[SI-LIST] Re: Traces don't cause EMI - really?
- From: "Salkow, Steven" <steven.salkow@xxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx, Larry Barnes <larry.barnes@xxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Mon, 20 Oct 2003 10:44:33 -0700
Earlier in my career, I had occasion to run various designs at Xerox
through FCC Class B EMI certification. Many of these were IBM PC like
motherboard systems where four layer PCBs were the rule. Hence all
signals were Microstrip. All of these systems were in metal enclosures
some of which had bays for installable devices such as Hard drives. Some
of these opening were left open initially, essentially covered by
plastic only panels. Measurements were made. Virtually none of these
initially passed FCC Class B without some work. Sometimes the Drive
areas could be left open whereas very small openings seemed to act as
wave guides for higher frequency emissions. Running the clock spread
spectrum (dithering) allowed many of these systems to pass as the
emissions were spread over a wider spectrum with respect to time hence
passed levels of Class B. Where it not for metal cases or some means of
effective containment, none of these systems would pass emissions.
FCC class B is more stringent than FCC Class A. If your electronics in
going in a good EMI envelope, spending a lot of extra time on
controlling board emissions may have diminishing financial rewards. But.
. .
Card to card coupling:
If you have two cards side by side, say in adjacent card slots (0.800
inches away) and one of those cards has a highly susceptible circuit,
you may need to revert to shielding to gain successful noise margins or
increase separation distance or both.
Steve Salkow
Lockheed Martin
-----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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