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

Vinu, I think it is more mind set than anything else.  Let's put some 
additional numbers to this:

A really well-mounted 0402 capacitor is going to exhibit 0.5nH or more 
mounted inductance.  At 1.5GHz that's 5Ohms.  Ignoring resonances, a 
typical 3 mil cavity even 1" on a side is going to exhibit impedance in 
the 100's of milliOhms.  So:

1) A single capacitor will still impose a substantial bump in the 
impedance for a single signal.
2) A typical plane cavity will impose a much smaller bump.

Since we are talking differential signaling, the even-mode signal 
components should shrink at all frequencies below Fknee, so we don't 
need tons of charge storage. Cavity is going to be more effective.  (But 
I would far prefer a contiguous return in the first place.)

Steve
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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-- 
Steve Weir
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