EMC Question of the Week: February 25, 2019
Differential-mode signals propagating down a transmission line are partially converted to common-mode noise when the transmission line,
- is unbalanced
- experiences a change in the balance
- experiences a change in the impedance
- any of the above
Answer
The correct answer is "b." Mode conversion occurs if and only if there is a change in the electrical balance of a transmission line. The common-mode voltage developed at any point in the signal path is equal to the differential-mode voltage at that point times the imbalance change (i.e. change in the current division factor) at that point.
Balanced sources driving balanced lines terminated in balanced loads do not experience mode conversion. Similarly, there is no mode conversion when single-ended sources drive unbalanced lines (e.g. coaxial cables or strip lines) terminated in unbalanced loads. In situations where mode conversion is undesirable, it is important to maintain the same level of electrical balance from the source all the way to the load.
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