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

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "Vinu Arumugham" <vinu@xxxxxxxxx>, "Scott McMorrow" <scott@xxxxxxxxxxxxx>
  • Date: Mon, 15 Sep 2008 12:48:54 -0700

Vinu,
Can you show me a capacitor that works at 3.125 Gb/S for decoupling?

Lee


----- Original Message ----- 
From: Vinu Arumugham 
To: Scott McMorrow
Cc: Lee Ritchey; Michael Rose; si-list
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
  

Other related posts: