Steve, "A single capacitor will still impose a substantial bump in the impedance for a single signal.", yes, but the same applies if that capacitor were in an AC coupling configuration. It certainly has an impact but I don't see it being "unusable" as Lee put it. "A typical plane cavity will impose a much smaller bump.", yes, but if the 1 sq. in. cavity has to support several links, then the difference between (1) and (2) is not as big. Thanks, Vinu steve weir wrote: > 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 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