The common mode inductance is wound on the upper and lower half rings of a magnetic ring, respectively, with the same number of turns but opposite coils. The common mode interference is the same, so the magnetic lines of force formed in the magnetic ring are superimposed on each other, and the inductance impedance is large to attenuate the interference. The magnetic lines of force formed by the differential mode signal in the magnetic ring cancel each other out and have no inhibitory effect. Only the coil resistance and small leakage inductance have a slight influence on the differential mode signal.
The common mode inductor is essentially a two-way filter. On the one hand, it filters out the common mode signal interference on the signal line, and on the other hand, it suppresses the signal line itself from emitting electromagnetic interference. Interfering signals are well suppressed by the common-mode inductance, while differential signals have little effect.