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

Vinu

Your statement: 

'"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. '

Is just not true.  There are significant electromagnetic differences 
between a capacitor used for AC coupling in a through configuration and 
one used for power plane shunt applications.

Hint: follow the magnetic fields


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:
> 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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