EMC Question of the Week: May 27, 2024

four pairs of differential signal traces routed on a circuit board

A differential signal propagates on two loosely coupled, parallel microstrip traces. Each trace has a 50-Ω characteristic impedance to the ground plane and the differential-mode impedance is 100 Ω. If the traces are routed closer to each other (so that they are no longer loosely coupled), the differential-mode impedance will be

  1. higher
  2. lower
  3. the same
  4. undefined


The best answer is “b.” Moving the traces closer reduces the inductance and increases the capacitance between them. Overall, this lowers the differential-mode impedance and makes the common-mode impedance slightly higher.

For loosely coupled traces, terminating each trace in its own characteristic impedance provides a matched termination for both differential-mode and common-mode propagation. For tightly coupled traces, three resistances are required to match both modes. However, tightly coupled traces are often terminated with their odd-mode impedance (half the differential-mode impedance). This provides a matched termination for the differential-mode signals, but not the common-mode. 

Note that the differential-mode impedance is the ratio of the voltage (between signal conductors) to the current in a forward-propagating differential-mode signal. For two identical signal conductors, the odd-mode impedance is always half the differential-mode impedance. If the signal conductors are not identical (e.g., have different impedances to ground), the definition of the differential-mode impedance is unchanged, while the definition of the odd-mode impedance is ambiguous. 

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