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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Vinu Arumugham <vinu@xxxxxxxxx>
  • Date: Wed, 17 Sep 2008 13:51:34 -0400

All,

To get back to the original intention of this thread I have several thoughts

There have been 3 alternatives proposed for bypassing a split plane

1) place capacitors across the split from power plane to power plane

2) use bypass capacitors to ground

3) use thin dielectrics

My thoughts on each case:

Case 1
Allows common mode noise to be injected from one power domain into the 
other.

Common mode noise crossing power domains may cause unintended problems 
in the other power domain.

If designed correctly, some of the common mode may be transfered back to 
the signal where it can be partially terminated and attenuated by the 
receiver.

Common mode noise rattles around 2 power cavities instead of 1.

Case 2
Most of the Common mode noise is reflected off the split plane boundary 
and localized to the transmitter signal traces and power cavity.

Most common mode energy rattles around originating power cavity until it 
is dissipated by losses in dielectric, copper and capacitors.

Some common mode energy is transferred from the near end to the far end 
cavity by slot mode waves.

Case 3
Most of the Common mode noise is reflected off the split plane boundary 
and localized to the transmitter signal traces and power cavity.

Most common mode energy rattles around originating power cavity until it 
is dissipated by losses in dielectric, copper and capacitors.

Some common mode energy is transferred from the near end to the far end 
cavity by slot mode waves.

Thinner dielectric decreases power cavity impedance and increases power 
cavity losses, thus attenuating common mode noise more quickly.

Thinner dielectrics also reduce the slot mode effect.


In all cases, there is no impact on the differential signal received at 
the receiver.  Whether or not allowing common mode noise to cross power 
domains is a good idea is uncertain.  Shunting the slot with capacitors 
will attenuate slot mode wave propagation, at the expense of increasing 
common mode noise in the far end cavity.  Does this make a difference?  
Who knows?

This is not a simple problem, and should be generally avoided.  When it 
cannot be avoided, I would probably:

A) Make sure that the power planes referenced are the digital power 
planes least sensitive to noise in the system.

B) Make sure that the power planes are adjacent to ground planes.

C) Use the thinnest power/ground dielectrics consistent with the cost 
constraints of the design.

D) Place at least 2 capacitors across the split, with differential pairs 
in between, to help contain slot mode waves.

E) Place a ground via fence around both split planes, possibly around 
the entire edge of the board.

F) Make sure that neither of the split planes has an unfiltered direct, 
or coupled path, to a cable assembly that flops around in, or exits the 
chassis.

Pray that there isn't an unintended radiator that has been overlooked.


Best 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,
>
> Agreed, except that sometimes one is forced to look at "poor design" 
> alternatives to check if there is enough margin to make it work. We 
> would not have this thread if Michael had not been forced to look at the 
> poor design alternative of referencing 3Gbps pairs to split power planes!
>
> Thanks,
> Vinu
>
> Scott McMorrow wrote:
>   
>> Vinu
>>
>> Again, you are correct for a poor design, but incorrect for a good 
>> design.  To correctly engineer the the AC coupling system, it is 
>> necessary to decompose the problem into two separate parts:  Via 
>> transitions, and capacitor transitions.  Both can be designed to have 
>> a 50 ohm broadband match that is reasonable to at least 10 GHz.  When 
>> that is the case, the AC coupling capacitor becomes nearly 
>> transparent, with extremely low loss.  This necessarily requires 
>> ground return vias to complete the 3D via transmission lines.
>>
>> 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,
>>>
>>> To keep the via stub small, signals are commonly routed on layers 
>>> near the bottom of the board and connect through vias to the AC 
>>> coupling capacitor mounted on the top side of the board. On dense 
>>> boards where there is no space for ground return vias, the AC 
>>> coupling capacitor seems to be operating with the same 
>>> poorly-referenced loop as the bypass capacitor.
>>>
>>> Thanks,
>>> Vinu
>>>
>>> Scott McMorrow wrote:
>>>       
>>>> 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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