Muriel,
One more opinion on the subject. I was going to reply to you yesterday but I
then I didn't get the time to do it..
I agree with Jim Knighten. The first thing that came to my mind was the paper
that he mentioned in his second reply:
"Investigation of fundamental EMI source mechanisms driving common-mode
radiation from printed circuit boards with attached cables" by:
D.M.Hockanson, J.L.Drewniak, T.H.Hubing, T.P.Van Doren, Fei Sha and M. J.
Wilhelm - IEEE Transactions on EMC, Vol.38, no.4, November 1996.
I think this is a very good and solid reference covering the subject (addressed
to common-mode radiation).
It also contains other useful references on the subject of common mode
disturbances.
The physical phenomena of the common-mode emissions are essentially related to
the finite (non-zero) impedance of each and every conductor. Every circuit
carrying time-varying (differential mode) voltages/currents generates common
mode voltages/currents.
This is deep-rooted in physics and specifically in Maxwell's equations.
Faraday's law is one of them: it says that every time you have a variable
current -> variable magnetic field, you have a voltage drop -> that voltage drop
drives the cm current.
Typical case: the voltage drop across the inductance of return paths in any
circuit drives a common mode current that flows across a stray capacitance (you
never have open circuits in rf !).
The higher the frequencies in your signals (differential mode voltage/current),
the higher the effects of stray inductances and capacitances in any circuit.
These are inevitable phenomena. One key factor in EMC is to minimize these
parasitic inductances and capacitances.
As for your example, a DC battery feeding a resistor doesn't have cm currents in
stationary conditions (constant voltage/current). But when you open or close the
circuit you have transients that generate cm currents.
Whether they are negligible or not, that depends on the stray impedances in your
circuit and on the wave-shape of the transients (the faster they are, the more
likely you will see cm currents).
For all the details go to the above mentioned paper....
Just my 0.2 cents..........
Regards,
Paolo Roncone
Compuprint s.p.a. - Italy
Muriel Bittencourt de Liz <[email protected]> on 04/05/2000 17.12.42
Please respond to Muriel Bittencourt de Liz <[email protected]>
To: EMC-PSTC List <[email protected]>
cc: (bcc: Paolo Roncone/IT/BULL)
Subject: Re: origin of common-mode currents
Group,
Thanks for all the answers to the question of common-mode emissions.
But, as I did once, most of you didn't understand what I meant.
I've already read books of EMC (like C.R. Paul), saw a lot of
homepages/magazines (RBItem, chapters of IEEE EMC society), read books
of electronics-area... etc... But all this references did not mention
anything about the physical phenomena of the common-mode emissions. I
don't know if I'm not being clear... So, please question me about more
details...
For example, in a simple circuit (a DC battery feeding a resistor), do I
have common-mode emissions? Are the common-mode emissions inherent from
any physical system? Can I model them in HF?
Thanks in advance.
Muriel
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