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

  • From: Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>
  • To: "'Istvan NOVAK'" <istvan.novak@xxxxxxxxxxxxxxxx>,Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Tue, 10 Feb 2004 18:18:42 -0800

OK,
My feedback :
a) Yes package resonance varies, in fact if I understand Larry's previous
comments, his package has so much perforation on the PCB that it has choke
points BELOW 100MHz. He has to use thin core just to bring the resonance
back to near 100MHz. My question is what will be harmed by the noise leaked
out to the system beyond the choke point ? It is a chicken and egg problem,
if nothing from the PCB side can bring in help to sustain the core noise
margin, nothing from outside can bring enough harm to impact the core
neither. What remains "harmful" is an EMI containment problem (through plane
stitching) and not a decoupling issue.
b) The plane capacitance is used for the image current of the non-ground
reference plane to AC coupled to the ground on the other side that the
stripline signal is referencing. It is a function of the etch rate and
locality of the signal.
c) If you can measure a converter that can provide "harmful" level of noise
to the silicn core supply, I think the last thing you worry about is
decoupling solution. You should fire the converter vendor. If it is what you
call "meet all the other functional spec", it falls back to EMI containment
rather than decoupling.

At the end of the day, throwing fancy caps or thin core is still neither
necessary nor the most effective.


-----Original Message-----
From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
Sent: Tuesday, February 10, 2004 5:55 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,

See my comments below.

Regards,

Istvan Novak
SUN Microsystems

> 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

Are you sure that the resonance frequencies of the package and PCB are
always exactly
the same?  If the PCB resonance lines up with the excitation frequency,
while the
package structural resonances dont, a relatively large attenuation through
the
package-board interface still cant block harmful radiation from the board
(before you ask: such situations were measured on real boards).

> 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.

Capacitance or inductance and thin laminate: OK lets ask a different
question.
You say you need plane capacitance.  Is the required amount of plane
capacitance
independent of how many traces the plane has to reference?  Do you want
more plane capacitance if the same plane area references faster and more
traces?

> c) External terminators,
> Ans : The resistance of the terminator is the damping factor
> d) Noise from the supply
> Ans : 200-400MHz noise from a supply ??????

Several hundred MHz noise may come from the converters, though
I agree that if a converter does that it is not the best converter, but it
still
can meet all the other functional spec (and again: it was measured on
real parts).


> e) External cable coupling
> Ans : ferrite beads and chokes
>
> Aside from the above, none of which is related to fancy decoupling caps or
> thin core PCB, where else ?
>
> Chris
>

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