Hello Muriel and Group, I'll give it a try.
I present a 1-day course in EMC system design 5-10x a year to Dutch engineers in little groups, and this is an extract. Imagine a digital circuit. 2 Ports A,B connected to each other. Input to gate A is a 10Mhz square wave or whatever. Please take a moment and draw the circuit on a paper, including the ground path. Connect a 2 wire cable to the input and one to the output. Draw the ground connections in bold. in this digital circuit one sees a source (A) and a load (B) , just as your battery and resistor. The current required to send a "1" (+5V) from gate A to gate B flows trough the output from gate A to the input of gate B, via the ground back to gate A. This is a closed loop, creating magnetic field, has an amount of self induction and radiates DM-emission. What you wanted was CM emission current, not DM however. I wrote that the current loop has induction, therefore some voltage drop will occur on the entire length of the loop. Also on the part that we call DGND or ground. Draw this voltage source Vcm between the ground connections of gate A and Gate B in the ground wire. Presto, CM source revealed. If both cable grounds wires are physically large, a current will flow from cable A (ground wire) to cable B (ground wire). As Murphy comes in, your cables resonate at the EMI frequency and behave like a dipole antenna having 50 ohms of resistance. Let me tell you that approx 5 uA is sufficient to create CM emission problems, then you need 250 uV of Vcm to have a major CM problem. That is not much. Remedy (1 of 15): connect the grounds of the cables both at output or both at the input, therefore excluding Vcm from ground wires. Clear Muriel ? Regards, Gert Gremmen, (Ing) ce-test, qualified testing =============================================== Web presence http://www.cetest.nl CE-shop http://www.cetest.nl/ce_shop.htm /-/ Compliance testing is our core business /-/ =============================================== >>-----Original Message----- >>From: [email protected] [mailto:[email protected]]On Behalf >>Of Muriel Bittencourt de Liz >>Sent: Thursday, May 04, 2000 5:13 PM >>To: EMC-PSTC List >>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 >> >>------------------------------------------- >>This message is from the IEEE EMC Society Product Safety >>Technical Committee emc-pstc discussion list. >> >>To cancel your subscription, send mail to: >> [email protected] >>with the single line: >> unsubscribe emc-pstc >> >>For help, send mail to the list administrators: >> Jim Bacher: [email protected] >> Michael Garretson: [email protected] >> >>For policy questions, send mail to: >> Richard Nute: [email protected] >> >> >>
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