According to the rule of thumb, four-layer boards are usually used in high-density and high-frequency situations. Multi-layer PCBs are more than 20DB better than two-layer boards in terms of EMC. Under the condition of a four-layer board, a complete ground plane and a complete power plane can often be used. Under this condition, it is only necessary to connect the ground wires of the circuits divided into several groups to the ground plane, and the working noise is specially handled