## EMC Question of the Week: June 10, 2024

For a differential trace pair, terminating each trace in the odd-mode impedance provides a matched termination for both the odd-mode and even-mode propagation provided the coupling between the traces is

- strong
- weak
- ideal
- intrinsic

## Answer

The best answer is “b.” If the traces are weakly coupled (Z_{M} is large relative to 2Z_{1}), the differential-mode impedance is the sum of the two trace impedances. In this case, matching each trace independently provides a good match for both differential-mode and common-mode propagation. The value of the matching impedance is Z_{1}, the odd-mode impedance.

On the other hand, if the traces are strongly coupled, the differential-mode impedance is not simply the sum of the two trace characteristic impedances. It is a function of the mutual impedance between the traces as well. In this case, terminating each trace with the odd-mode impedance still matches the differential-mode (odd-mode) propagation. However, those two impedances in parallel are no longer the correct match for the common-mode (even-mode) propagation.

Have a comment or question regarding this solution? We'd like to hear from you. Email us at