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

Steve,

"A single capacitor will still impose a substantial bump in the 
impedance for a single signal.", yes, but the same applies if that 
capacitor were in an AC coupling configuration.
It certainly has an impact but I don't see it being "unusable" as Lee 
put it.

"A typical plane cavity will impose a much smaller bump.", yes, but if 
the 1 sq. in. cavity has to support several links, then the difference 
between (1) and (2) is not as big.

Thanks,
Vinu

steve weir wrote:
> 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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