EMC Question of the Week: November 3, 2025
A 100-MHz signal generator in a shielded enclosure drives a matched load on a circuit board through a 50-cm coaxial cable. On the board, the signal travels 8 cm on either a pair of coplanar traces or a microstrip trace. Common-mode current and radiated emissions are reduced by choosing the
- coplanar traces
- microstrip trace
- path with the lowest inductance
- path with the better impedance match
Answer
The best answer is “b.” The common-mode voltage that drives the cable relative to the board is determined by the change in the electrical balance at the interface. It is completely independent of the transmission line inductance or characteristic impedance. Since the coaxial cable is perfectly unbalanced, the microstrip trace (which is very unbalanced) generates relatively little common-mode voltage. The coplanar traces (which are balanced) generates a common-mode voltage equal to half the signal voltage at the interface.
In this experiment, the common-mode voltage can be determined precisely from the signal voltage and the computed imbalance factors of the circuit board transmission lines. The common-mode current on the cable is this common-mode voltage divided by the input impedance of the dipole antenna formed by the source/cable and the circuit board.
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