[SI-LIST] Re: plane-to-plane decoupling
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Vinu Arumugham <vinu@xxxxxxxxx>
- Date: Tue, 16 Sep 2008 20:04:52 -0400
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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>
>
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