FreeLists / si-list / [SI-LIST] Re: EMC Analysis

["Tang, George" <George.Tang@xxxxxxxx>]

What you measure at far field is the radiated power.  Assume that you
are measuring 200 MHz clock fundamental signal and not the harmonics of
the clock.  This 200 MHz signal needs a radiating antenna, which is
usually one or more cables (could even be the 60Hz power cord).  The
test person is required to maximize the radiation by rotating the DUT on
a turntable and moving the cables into different orientations.  By
keeping both clocks running, you should see the radiation go up by about
2 dB, unless you hit a chassis or cable resonance.  If you do hit a
resonance (which occurs often), all bets are off. =20

George=20



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Hassan O. Ali
Sent: Monday, March 13, 2006 8:34 AM
To: darshanmehta2k@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: EMC Analysis


Nobody knows :-)

Seriously, for EMC purposes, the electric (E) field radiation levels are
measured in the=20
far-field e.g. at a distance of 10 meters from the radiation source. If
you have two (or=20
more) electromagnetic field radiation sources, the sum of their radiated
E-field at a=20
far-field point is a spacial and temporal vector sum which depends on
many things.=20

For example, if the two radiation sources have orthogonal radiation
patterns e.g. the=20
point of maximum radiation of one source lies along the x-axis and the
other along the y-
axis, if the far-field point where you measure the E-field is along the
x-axis, the=20
total measured E-field will probably be just the maximum E-field of the
first source=20
(certainly not the linear sum of the two maxima).

You also mentioned "system level" - and I'm sure you know that all
systems are not=20
created equal! A system can have other factors that affect field
interactions making it=20
so difficult to determine the best way to aggregate radiated E-field
from multiple=20
sources. A printed circuit board with many nets, operating at various
frequencies, is a=20
very complex radiation system by itself. When you have many of those
boards, say in a=20
shelf, you've compounded the problem even more. That's why I said at the
beginning,=20
nobody would know how to sum the radiated fields unless you're very
specific on many=20
things.=20

In the 90's there were many tools that were claiming that they could
predict radiated=20
emission levels from printed circuit boards. Those tools failed
primarily because of the=20
prediction complexity that I've mentioned above.

Best regards.

Hassan.



On Mar 13, Darshan Mehta <darshanmehta2k@xxxxxxxxx> wrote:
>=20
>   Hello Everybody,
> =20
>  I have one very basic question about EMC Analysis. If we have 2 nets
in design, CLK1=20
and CLK2, both switching at 200 MHz. If we see Electro-Magnetic
radiation of 100 dB from=20
CLK1 and 105 dB radiation from CLK2, what will be the EMI radiation
level at System=20
level? Thanks in Advance!
> =20
>  Best Regards,
>  Darshan Mehta
>  =20
>       =09
> ---------------------------------
> Yahoo! Mail
> Bring photos to life! New PhotoMail  makes sharing a breeze.=20
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