Dear all, I'm sure there are some people at EMC-PSTC who just knows the answer to question which worry me since long time, so I decided to ask you. Let us imagine 10cm 2 wire cable. At the right side first wire is grounded, and the second one is left opened. At the left side both wires are connected together and driven by ideal 1V AC source (in reference to this ideal ground). Assuming wires are not too close to each other I suppose there is 1V at this opened wire end. Now we put the ferrite tube on both these wires. When the ferrite is inductive it is 1:1 trafo giving me 0V at this opened wire end. My question is: What the voltage will be when the frequency is such that ferrite is only resistive and 0 inductive ? Is it still trafo or the wires are just two separate resistors ? I came to that (simplified) question when I was trying to imagine what is better: to put the three RS485 wires (A,B and GND) by the single ferrite tube (200 ohm) or to put each of them by multilayer ferrite bead (1k ohm each) to be more resistive to EN 61000-4-2, EN 61000-4-3, EN 61000-4-4. The RS485 was opto-isolated so I was not interested in frequencies below 10MHz. If resistive ferrite is still trafo single tube looks better, but if not the ferrite beads (higher R) seems to be better. I suppose single tube is better, but don't know exactly why. Thanks in advance for any comments. Best regards Piotr Galka
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