Common Mode and Differential Mode Signals

A differential mode signal is one that exists between the conductors of a cable. At any given point along the cable, current flowing on  one conductor is precisely balanced by current flowing in the other direction on the other conductor. The intentional signals carried by cables are differential mode signals – the audio or video signal in a home audio system, Ethernet signals on CAT5/6 cable, and the RF signal carried by the feedline connecting our antennas to our transceivers.

A common mode signal is one that places equal voltage on all conductors – that is, the voltage between the two ends of the cable are different, but there is no voltage between the conductors. Antenna action produces a common mode voltage and current along a cable. The antenna current induced on audio and video wiring is a common mode signal. That is, with "ideal" cable, there is no differential voltage between the signal conductors as a result of this antenna action. If the cable is shielded, nearly all of this current flows on the shield (and skin effect causes it to flow on the outside of the shield). If the shield is ideal (that is, if the current is distributed with perfect uniformity around it), the field inside the shield will be zero, and thus none of this antenna current will flow inside the cable. Conversely, when a cable shield is carrying differential mode current, as in the case of coax, skin effect will cause that differential mode current to flow on the inside of the shield.

The real world is not ideal, so most interfering signals will simultaneously exist in both common mode and differential mode, but in most real world conditions, one or the other mode dominates.

Several cable defects (essentially manufacturing tolerances) certainly can and do convert this  "common mode" antenna current to a differential signal (that is, a voltage between the signal conductors), but that is rarely the most powerful coupling  mechanism. One common defect that affects both balanced and unbalanced cables is imperfect construction of cable shields. In even the best "real world" balanced twisted pair cables, there are imbalances in the capacitance between "red" and "black" conductors to the shield on the order of 5%. [B. Whitlock, JAES, June 1995] In balanced paired cables that use "foil/drain" shields, there is even more imbalance in the inductive coupling between each conductor and the shield. Noise (or RFI) coupled by this mechanism is called "shield-current-induced noise," or SCIN. [N. Muncy, JAES, June 1995] All three of these mechanisms convert shield current to a differential signal at system input and output terminals. If the cable is an unshielded pair (loudspeaker cable, for example), RF will be induced approximately equally on both conductors (but, depending what the input circuit of the equipment looks like at RF, current flow into the equipment may not be equal on both conductors). This can also produce a differential voltage at the input (or output) terminals.

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