[SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc

From: steve weir <weirsp@xxxxxxxxxx>
To: Chris.Cheng@xxxxxxxxxxxx, Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>,"'Istvan NOVAK'" <istvan.novak@xxxxxxxxxxxxxxxx>,Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>
Date: Tue, 10 Feb 2004 18:00:18 -0800
Chris, as I said, I view it as an EMI, not power concern.  That could 
change on large boards, particularly with high K materials as lamda / 2 
gets down below the IC PDS cut-off.

Regards,

Steve.

At 05:46 PM 2/10/2004 -0800, Chris Cheng wrote:
>Steve & Ray,
>Good point, there will be noise leaking out from the core through the
>package. It will be "grossly inadequate" or attenuated to not affecting back
>into other chips core through their package. So is this a matter of
>containment or decoupling ? Like you said below, a nice faraday cage formed
>by top and bottom ground planes stitched with ground via may just be as
>effective as your decoupling solution.
>Is it the wild Von Karmann wind that can knock the bridge off or the blow
>that takes out the birthday cake candle ?
>I believe it's just the later.
>
>-----Original Message-----
>From: steve weir [mailto:weirsp@xxxxxxxxxx]
>Sent: Tuesday, February 10, 2004 2:13 PM
>To: Chris.Cheng@xxxxxxxxxxxx; 'Istvan NOVAK'; Chris Cheng
>Cc: si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: Stack up for EMI reduction, plane resonance
>and u-s trip radiation etc etc
>
>
>Chris,
>
>There is one point that I disagree on.  Even though the package cuts off
>with at least a -2 slope, there is quite of bit of high frequency energy
>that still passes between the PWB and the IC.  It is just grossly
>inadequate to power the IC.  But it has lots of potential to aggravate EMI
>problems.
>
>No one believed Von Karmann when he theorized that wind was the source of
>energy that sent the Tacoma Narrows bridge into destructive
>resonance.  But, we all have learned that Von Karmann was right.
>
>So now we have these resonant cavities in the form of PWB's with very low
>damping coefficients.  The IC's don't provide much damping, because as you
>note the packages appear reactive, not resistive.  As long as the energy
>stays in the cavities and sloshes around at frequencies higher than the IC
>cut-off(s), it probably isn't any big deal.   But we have these:  board
>edges, vias, and components, all more than willing to provide radiation
>paths for that energy.
>
>One of the hotter debates was the 20H rule.  Amidst that debate came the
>notion of ground fences on the outside of the board.  While I like those
>for ESD, they can do just as much harm as good for EMI.
>
>At 01:22 PM 2/10/2004 -0800, Chris Cheng wrote:
> >Istvan,
> >
> >You got me on this one, I really need to figure out where can the
>200-400MHz
> >noise on PCB comes from ?
> >Is it :
> >a) Core noise, IC internal switch noise which propagate through the package
> >power pins to the PCB
> >Ans : Beaten to death, package is the choke point. EMI noise radiates from
> >package not PCB
>
>Agreed
>
> >b) I/O switching noise, comes out from signal pins needs a return path the
> >I/O power
> >Ans : Managing the return path and reference plane not the decoupling caps.
> >Yes, the plane CAPACITANCE not inductance provides the return path for the
> >image current return through the opposite reference ground plane.
>
>This is where as system integrators, we end-up applying band-aids due to
>poor IC design.
>
>Minor nit, I disagree with your characterization of plane
>capacitance.  Sure, without capacitance we would not have a coupling
>mechanism, but at the frequencies of interest, the behavior over even
>fairly short distances is of a transmission line, not a capacitor.  This is
>especially true with high K materials, and thin laminates.
>
>
> >c) External terminators,
> >Ans : The resistance of the terminator is the damping factor
>
>Agreed.
>
> >d) Noise from the supply
> >Ans : 200-400MHz noise from a supply ??????
>
>It's all those TWT's that they like to use in quarter bricks!!!  ( Just
>kidding, everyone knows it is the Klystrons. )
>
> >e) External cable coupling
> >Ans : ferrite beads and chokes
>
>Moating near I/O, shunt devices such as feed-through caps, or X2Ys are
>pretty effective too, adequate bonding of the PWB to the chassis, etc.
>
>
> >Aside from the above, none of which is related to fancy decoupling caps or
> >thin core PCB, where else ?
>
>It is a matter of impedance.  Either we get the decoupling capacitors
>significantly closer to the package than lambda / 4, or we have stuck the
>characteristic impedance of the planes between the IC and the caps.  For
>thick dielectric where that impedance can be an ohm or more, that is often
>way too much.  So, get close, or pay for fancy thin dielectrics.
>
>What fancy decoupling capacitors can do is make it easier to stay close to
>the IC by using fewer devices.  But we are still stuck drilling enough via
>holes to attach those devices.
>
>Steve
>
> >Chris
> >
> >-----Original Message-----
> >From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
> >Sent: Monday, February 09, 2004 7:58 PM
> >To: Chris Cheng; si-list@xxxxxxxxxxxxx
> >Subject: Re: [SI-LIST] Stack up for EMI reduction, plane resonance and
> >u-strip radiation etc etc
> >
> >
> >Chris,
> >
> >Well, it depends on the nature of the devil; if you are
> >concerned by noise getting from the PCB into the
> >package through its power/ground pins, you are
> >correct: the package resonance will filter out noise
> >above the cutoff frequency.  If you also want to
> >reduce the noise on the PCB itself, the active devices
> >will not reduce the noise for the same reason, because
> >the package separates the silicon from the PCB.
> >
> >Regarding parallel plate capacitance: this was discussed
> >several times on the list, and I dont want to repeat
> >myself.  But I think we are saying the same thing.
> >When you say parallel plate capacitance, I say
> >inductance.  For a board of a few inches in size or
> >bigger, the lowest series resonance of the board plates
> >is 100MHz or lower. Above that frequency the impedance
> >is mostly inductive.  If you need a certain amount of parallel
> >plate capacitance, we like it or not, it comes with a certain
> >amount of inductance above the series resonance.  If
> >you need more parallel plate capacitance, you get it
> >together with lower inductance.
> >
> >Regards,
> >
> >Istvan Novak
> >SUN Microsystems
> >
> >----- Original Message -----
> >From: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>
> >To: "'Istvan NOVAK'" <istvan.novak@xxxxxxxxxxxxxxxx>; "Chris Cheng"
> ><Chris.Cheng@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> >Sent: Monday, February 09, 2004 4:33 PM
> >Subject: RE: [SI-LIST] Stack up for EMI reduction, plane resonance and
> >u-strip radiation etc etc
> >
> >
> > > Yes, once again the devil is in the details. It is one thing to stick an
> > > impedance probe to measure the power plane impedance at a random
>location
> >on
> > > the PCB. It is another thing to measure it on the real load side (i.e.
> >after
> > > the package). Have you done that ? Are you convince you can even see any
> > > effect at 200-400MHz on PCB through the package ? Your colleague Larry
>and
> > > me don't think so.
> > > As for I/O return current related noise on PCB, it is the parallel plate
> > > capacitance that sandwich the stripline which is responsible for the
> > > decoupling/return of the current (at least at 200-400MHz). Not the thin
> >core
> > > power/gnd pairs or fancy decoupling caps.
> > >
> > > -----Original Message-----
> > > From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
> > > Sent: Sunday, February 08, 2004 3:07 PM
> > > To: Chris.Cheng@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > > Subject: Re: [SI-LIST] Stack up for EMI reduction, plane resonance and
> > > u-strip radiation etc etc
> > >
> > >
> > > Chris,
> > >
> > > I am not speaking for Zhangkun, but in many of the real boards I have
> >looked
> > > at by measurements and simulation, you can see the evidence of
> >antiresonance
> > > between the plane capacitance and inductances of capacitors.  Chips (at
> > > least on those boards I have looked at) did SHIFT the resonance
>frequency
> > > slightly, but did not make the peak go away.  You are correct in saying
> >that
> > > if you sprinkle the board with capacitors, the resonance peak is
> >suppressed.
> > > But as you said in one of your recent postings, the devil is in the
> >details:
> > > sometimes you may need so MANY capacitors over the board area to
> > > sufficiently suppress the resonance that it becomes a pain.
> > >
> > > Regards,
> > >
> > > Istvan Novak
> > > SUN Microsystems
> > >
> > > ----- Original Message -----
> > > From: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>
> > > To: <si-list@xxxxxxxxxxxxx>
> > > Sent: Monday, February 02, 2004 10:15 PM
> > > Subject: [SI-LIST] Stack up for EMI reduction, plane resonance and
>u-strip
> > > radiation etc etc
> > >
> > >
> > > > Finally......
> > > >
> > > > Zhangkun,
> > > >
> > > > I am also curious about these 200-400MHz plane resonace. If you
>sprinkle
> >a
> > > > PCB with a wide range of caps with different values and at different
> > > > location and with high power loading (ie real IC chips) at different
> > > > location, do you still see pronounced peaks at 200-400MHz ? I have no
> > > doubt
> > > > a bare power/gnd plane pair can resonate at those frequencies, but
>I've
> > > > never seen that case once realistic caps and loading (IC chips) is
> >placed
> > > on
> > > > the PCB. Are these simulation results or measurements based on a real
> > > system
> > > > with chips and caps ?
> > > >
> > >
> > >
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References:
- [SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc
  - From: Chris Cheng
[SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc

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