Vinu Your statement: '"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. ' Is just not true. There are significant electromagnetic differences between a capacitor used for AC coupling in a through configuration and one used for power plane shunt applications. Hint: follow the magnetic fields 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: > 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