The motherboard inside the computer is mixed with a variety of high-frequency circuits, digital circuits and analog circuits. When they work, a large number of high-frequency electromagnetic waves interfere with each other, which is EMI. EMI can also be emitted outward through the wiring of the mainboard or external cables, causing electromagnetic radiation pollution and affecting the normal operation of other electronic equipment.

The chip on the PC board is both an electromagnetic interference object and an electromagnetic interference source. In general, we can divide these electromagnetic interferences into two categories: series-mode (differential mode) and common-mode (grounded). Take the two PCB wiring on the motherboard (connecting the main board components of the conductor) for example, the so-called string mode interference, refers to the interference between the two wiring; Common-mode interference is caused by the potential difference between the two lines and the PCB ground wire. Series mode interference current acts between two signal lines, and its conduction direction is consistent with waveform and signal current. Common-mode interference current acts between the signal line and the ground wire. The interference current flows through half of the two signal lines in the same direction, and the ground wire is the common loop.

If the common-mode current generated by the board is not filtered by attenuation (especially common-mode current in the wiring of high-speed interfaces such as USB and IEEE 1394), then common-mode interference current can easily pass through the interface data lines to generate electromagnetic radiation -- common-mode radiation in the cable due to common-mode current. FCC of the United States, CISPR22 of the International Special Committee on Radio Interference and GB9254 of China and other standards and specifications have relevant restrictions on the common mode conducted interference and radiation emission of the communication port of information technology equipment. In order to eliminate the input interference signals and various interference induced by signal lines, we must reasonably arrange the filter circuit to filter the interference of common mode and series mode, common mode inductance is a part of the filter circuit.

Common-mode inductor is essentially a bidirectional filter: on the one hand, it filters out common-mode electromagnetic interference on the signal line, on the other hand, it inhibits itself from sending out electromagnetic interference, so as to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment.