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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: leeritchey@xxxxxxxxxxxxx
  • Date: Mon, 15 Sep 2008 15:53:24 -0400

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
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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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>>>>>         
>>>>>           
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>>>>
>>>>       
>>>>         
>>>
>>>
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>


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