[SI-LIST] Re: Traces don't cause EMI - really?
- From: "Chan, Michael (Eng Hou)" <michael.chan@xxxxxx>
- To: <jeff.loyer@xxxxxxxxx>, <MikonCons@xxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Oct 2003 13:53:35 -0500
Jeff:
There is differential mode and common mode radiation. In =
differential mode, even=20
though you might have a perfect termination bus but if the loop area is =
large, then you
still have a chance to radiate a lot and fail FCC ( i.e. you route your =
traces on outer
layer ( microstrip ) and the traces are far away from power/reference =
plane ).=20
Just my 2cents.
Regards,
MC
-----Original Message-----
From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
Sent: Thursday, October 16, 2003 12:13 PM
To: MikonCons@xxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?
Hi Mike,
In your posting, you said:
"I designed a special PCB in support of a worldwide seminar tour for =3D
Hewlett-Packard that clearly demonstrated unacceptable (i.e., >FCC Class =
=3D
B) radiation from single, terminated, 50-Ohm traces on a 10-inch PCB."
Just to be clear - are you saying that if I connected 2 properly =3D
designed chips (driver and receiver) together with many properly =3D
designed single-ended transmission lines, they would likely fail FCC =3D
standards?
Sorry to force you to restate something that you were so clear about, =
=3D
but the statement is a bit surprising to me. My impression has been =3D
that FCC failures were inevitably due to an error in the design - =3D
impedance mismatches, crossing a split plane, changing reference planes, =
=3D
etc. =3D20
I would expect differential routing to radiate significantly less EMI in =
=3D
the presence of these errors, but I wouldn't expect a properly designed =
=3D
board with many single-ended signals to fail EMI.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of MikonCons@xxxxxxx
Sent: Monday, October 13, 2003 12:20 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?
In a message dated 10/13/2003 7:17:10 AM Pacific Standard Time,=3D20
leeritchey@xxxxxxxxxxxxx writes:
I've tried several times to measure significant emissions from a single
transmission line travelling over a plane at the distances above the =3D
plane
one uses to create signal paths for logic circuits, the topic of concern =
=3D
in
this thread, with no luck.
I welcome measurements from others that supports the statement that =3D
traces
over planes are significant sources of EMI. So far, no one has =3D
presented
such evidence. Lacking such evidence, how can anyone make such claims?
True, this thread concerns differential signals, but how are they =3D
different
from single ended signals?
True, Doug Brooks' paper didn't set out to measure emissions from traces =
=3D
on
outer layers, but it did state that measured emissions were 30+ db below
what a mono pole would emit. =3D20
In all my measurements, the lead frames of the ICs are the big =3D
radiators.=3D20
They make nice antennas. They stick up above the PCB and they have
significant transient currents flowing through them, especially the =3D
power
leads. Check out a PLCC with a near field probe sometime.
Time for some measurements from those who want this discussion to =3D
seriously
come to any conclusion. Anyone have any?
Lee
**************
WOW! I just got a chance to review the latest SI list inputs and =
I'm=3D20
astounded at such comments. Lee, the "evidence" has been around for =3D
decades. The IEEE=3D20
Proceedings published a special edition circa 1989 describing over =
40=3D20
different surface structures on PCBs that are efficient antennas, many =
=3D
of which are=3D20
unintentionally formed by poor trace layouts. I designed a special PCB =
=3D
in=3D20
support of a worldwide seminar tour for Hewlett-Packard that clearly =3D
demonstrated=3D20
unacceptable (i.e., >FCC Class B) radiation from single, terminated, =3D
50-Ohm=3D20
traces on a 10-inch PCB. The same board was used to demonstrate both =3D
radiation and=3D20
crosstalk reductions of 6 to 14 dB by the use of guard traces between =
=3D
signal=3D20
traces (which you recently declared as totally unnecessary). I expanded, =
=3D
updated, and presented both analyses and confirming measurements at =3D
multiple client=3D20
companies (including IBM, Motorola, AMD, and Johns-Hopkins), and =3D
presented a=3D20
paper at SuperDesignCon 95 illustrating these same effects. IBM =3D
published an=3D20
excellent paper circa 1998 at a Southern California conference (don't =
=3D
recall=3D20
which) that compared the radiation from traces as a function of their =
=3D
distance=3D20
from the PCB edges. The bottom line is that ANY trace with current =3D
flowing on=3D20
it generates EM fields and WILL radiate at some levelof efficiency. =3D
That's why=3D20
one should always look to burying high-speed traces unless they are very =
=3D
short=3D20
relative to the rise time of the signals they conduct.
Surface routed differential signal traces offer opposing polarities of =
=3D
fields=3D20
which tend to cancel, leaving only the effect of a small loop antenna to =
=3D
radiate (but they still do radiate). That's why I also favor tightly =3D
coupled=3D20
(i.e., closely spaced) pairs for such applications (which, if I recall =
=3D
correctly,=3D20
you also seem to dislike). Differential pairs are definitely different =
=3D
from=3D20
individual traces.
I have very little time for technical activities (what with the trout =
=3D
season=3D20
coming to an end soon), but I could not let this thread slip by.
Respectfully to all,
Mike
Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
*** Serving Your Needs with Technical =3D
Excellence ***
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