[SI-LIST] Re: FW: Re: Sanmina patent on dielectric thicknesses under 4 mi ls
- From: Ritchey Lee <leeritchey@xxxxxxxxxxxxx>
- To: chris.cheng@xxxxxxxxxxxx
- Date: Tue, 27 Nov 2001 17:22:50 -0800
Same here. It's the plane capacitor that does all the work. Lots of good
reseach that backs this up.
Lee
Chris Cheng wrote:
> Chris,
> I suggest you do the following experiment (I've done it so
> many times to toy with EMC engineers).
> Take a system board with your favorite sub nf caps, do an
> EMI scan, replace every single sub nf caps with .1uf or
> .01uf on the same PCB and redo your scan. Let's see if they
> make any difference.
> I never lost my bets but you can be the first one.
>
> -----Original Message-----
> From: Chris Padilla [mailto:cpad@xxxxxxxxx]
> Sent: Tuesday, November 27, 2001 3:41 PM
> To: chris.cheng@xxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Sanmina patent on dielectric thicknesses
> under 4 mi ls
>
> I use plenty of sub nf caps to help curb emission problems in a lot of my
> systems and they do work just fine and are necessary.
>
> However, you point specifically to core noise (presumabley on some large
> ASIC or processor or something) and while that is not an area I
> specifically deal with, it certainly comes into play. EMI is a system
> level phenomenon and the size of your PCB most certainly can resonate at
> frequencies well above 100 MHz.
>
> I fear I am missing your point here....
>
> ----->Chris
>
> EMC Engineer
> Cisco Systems
>
> >Ron,
> >May I ask what is the intend use for the sub nf caps ?
> >
> >By no means I claim there is absolutely no need for sub-nf caps,
> >however, I always question the need for sub-nf caps for EMI or
> >noise decoupling in a digital system. I believe the place for
> >such decoupling is either on die or package not on the PCB.
> >In fear of sounding like a broken record again, I claimed :
> >a) High frequency core noise cannot propagation through the
> >package back to the PCB. Same reason you have sso limitation
> >on a package, its just as bad to go out of than go into a
> >package. Core noise above 100MHz can not be decoupled
> >outside the package.
> >b) I/O switching noise is related to the image current
> >return path which exist between the power plane sandwiching
> >the signal layers and dictated by the impedance control
> >of the stackup.
> >
> >I've used Zycon before thinking they can lower EMI or
> >power noise but the reality is they don't and the reasons
> >are stated above.
> >
> >If you have other reasons to use distributed sub nf
> >decoupling caps, I will keep by mouth shut. And I practice
> >what I preach. I never use decoupling below 1nf in any of
> >my systems and they pass class B just fine.
> >
> >
> >
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