[SI-LIST] Re: Traces don't cause EMI - really?

Lee I am astounded at your comments. If you would glance at Figure 4
(Emissions from a reference PCB without cables) you will see that
the Y-axis is numbered and (although not listed) is intended to be in
dBuV. If you look at the FCC Class B limit (superimposed on the
graph) I think you'll agree that the reference board actually FAILED
Class B at around 300MHz. I'd say that was pretty significant.
So, again, you are wrong to assert that the detected emissons
are not large enough to cause concern.

I have modeled this board in Ansoft HFSS and got the same
results.

Did you read the Conclusion?

CONCLUSION: A multilayer PCB with a solid ground plane produced
radiated emissions similar to a dipole antenna.
SO Traces over planes DO cause emissions.
The emissions are from the PCB/trace structure.
Change the dimensions, add cables and the profile changes.

Counter-challange - Prove to me that traces DO NOT radiate.




-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey
Sent: Sunday, October 19, 2003 5:06 PM
To: Larry Barnes; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


I downloaded a copy of this paper a few minutes ago.  The conclusions of
the paper do not state that microstrip traces emit significant EMI.  All
that is stated is that the analysis and chamber measurements agree within
measurement accuracy of what little energy is detected.  The actual amount
is not stated in the paper.  There is certainly no claim that the detected
emissions are large enough to cause concern among EMC engineers.

This is likely one of those cases where the difference is between
detectable and significant.  There is no claim in this paper that the
emissions are significant, only detectable.

If we are to accept that surface traces are significant sources of EMI,
there needs to be credible demonstration of this.  The reason is, designers
are expected to expend cost avoiding microstrip traces.  To do this, there
needs to be demonstrated emissions that are worth the extra cost to control.

My challenge to the proponents of this claim is to provide this
demonstration.

I and many others have designed hundreds, maybe thousands of PCBs with
traces on outer layers with all the products successfully passing all
emissions tests.  That could not happen if these claims are true.

Lee


> [Original Message]
> From: Larry Barnes <larry.barnes@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 10/18/2003 5:42:14 AM
> Subject: [SI-LIST] Re: Traces don't cause EMI - really?
>
> I just couldn't resist after all this discussion.  I would refer you to =
> a study done by Dr. Dave Hill of NIST.  I demonstrates that microstrips =
> indeed radiate.  The title of the paper is "Radiated Emissions and =
> Immunity of Microstrip Transmission Lines: Theory and Reverberation =
> Chamber Measurements"  IEEE Trans on EMC  May 1996.=20
>
> Larry
>
>
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> Lawrence C. Barnes
> QLogic Corporation
> Aliso Viejo, CA  92656
>
>
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