EMC Question of the Week: February 12, 2018
Some of the power in a differential-mode signal propagating down a transmission line is converted to common-mode when,
- the transmission line is not balanced
- there is a change in the electrical balance of the transmission line
- there is a discontinuity in the ground structure
- there is a change in the characteristic impedance of the transmission line
Answer
The correct answer is "b". Mode conversion occurs when there is a change in the electrical balance of the signal carrying conductors. At the point of the change in the balance, a common-mode voltage is created that is exactly equal to the differential-mode voltage at that point times the magnitude of the change in the electrical balance.
Differential signals can propagate without mode conversion along balanced or unbalanced transmission lines. A discontinuity in the ground structure will only produce mode conversion if it changes the balance of the transmission line. For example, suppose a differential signal is propagating on a pair of identical circuit board traces above a ground plane. If the signal leaves the board on a twisted-wire-pair leaving the ground plane behind, there is no conversion from differential mode to common mode as long as the trace pair and the twisted-wire-pair were both balanced.
Finally, changes in the characteristic impedance will cause reflections of the differential-mode signal, but they will not result in mode conversion if there is no corresponding change in the electrical balance.
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