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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Vinu Arumugham <vinu@xxxxxxxxx>
  • Date: Mon, 15 Sep 2008 19:39:18 -0400

Vinu,
The performance of mounted capacitors in the through or shunt 
configurations are  considerably different, and have to do with 
impedance matching and 3D position of the reference plane.  In the DC 
block / AC coupling roll, the capacitor and PCB form a nice transmission 
line system that can be well-matched, with well-contained magnetic 
fields.  In the power / ground plane bypass application, the capacitor 
is in a system with huge impedance mismatches, and with poorly contained 
magnetic fields. 

If you perform 3D full-wave modeling of the two structures, you will see 
significant differences.  Capacitors placed across a slot (which is what 
we're talking about) have significant performance degradation due to 
back-scattering at the slot boundary, unless the return path is 
transitioned correctly.  Otherwise, you excite the cavity and slot 
resonances.  DC block capacitors do not have that problem, since the 
return path does not break, if designed correctly.  Bypass capacitors 
utilize vias that transition across multiple cavity apertures.  At ultra 
high frequencies, most of the energy stays in the cavities, where it is 
dissipated by losses.

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, Scott,
>
> I don't see the difference whether you want to look at the capacitor 
> as a series or shunt element. A decoupling capacitor may look like a 
> shunt element when it is part of a PDN but that PDN could serve as a 
> series element in the signal return path. So, if a capacitor is 
> acceptable in an AC coupling role in the signal path, the same 
> capacitor should be acceptable as part of a PDN that is a return path 
> for that signal. In other words, think of it as an AC coupling 
> capacitor for the return path instead of the signal path (US Patent 
> 7262974).
>
> For this application, the capacitor only needs to support ~10mA of 
> switching current at 1.5GHz, and a few tens of mV drop across its 
> impedance would be acceptable.
>
> Thanks,
> Vinu
>
> Lee Ritchey wrote:
>> 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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>>>>>>>    
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>


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