[SI-LIST] AW: Re:AW: A common mode current on differential PCB pair cause EMI?
- From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
- To: Tesla <emcesd@xxxxxxx>
- Date: Wed, 19 Dec 2012 09:17:24 +0000
Hi Tesla,
I dont think common modes can be assigned evil or not. Fact is, that common
modes can radiate, thus they can be dangerous (they dont have to). And another
fact is, that you will always have common modes in your system (not necessarily
on every differential trace, but there are many other signals).
As long as there are no resonances of any kind, radiation will be very limited,
and this is the tricky part as there is no ideal GND plane, there are via
openings, there are multiple different currents in your system, there are
discontinuities along the traces, there are slots in GND and Power planes, and
Planes do form a large capacitor, and many other things can influence the
common modes.
EMI problems have been there in the past, and will be in the future, and most
of these problems are caused by the PCB Layout, and not by the signals itself.
With proper Design, the EMI can be reduced to an acceptable minimum.
BR
Gert
Von: Tesla [mailto:emcesd@xxxxxxx]
Gesendet: Mittwoch, 19. Dezember 2012 09:56
An: Havermann, Gert
Cc: si-list@xxxxxxxxxxxxx
Betreff: Re:[SI-LIST] AW: A common mode current on differential PCB pair cause
EMI?
Hi, Gert
Thanks.
The devil is Mode Conversion.
if we want the common mode to radiate, Do we have to let the common mode fly
through geometrical features ,e.g. plane resonance, slot antennas, via filters ?
If there are not "standing wave" for common mode current, common mode is a
friend, not a evil?
Tesla
At 2012-12-19 16:06:04,"Havermann, Gert"
<Gert.Havermann@xxxxxxxxxxx<mailto:Gert.Havermann@xxxxxxxxxxx>> wrote:
>Tesla,
>
>it's not the common mode itself that creates high EMI propability. The problem
>is Mode Conversion!
>Common modes do not cancel out, thus they "radiate" and this in not only a
>crosstalk issue, but also an EMI issue.
>If your diff. pair is out of balance (regardless if it is microstrip or
>stripline), then common modes (from aggressors) that couple into that line are
>converted to differential and start to distort the signal. Vice versa, the
>signal itself is partially converted into common mode which then couples into
>other traces or geometries.
>If standing waves or other resonances occur due to impedance mismatch or due
>to geometrical features (e.g. plane resonance, slot antennas, via filters)
>then these common modes will be radiated, or EM waves are coupled into the
>system.
>
>BR
>Gert
>
>
>----------------------------------------
>Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339
>Espelkamp; Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB
>8808; Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning,
>Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost
>
>
---------------------------------------------------------
Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 Espelkamp;
Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB 8808;
Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning,
Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost
-----Ursprüngliche Nachricht-----
>Von: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
>[mailto:si-list-bounce@xxxxxxxxxxxxx] Im Auftrag von Tesla
>Gesendet: Mittwoch, 19. Dezember 2012 04:02
>An: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
>Betreff: [SI-LIST] A common mode current on differential PCB pair cause EMI?
>
>HI, Experts.
>
>Suppose a differential microstrip trace pair on PCB with complete GND
>reference plane.
>I was told that the common mode on that pair casue EMI issue.
>But i think the differential pair, also are two independant microstrip traces.
>If common mode on this pair cause EMI, then a single TML also cause EMI. But a
>micrstrip does not cause much EMI in my limited knowledge(This assumption may
>be wrong).
>So i think the common current on PCB with complete GND reference plane does
>not cause EMI,it may be harmful to the differential amplifier in receiver.
>
>Thanks.
>
>Tesla
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