[SI-LIST] Re: Traces don't cause EMI - really?
- From: "Vishram Pandit" <vishrampandit@xxxxxxxxxxx>
- To: bdewitt@xxxxxxxxxxxxx, leeritchey@xxxxxxxxxxxxx,si-list@xxxxxxxxxxxxx
- Date: Mon, 20 Oct 2003 16:54:18 +0000
Here is my 2c worth ...
The total EMI comprises trace radiation and edge radiation. Trace radiation
is influenced by the signal quality, and hence is dependent on the matching,
return path discontinuity, etc. Edge radiation comes from the structure,
Ldi/dt, ground bounce, etc. I have done several simulations for
simulataneously switching traces (SSO), and found out that the trace
radiation is much lower compared to the Edge radiation. The simulator can
give me the radiation from traces as well as from the different edges.
Due to the switching lines, noise is generated on the Power/Ground. The
noise on different layers propagate through the structure and pollutes the
entire board. It travels through vias, traces, and different shapes on the
board. Finally, it radiates from the edge of the board. In my simulations, I
have observed so-called hot-spots on intermediate layers due to the
swtichingtraces. This is wholly dependent on the structure. Decopling
capacitors can be placed to mitigate the noise at hot-spots. If you put the
ground all along the edges of the board and multiple vias, you can reduce
theedge radiation. But the reflected waves will be radiated from the
structure again. In summary, we can still see higher radiation for the board
with perfectly matched traces with continuous reference palne, because of
thePower/Ground and the structure noise.
Thanks,
Vishram Pandit
Senior Member Technical Staff (SI/EMC)
Hughes Network Systems
>From: "Brent DeWitt" >Reply-To: bdewitt@xxxxxxxxxxxxx >To: , >Subject:
[SI-LIST] Re: Traces don't cause EMI - really? >Date: Sun, 19 Oct 2003
21:27:58 -0600 >>It seems this has become something of a tempest in a
teacup.As I mentioned >in an earlier post, I believe much of the energy of
the dispute has >developed over differences in terminology. >>First, in the
vast majority of cases, I agree with Mr Ritchey. In the >twenty five years
orso I've been involved in EMC, I've never seen the >radiation from surface
trace fail a product. That said, I've only worked >on old/slow boards with
fundamental clocks less than 1.5 GHz, although I've >worked with 900 MHz
intentional radiators to their 10th harmonic. Rules of >thumb only work
untilthe thumb is too fat to see the problem underneath it. >As I mentioned
in an earlier post, resonant patch antennas and other >intentional radiating
pcb structures are nothing more than fat, well >designed traces. Somewhere
between them and our ideal EMC designs lies >practicality. As frequency
increases, I find myself needing increasingly >closer inspection of detail.
>>In somewhat oblique support of Mr. Ritchey, most failing EMC issues I've
>observed have been associated with L di/dt induced voltages on the
reference>plane caused by surface traces. I/O cables, using said poorly
controlled >reference planes out to the world, are often a major emissions
issue, but >that is entirely another subject for discussion. >>Finally, I
believe Mr. Ritchey is correct, but using the following >assumptions: >>-
Thetrace structure and geometry does not approach a resonant structure at
>the fundamental or appreciable harmonics of the operating frequency. >- The
reference plane structure supporting the return currents of the >surface
trace does not significantly contribute to reference plane resonance >and
induced voltage on attached cables. >>Respectfully, >>Brent DeWitt
>>-----Original Message----- >From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey >Sent:
Sunday,October 19, 2003 6:06 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
accuracyof 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
sourcesof 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
costto 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 >>To:
>>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
Emissionsand = >>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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