The concept of differential mode versus common mode coupling is fundamental to an understanding of EMC.

Consider two PCB assemblies interconnected by a pair of wires, and mounted over a metal structure such as a chassis. The wires carry signal currents in differential mode (go and return) along the pair and back, and the wires should, if properly designed, be in close proximity. A radiated field can couple to this system and induce differential mode interference between the two wires; similarly, the differential current will create a radiated field of its own. The interaction with the field is largely determined by the length of the wires and their separation distance, or the area of the whole loop, which is under the designer's control and can be suitably minimised. No external structures are involved in the circuit.

The same wires also carry currents in common mode, that is, flowing in the same direction on each wire. These currents may have nothing at all to do with the intended signal currents. They may be induced by an external field coupling between the loop formed by the wires, the chassis and the various impedances (mostly parasitic and not controlled on the circuit schematic) connecting the assemblies to the chassis. Or they can be generated by internal noise voltages between the chassis reference point and the wire terminals, and be responsible for radiated emissions. Critically, the current return path is potentially large and certainly ill-defined, and includes structures (such as the chassis) which are not designed as part of any circuit.

Incoming interference generates current flow principally in common mode; differential voltages are created within the circuit, and hence cause susceptibilities, when this current takes different paths through different impedances within the circuit structure.

The concept can be scaled down; for instance, the same equivalent circuit applies to a single PCB assembly. Here, each circuit track pair (signal and return) takes the place of the cable between two assemblies. Alternatively, it can be scaled up, to apply to external cables between two enclosures in an installation.

A video demonstration of differential and common mode noise can be found below,

Cable Modes

Differential-mode current, I_DM, is the current which flows in one direction along one cable conductor and in the reverse direction along another. It is normally equal to the signal or power current, and is not present on the shield. It contributes little to the net radiation because the total loop area formed by the two conductors is small; the two currents tend to cancel each other.

Common-mode current I_CM flows equally in the same direction along all conductors in the cable, including the screen if it is present, and may or may not be quite unrelated to the signal currents. Any ground noise due to circuit operation which appears between the point of the cable connection and the ground reference of the circuit will contribute to this. A very typical source of such noise is microprocessor clock and data currents flowing in the 0V rail of the PCB; without further treatment, this noise can then appear on interfaces such as low-speed data lines or analogue transducers.

That part of the intended signal current which does not return via the cable because of stray leakage paths, does appear as a common mode component. In this particular case the conversion mechanism is known as “Longitudinal Conversion Loss” (LCL). It is particularly significant for telecom ports which carry signals within the interference frequency range being measured, such as LAN and other high-speed data networks. Perfectly balanced lines carrying a perfectly balanced differential signal, would not create any common mode component. In reality, all such lines and their drivers exhibit a certain amount of imbalance, and this results in some degree of common mode conversion.

Common mode cable current returns via the installation's ground network and therefore the radiating loop area is large and uncontrolled. As a result, even a small I_CM can result in large emitted signals; and conversely, large common mode currents can be induced by relatively small radiated fields.

The following video illustrates the return current path within a transmission line and shed light on the generation of common-mode current in two distinct scenarios. The first case involves wires emerging from a shielded cable, while the second case explores wires exiting a shielded enclosure.

Common mode on supplies

Two different modes can be identified when we look at power supply connections; either of these could be called common mode.

In one case (A), the return path is the chassis and/or safety earth connection; for safety class I apparatus, that is with a safety green-and-yellow wire in the mains lead, this will be the return and any accessible metalwork of the unit, usually including the chassis, will be connected to it. Common mode disturbances are then measured between Live and Neutral, taken as one line, and the safety earth. The same approach can be applied to DC supplies when both + and - lines are isolated from the metalwork.

In the other case (B), all three lines of a safety class I apparatus - L, N and E - are considered together; common mode disturbances then appear between all three lines and an external ground reference. Even if the safety earth line is at some point connected to the ground reference, at the apparatus end the impedance of the cable "gets in the way" of this connection. Other external connections, and/or stray capacitance, complete the common mode path.

Distinguishing these two approaches is important. For instance, Y-capacitors from live to earth and neutral to earth will have an effect for case (A), but may have no effect for case (B). Case (A) is relevant for conducted RF emissions tests, where the safety earth connection to the ground reference plane is an integral part of the LISN terminating impedance. It's also one of the coupling modes for the surge immunity test.  Case (B) is relevant for radiated emissions and susceptibility, where the mains supply cable acts as a single antenna structure, and also for EFT/Burst and conducted RF immunity, since in these tests the stress is applied to the supply input on all lines together.