The motherboard inside a computer contains a mix of high-frequency, digital, and analog circuits. When these circuits operate, they generate a large amount of high-frequency electromagnetic waves that interfere with each other-this is electromagnetic interference (EMI). EMI can also be emitted outwards through motherboard wiring or external cables, causing electromagnetic radiation pollution and affecting the normal operation of other electronic devices.
The chips on the PC board are both objects and sources of electromagnetic interference during operation. Generally speaking, we can divide these electromagnetic interferences into two categories: differential-mode interference (DMO) and common-mode interference (CMI). Taking two PCB traces (wires connecting various components on the motherboard) as an example, differential-mode interference refers to interference between the two traces; while common-mode interference is caused by the potential difference between the two traces and the PCB ground line. Differential-mode interference current acts between the two signal lines, and its conduction direction is consistent with the waveform and signal current; common-mode interference current acts between the signal line and the ground line, with half of the interference current flowing through each signal line in the same direction, and the ground line serving as a common loop.
If the common-mode current generated by the circuit board is not attenuated and filtered (especially the common-mode current on high-speed interface lines such as USB and IEEE 1394), then the common-mode interference current can easily generate electromagnetic radiation through the interface data lines-common-mode radiation generated in the cable due to the common-mode current. Standards such as the US FCC, CISPR22 of the International Special Committee on Radio Interference, and my country's GB9254 all have relevant restrictions on common-mode conducted interference and radiated emissions from communication ports of information technology equipment. To eliminate interference signals input to signal lines and various induced interferences, we must properly arrange filtering circuits to filter common-mode and common-mode interference; common-mode inductors are a component of these filtering circuits.
