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

Vinu

Forget about "inductance" and follow the field patterns in 3D and you 
will quickly see the difference.  A bypass capacitor operates in a 
poorly-referenced loop.  When designed correctly, an AC coupling 
capacitor is operating as one half of a transmission line, where the PCB 
plane serves as the other half of the line. The differences are 
significant.  As a result, a very reasonable 50 ohm match for a coupling 
capacitor can be engineered, with good return loss out to 10 GHz.


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:
> 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
>>>>>>>>>>>>                                  
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