[SI-LIST] Re: plane-to-plane decoupling
- From: "Tony Luan" <luant@xxxxxxxxxxx>
- To: "Chris Cheng" <Chris.Cheng@xxxxxxxx>, "Vinu Arumugham" <vinu@xxxxxxxxx>, <leeritchey@xxxxxxxxxxxxx>
- Date: Mon, 15 Sep 2008 19:09:33 -0700
Just like Lee and Scott said, simple Decap between power islands to GND
does not have any effects at 3GHz, I think the decoupling effect will be
very trivial above 1GHz.
We may put AC coupling caps head to head with Decaps ( pwr to GND) and
make one signal AC coupling cap and one Decap as a pair, make their vias
coupled strongly. Or just route microstrip accompanied by Decaps. In
this way, we may see return current goes to Decaps instead of interplane
capacitance. If that is case, every lane needs their own Decaps.
High speed differential signal cross gap may cause severe EMI problem.
If the channel has some discontinuities and asymmetries, there will be
large common mode - differential mode conversion. Common mode noise can
hurt differential signal also.
Thanks
Tony
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Chris Cheng
Sent: Monday, September 15, 2008 6:46 PM
To: Vinu Arumugham; leeritchey@xxxxxxxxxxxxx
Cc: Scott McMorrow; Michael Rose; si-list
Subject: [SI-LIST] Re: plane-to-plane decoupling
Couple of questions I am curious :
a) The original question refer to multiple diff pairs. Assuming a series
AC cap is good enough for 1/2 of a pair. Does that mean it is good
enough for multiple diff pairs ? If not, what is the correct ratio ?
b) The series AC cap is inline with the signal path. How does one put a
return plane AC cap inline with the signal path ? If the cap is offset
to the side, what does the loop inductance impact will be ? Extrapolate
that to multiple pairs per cap, how does the loop inductance look like ?
c) Finally, do we really care that much of the common mode return ? How
does the differential mode impact look like ?
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Vinu Arumugham
Sent: Monday, September 15, 2008 4:02 PM
To: leeritchey@xxxxxxxxxxxxx
Cc: Scott McMorrow; Michael Rose; si-list
Subject: [SI-LIST] Re: plane-to-plane decoupling
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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>
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