[SI-LIST] Re: plane-to-plane decoupling

Vinu,

I stand by my statement and have data to support it.

Lee


> [Original Message]
> From: Vinu Arumugham <vinu@xxxxxxxxx>
> To: Scott McMorrow <scott@xxxxxxxxxxxxx>
> Cc: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>; Michael Rose
<mrose@xxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx>
> Date: 9/15/2008 4:26:45 PM
> Subject: [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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