In all circuits both common-mode (CM) and differential-mode (DM) currents are present. Both types of current determine the amount of RF energy propagated between circuits or radiated into free space. There is a significant difference between the two. Given a pair of transmission lines and a return path, one or the other mode will exist, usually both. Differential-mode signals carry data or a signal of interest (information). Common-mode is an undesired side effect from differential-mode transmission and is most troublesome for EMC. For purposes of this discussion, reference to differential (twisted) pair transmission lines is not presented. When using simulation software to predict emissions, DM analysis is usually the form of analysis used. It is impossible to predict radiated emissions based solely on DM (transmission line) currents.
Differential-mode current is the component of RF energy present in both the signal and return paths that is equal and opposite to each other. If a 180° phase shift is established precisely, RF differential-mode currents will be canceled. Common-mode effects may, however, be developed because of ground bounce and power plane fluctuation caused by components drawing current from a power distribution network. Differential-mode signals will (1) convey desired information and (2) cause minimal interference as the fields generated oppose each other and cancel out if properly set up.
Common-mode current is the component of RF energy that is present in both signal and return paths, often in common phase to each other. The measured RF field due to CM currents will be the sum of the currents that exist in both the signal and return path. This summation could be substantial. Common-mode currents are generated by any imbalance in the circuit. Radiated emissions are the result of such imbalance. Poor flux cancellation may be caused by an imbalance between two transmitted signal paths or excessive impedance in the return path. If DM signals are not exactly opposite and in phase, magnetic flux will not cancel out. The portion of RF current that is not canceled is identified as “common-mode” current. Common-mode signals (1) are the major sources of cable and interconnect EMI and (2) contain no useful information.
Common-mode currents begin as the result of currents mixing in a shared conductive path such as power and return planes within a PCB, cable assemblies between systems, or the chassis enclosure. Typically this happens because RF currents flow through both intentional and unintentional paths. The key to preventing CM energy is to understand and manage RF return currents.
In any return path RF current will attempt to couple with RF current in the source path (magnetic flux traveling in opposite directions to each other). Flux that is coupled to each other (i.e., 180° out of phase) will cancel, permitting the total magnitude of flux to approach zero. However, if an RF current return path is not provided with symmetry to the source through a path of least impedance, residual common-mode RF currents will be developed across the source of impedance. There will always be some CM currents in any transmission line or system as a finite distant spacing must be present between the signal trace and return path (flux cancellation approaches 100%). The portion of DM return current that is “not” canceled becomes residual CM current.
To make a DM-CM comparison for both magnetic and electric field sources, consider a pair of parallel wires carrying a DM signal. Within this wire, RF currents flow in opposite directions. As a result, all RF fields are contained between the wire pair. This parallel set of wires will act as a balanced transmission line that delivers a clean differential (signal-ended) signal to a load.
Using this same wire pair, look at what happens when CM voltage is placed on this wire. No useful information is transmitted to the load since the wires carry the same voltage potential. This wire pair now functions as a driven dipole antenna with respect to ground. This antenna radiates unwanted CM fields with extreme efficiency. Common-mode currents are generally observed in I/O cables. This is why I/O cables radiate RF energy as well.