EMC Question of the Week: October 27, 2025
A differential signal propagates on a pair of microstrip traces then transitions to a twisted wire pair when it leaves the circuit board. Which of the following would cause a differential-mode to common-mode conversion at the interface?
- change in the characteristic impedance
- loss of ground reference
- either of the above
- none of the above
Answer
The best answer is “d.” Mode conversion only occurs if there is a change in the electrical balance at the interface. As long as the trace pair is balanced and the wire pair is balanced, there is no mode conversion.
A change in the characteristic impedance would cause some of the differential-mode signal to be reflected at the interface, but the reflected signal would still be differential-mode. The loss of the ground plane at the interface has no impact on the mode conversion as long as both sides of the signal see the same change in the impedance.
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