EMC Question of the Week: August 31, 2020

Equations for Common-Mode and Differential-Mode Components

In a multiwire transmission line, parallel signal conductors carry currents I1 and I2, respectively, where I1 and I2 point in the same direction. The common-mode current on the wire pair is 

  1. I1 - I2
  2. I1 + I2
  3. (I1 - I2)/2
  4. (I1 + I2)/2


The correct answer is "b". This question is similar to last week's question, but it asks for the common-mode current instead of the differential-mode current. The differential-mode current was (I1 - I2)/2 if and only if the wire pair is balanced. The common-mode current is the sum of the currents on the wires. It is not divided by 2, and there is no requirement that the wire pair be balanced.

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