EMC Question of the Week: September 7, 2020

Circuit Board to Wire Harness Connection with Multiple Twisted Wire Pairs

In order to prevent differential-mode to common-mode conversion, what property of an electrical connector needs to be matched to the signal conductors on both ends?

  1. wave velocity
  2. characteristic impedance
  3. electrical balance
  4. all of the above


The correct answer is "c". In transmission lines, conversion between differential and common modes occurs if (and only if) there is a change in the electrical balance. Changes in the wave velocity and characteristic impedance may affect the differential signal propagation (e.g. by causing reflections), but they do not result in mode conversion.

Of course, at frequencies where the connector is electrically very short (i.e. compared to a wavelength), The common-mode voltage produced at one end of the connector can be largely cancelled by the common-voltage produced at the other end of the connector. For this reason, maintaining the correct level of balance (or imbalance) in a short connector may not be critical at low data rates. 

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