[SI-LIST] Re: plane-to-plane decoupling
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Mon, 15 Sep 2008 14:14:51 -0700
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
> >>>>>
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