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

  • From: Richard Jungert <r_jungert@xxxxxxxxxxx>
  • To: Scott McMorrow <scott@xxxxxxxxxxxxx>, <leeritchey@xxxxxxxxxxxxx>, <mrose@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 15 Sep 2008 12:06:39 -0700




Hello.

If anyone is interested in putting 100 ohm differential shielded cables ( also 
ribbonized ) and connectors on backplane for 3.125gb/s signaling please let me 
know. I have the solution in a PDF file on my computer ready to go.  Put the 
sensitive signals on small minature differential coax cable and zip the problem 
is solved. Run them 3 feet long if you want no problem.  Go Board to connector 
to differential cable to connector and back to board. It works great.

Richard Jungert

> Date: Mon, 15 Sep 2008 14:45:19 -0400
> From: scott@xxxxxxxxxxxxx
> To: vinu@xxxxxxxxx
> CC: leeritchey@xxxxxxxxxxxxx; mrose@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: plane-to-plane decoupling
> 
> 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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