impose a much smaller bump. Since we are talking differential signaling, the even-mode signal components should shrink at all frequencies below Fknee, so we don't need tons of charge storage. Cavity is going to be more effective. (But I would far prefer a contiguous return in the first place.) Steve Vinu Arumugham wrote: > 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 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>>>>>>> >>>>>>> >>>>>> ------------------------------------------------------------------ >>>>>> 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 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