Differential-mode (DM) and Common-mode (CM)
Wanted signals are always DM: they flow along the ‘send’ conductor, and flow back along the ‘return’ conductor(s). In single-ended signalling, all the return currents share a common conducting structure, usually the 0V of the DC power distribution system. In balanced (or ‘differential’) signalling there is a dedicated conductor for the return current path as well as for the send path, and for good signal quality and EMC the two are routed together as a twisted pair.
However, unavoidable imbalances in the physical realisations of interconnections in PCBs and cables cause CM voltages and currents to arise, as shown by Figure 1. CM currents flow out on both send and return conductors at the same time, and return via another route, often the safety earth structure or the mains power distribution network. CM currents are typically measured in μA, whereas DM currents are in tens or hundreds of mA, maybe even Amps – but the much larger loop areas associated with CM noise currents and voltages makes them more important for EMC than the DM signals that originated them. Above about 1MHz most unwanted emissions are mostly CM.
A great deal of RF interconnect design is concerned with making cables and PCB traces that have better balance, to reduce the ‘longitudinal conversion loss’ (LCL) that converts the wanted signal energy into unwanted CM noise. The better the LCL, the further the wanted signal will propagate with an acceptable quality, or the higher the frequency that can be sent with acceptable quality over the same distance – hence the computer networking industry’s progress from Cat 5 to Cat 6 and eventually to Cat 7 cables for Ethernet, each increase in Category is accompanied by better balance, resulting in better LCLs at higher frequencies and reduced generation of CM noise for a given type of signal.
Because of the existence of DM and CM signals and noises, we need to be able to apply filtering techniques to both of them. Below 1MHz, we are more likely to be concerned just with filtering DM signals and noise. But at higher frequencies we can use DM filtering to reduce the amounts of RF present in conductors, so as to reduce the amount of CM noise currents and voltages created by the imbalances in the interconnects. We also use CM filtering to reduce the amounts of CM noise present.
