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

Scott,

On many high density 3Gbps layouts,  the AC coupling capacitor has more 
loop inductance than the  0.5nH  Steve cited for a decoupling capacitor. 
So it is not clear to me how the AC coupling capacitor can outperform 
the decoupling capacitor.

Thanks,
Vinu

Scott McMorrow wrote:
> 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
>>>>>>>>>>>                                  
>>>>> ------------------------------------------------------------------
>>>>>    
>>>>>>>>>>>     To unsubscribe from si-list:
>>>>>>>>>>>     si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the 
>>>>>>>>>>> Subject
>>>>>>>>>>>                               
>>>>> field
>>>>>    
>>>>>>>>>>>     or to administer your membership from a web page, go to:
>>>>>>>>>>>     http://www.freelists.org/webpage/si-list
>>>>>>>>>>>
>>>>>>>>>>>     For help:
>>>>>>>>>>>     si-list-request@xxxxxxxxxxxxx with 'help' in the Subject 
>>>>>>>>>>> field
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>     List technical documents are available at:
>>>>>>>>>>>                     http://www.si-list.net
>>>>>>>>>>>
>>>>>>>>>>>     List archives are viewable at:                 
>>>>>>>>>>> http://www.freelists.org/archives/si-list
>>>>>>>>>>>     or at our remote archives:
>>>>>>>>>>>             http://groups.yahoo.com/group/si-list/messages
>>>>>>>>>>>     Old (prior to June 6, 2001) list archives are viewable at:
>>>>>>>>>>>              http://www.qsl.net/wb6tpu
>>>>>>>>>>>                                                       
>>>>>>>>>>                              
>>>>> ------------------------------------------------------------------
>>>>>    
>>>>>>>>>>     To unsubscribe from si-list:
>>>>>>>>>>     si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the 
>>>>>>>>>> Subject
>>>>>>>>>>                           
>>>>> field
>>>>>    
>>>>>>>>>>     or to administer your membership from a web page, go to:
>>>>>>>>>>     http://www.freelists.org/webpage/si-list
>>>>>>>>>>
>>>>>>>>>>     For help:
>>>>>>>>>>     si-list-request@xxxxxxxxxxxxx with 'help' in the Subject 
>>>>>>>>>> field
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>     List technical documents are available at:
>>>>>>>>>>                     http://www.si-list.net
>>>>>>>>>>
>>>>>>>>>>     List archives are viewable at:                 
>>>>>>>>>> http://www.freelists.org/archives/si-list
>>>>>>>>>>     or at our remote archives:
>>>>>>>>>>             http://groups.yahoo.com/group/si-list/messages
>>>>>>>>>>     Old (prior to June 6, 2001) list archives are viewable at:
>>>>>>>>>>              http://www.qsl.net/wb6tpu
>>>>>>>>>>                                             
>>>>>>>>>     
>>>>>>>>> ------------------------------------------------------------------ 
>>>>>>>>>
>>>>>>>>>     To unsubscribe from si-list:
>>>>>>>>>     si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the 
>>>>>>>>> Subject
>>>>>>>>>                       
>>>>> field
>>>>>    
>>>>>>>>>     or to administer your membership from a web page, go to:
>>>>>>>>>     http://www.freelists.org/webpage/si-list
>>>>>>>>>
>>>>>>>>>     For help:
>>>>>>>>>     si-list-request@xxxxxxxxxxxxx with 'help' in the Subject 
>>>>>>>>> field
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>     List technical documents are available at:
>>>>>>>>>                     http://www.si-list.net
>>>>>>>>>
>>>>>>>>>     List archives are viewable at:                 
>>>>>>>>> http://www.freelists.org/archives/si-list
>>>>>>>>>     or at our remote archives:
>>>>>>>>>             http://groups.yahoo.com/group/si-list/messages
>>>>>>>>>     Old (prior to June 6, 2001) list archives are viewable at:
>>>>>>>>>              http://www.qsl.net/wb6tpu
>>>>>>>>>                                 
>>>>>> ------------------------------------------------------------------
>>>>>> To unsubscribe from si-list:
>>>>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject 
>>>>>> field
>>>>>>
>>>>>> or to administer your membership from a web page, go to:
>>>>>> http://www.freelists.org/webpage/si-list
>>>>>>
>>>>>> For help:
>>>>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>>>>
>>>>>>
>>>>>> List technical documents are available at:
>>>>>>                 http://www.si-list.net
>>>>>>
>>>>>> List archives are viewable at:             
>>>>>> http://www.freelists.org/archives/si-list
>>>>>> or at our remote archives:
>>>>>>         http://groups.yahoo.com/group/si-list/messages
>>>>>> Old (prior to June 6, 2001) list archives are viewable at:
>>>>>>          http://www.qsl.net/wb6tpu
>>>>>>             
>>>>>
>>>>>       
>>>>
>>>>
>>>>
>>>> ------------------------------------------------------------------
>>>> To unsubscribe from si-list:
>>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>>>
>>>> or to administer your membership from a web page, go to:
>>>> http://www.freelists.org/webpage/si-list
>>>>
>>>> For help:
>>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>>
>>>>
>>>> List technical documents are available at:
>>>>                 http://www.si-list.net
>>>>
>>>> List archives are viewable at:             
>>>> http://www.freelists.org/archives/si-list
>>>> or at our remote archives:
>>>>         http://groups.yahoo.com/group/si-list/messages
>>>> Old (prior to June 6, 2001) list archives are viewable at:
>>>>          http://www.qsl.net/wb6tpu
>>>>  
>>>>
>>>>   
>>>
>>>
>>
>>
>

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: