[SI-LIST] Re: [SI-LIST]6 layers stackup - proof
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Dan Smith <Dan.Smith@xxxxxxxxx>
- Date: Wed, 27 Feb 2008 10:32:34 -0800
Dan, actually you are making our point for us. A greater capacitance=20
to inductance ratio makes for a wider bandwidth bypass cap that results=20
in lower noise. That is exactly what the X2Y(r) caps do, and is aptly=20
demonstrated in the video.
Regards,
Steve.
Dan Smith wrote:
> All,
> The howard johnson video had one fundamental flaw in how he (and steve)=
came to their conlusion. This flaw was that the 0402 caps vs. the x2y c=
aps were not apples to apples comparisons. Why? The reason is comparing=
0402 cpacitors of 220nf with x2y caps of 100nf are not the same thing! =
Sure, two 100nf caps in parallel equate to 200nf (which is what dr. johns=
on stated in the video as to why they are the same thing) but I challenge=
anyone to perform a noise experiment where you have 1 200nf cap vs. 2 10=
0nf caps and try and get the same noise measurement. The resonance frequ=
ency of 200nf is about 16mhz and the resonance frequency of the 100nf is =
about 25mhz. thus the impedance due to these capacitors above 20mhz is g=
reater for the 200nf case - this is just fundamental capacitor science. =
This is exactly what happened in howard johnsons video. This is not rock=
et science just basic ohms-law. All this gobbly-gook talk of via incucta=
nce, etc. is in the small noise (no pun int
> ended) of why the results in that video differed. See attachement.
>
> Dan
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx=
] On Behalf Of Scott McMorrow
> Sent: Wednesday, February 27, 2008 9:34 AM
> To: Lee Ritchey
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: [SI-LIST]6 layers stackup
>
> Lee
> Fortunately for us, the physics are clear. If the correct land/via
> patterns are used, 50 mils of via reach will not swamp out the
> advantages of some types of low inductance capacitors. In fact, the
> difference will asymptotically approach the inductance ratio of the
> coupled via systems themselves at infinite via length.
>
> For the case of X2Y capacitors with 6-vias vs. 0402 with 4-vias, this
> ratio is 2:1. That is, the 6-via pattern of the X2Y is 2 times as
> efficient as the 4-via 0402 pattern. Even if we normalize for number o=
f
> vias, the X2Y pattern is still 1.33 times more efficient per via drill.=
> However, in real PCBs we are not dealing with infinitely long vias, so
> the performance advantage is much better, and typically 3:1 or 4:1
> depending on the 0402 via pattern (2-via or 4-via) used.
>
> However, if you incorrectly design your board such that a PRF occurs
> within a few octaves of where you need low PDS impedance, that is a
> different issue altogether, which can only be resolved by moving the PR=
F
> with the correct spatial distribution of capacitors. Low inductance
> capacitors are also more efficient in dealing with this problem.
>
>
> 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(r) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>
> Lee Ritchey wrote:
> =20
>> Tom,
>>
>> Well put!
>>
>>
>>
>> =20
>>> [Original Message]
>>> From: Tom Biggs <tbiggs@xxxxxxxxxxxxxxxxxxxxx>
>>> To: <si-list@xxxxxxxxxxxxx>
>>> Date: 2/26/2008 6:16:04 PM
>>> Subject: [SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
>>>
>>> You can take the bulldozer analogy two ways. It depends on whether yo=
u
>>> care about the weight of the bulldozer with passengers, or just the
>>> weight of the passengers.
>>>
>>> Steve's test fixture was geared toward 'weighing the passengers'. Lee=
's
>>> was toward 'weighing the bulldozer'. So they each served their purpos=
e.
>>> His whole point was that the vias going down 50 mils on a board are
>>> going to swamp out the advantages of low inductance caps. His test
>>> fixture, by design, had vias that go down 50mils. I'm sure he'd agree=
>>> that this would be a bad fixture for measuring the cap itself, which =
was
>>> not his goal.
>>>
>>> The appropriate land pattern to use for the low-inductance caps is a
>>> separate issue. I'd be curious to see Lee's board with Steve's land
>>> pattern.
>>>
>>> -tom
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxx=
rg]
>>> On Behalf Of Muranyi, Arpad
>>> Sent: Tuesday, February 26, 2008 1:31 PM
>>> To: si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
>>>
>>> Can't resist to illustrate this with an example:
>>>
>>> If you want to compare the weight of an ant and a cricket and you put=
>>> them on top of the same bulldozer, you will not see much difference i=
n
>>> their weight...
>>>
>>> Arpad
>>> ---------------------------------------------------------------------=
---
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxx=
rg]
>>> On Behalf Of Lee Ritchey
>>> Sent: Tuesday, February 26, 2008 11:16 AM
>>> To: Charles Grasso; Scott McMorrow
>>> Cc: Steve Weir; QU Perry; si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: [!! SPAM] Re: 6 layers stackup
>>>
>>> What does the test vehicle have to do with it? Both capacitors are
>>> seeing
>>> the same stackup. It's apples and apples. Why 26 layers? Lots of
>>> PCBs
>>> have 26 layers, pretty much all of them in terabit routers. This PCB=
>>> was used to test may things besides these two capacitors.
>>>
>>> What is being presented is the difference between the two capacitors
>>> under the same set of test conditions and it is not much.
>>>
>>> There are two sets of tests. One with the capacitors connected to th=
e
>>> first two planes inside the PCB, which is the lowest added inductance=
>>> and the other is with the capacitors attached to two planes further d=
own
>>> in the PCB.
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>
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> -- Binary/unsupported file stripped by Ecartis --
> -- Type: application/msword
> -- File: X2Y_wool.doc
> -- Desc: X2Y_wool.doc
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--=20
Steve Weir
Teraspeed Consulting Group LLC=20
121 North River Drive=20
Narragansett, RI 02882=20
California office
(408) 884-3985 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business=20
(401) 284-1840 Fax=20
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(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property o=
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