Radiation generation: Currents cause radiation, not voltages, static charges create electrostatic fields, constant currents create magnetic fields, and time-varying currents create both electric and magnetic fields. There are common mode currents and differential mode currents in any circuit. Differential mode signals carry data or useful signals. Common mode signals are a negative effect of differential mode.

Differential mode current: equal in magnitude and opposite in direction (phase). Due to the distributed capacitance, inductance of the trace, discontinuous impedance of the signal trace, and the signal return path flows through unexpected paths, etc., the differential mode current will be converted into a common mode current Common mode current: the magnitude is not necessarily equal, the direction ( phase) are the same.

The external interference of the equipment is mostly common mode, and differential mode interference also exists, but the common mode interference intensity is often several orders of magnitude larger than the differential mode intensity. External interference is also dominated by common mode interference. Common mode interference itself generally does not cause harm to the equipment, but if the common mode interference is converted into differential mode interference, it will be serious, because the useful signals are all differential mode signals.

The magnetic field of the differential mode current is mainly concentrated in the loop area formed by the differential mode current, while outside the loop area, the magnetic lines of force will cancel each other; the magnetic field of the common mode current is outside the loop area, and the magnetic field generated by the common mode current is in the same direction.

Many EMC designs of PCBs follow the above theory.

The ways to suppress interference on the PCB board are: reducing the area of the differential mode signal loop.

Reduce high frequency noise return (filtering, isolation and matching).

Reduce common mode voltage (ground design).