FreeLists / si-list / [SI-LIST] Re: Why is capacitor with high ESR

[steve weir <weirsp@xxxxxxxxxx>]

Chris, I agree that 7 will work better than 5, although it might be a 
little bit less representative of what people actually build.

Where we disagree is whether 7 will actually be better than 6.  I don't 
think it will for the reasons stated.  Do you think you can convince your 
company to build two test boards, one as 7 and one as 6?  The alternative 
is to run models, assuming we want to trust them.

Regards,


Steve


At 10:12 AM 12/6/2004 -0800, Chris Cheng wrote:
>Steve,
>I would think
>
>7)
>Gnd
>Power
>Signal
>Signal
>Gnd
>Gnd
>
>with the fence and sea of vias and decoupling in between will be better than
>6)
>
>For 4) it is obvious there is no edge containment.
>For 5) while the edge containment can help the first five layers, the bottom
>power plane can still have noise current which solely relie on decoupling
>caps that may or may not be effective and you will need a case like 6) to
>ensure the complete edge containment (since you can short the gnd/power
>planes with vias).
>
>-----Original Message-----
>From: steve weir
>To: Chris Cheng; 'sguzek@xxxxxxxxx '; 'Si-List '
>Sent: 12/5/2004 11:30 PM
>Subject: RE: [SI-LIST] Re: Why is capacitor with high ESR
>
>Chris,
>
>No, I don't see a lot of value from the very low R's in existing
>caps.  Yes, lots of ground vias do divide the cavities up and cause a
>lot
>of scattering.  Unfortunately, even a good number of those vias rise to
>the
>surface to meet decoupling caps which then become a source of
>radiation.  This is not to mention the other signal vias that do the
>same
>thing.
>
>We could have some fun constructing some ML boards with a stackup having
>at
>least two ground planes one close to each end of the stack-up that
>allows
>for a fence.  We could add more layers, but I think the following 6
>layer
>is adequate for the thought-problem / experiment:
>
>Gnd
>Power
>Signal
>Signal
>Gnd
>Power
>
>1.  Oscillator with nice fast CMOS drivers in the middle as our noise
>source, aside from local decoupling in the middle of the board, it will
>be
>an open cavity.
>2.  Same as 1. but with ground fence only at the board edges.
>3.  Same as 1. but with fence using DET with the best dissipative caps
>that
>we can get.
>4.  Same as 1, but with lots of ground  and power vias distributed
>around
>the board many connecting to decoupling caps on the surface.
>5.  Save as 4, but with fence as in 2.
>6.  Same as 4, but with DET as in 3.
>
>I believe that we agree that between 1, 2, and 3, that 3 will have the
>lowest radiation.
>I believe that we agree that between 1 and 4, 2 and 5, and 3 and 6,
>4/5/6
>will have lower radiation than 1/2/3 respectively.
>
>What I think you will find interesting is that of 4, 5, and 6, that 6
>offers considerable improvement over both 4 and 5.  This has been the
>subject of much of Istvan's work on the benefits of DET.  The impinging
>energy only hits the vias once on its way out to the board edge where it
>is
>absorbed.  The "ice-cube trays" of 4. help to remove a lot of the
>coherency
>from the noise, but we are pretty much stuck with the dielectric losses
>to
>dump the HF energy into heat.  What does not become heat escapes on its
>way
>to Zontar.
>
>I hope that we can agree that an alternative demonstration is to break
>up
>the Vcc plane into sections that are tied together with lossy decoupling
>
>networks.  Would you be surprised to find that the EMC performance of
>such
>a board with a thick cavity is much better than the same geometry board
>where the Vcc has not been divided, sic 1, or 4 from above?
>
>Regards,
>
>
>Steve.
>
>
>At 10:41 PM 12/5/2004 -0800, Chris Cheng wrote:
> >Steve,
> >There are zillions of ground vias spread all over due to chips and
>bypasses
> >and passives. On top of that you have most of the decoupling acting as
>LR
> >load at EMI frequencies all over the places. Do you still think those
> >resonance peaks that one predicts purely based on the rectangular
>geometry
> >of the board will remain in the same frequency location with the same
> >amplitude ? Or they will be spread out with tiny little peaks based on
>the
> >locality of the ground via density of the chips and passives ? To take
>the
> >transmission line analogy, in stead of a single transmission with a
>source
> >and a load at the ends, what if there are many many shorts and L/R load
> >along the way, can the energy be still concentrated ?
> >
> >


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