Stacking design of PCBs for emcation

EMC design is most suitable for four-layer PCBs. From the perspective of local sensitive circuits in EMS, metal enclosures or metal enclosure shielding can solve interference problems. From an EMI perspective, sometimes four-layer boards cannot meet the requirements of radiation limits, so the number of layers should be increased because multi-layer boards can generate high du/dt signals, and di/dt ensures a small area in the signal loop transmission process, providing low impedance backflow for high-speed signals.

The basic principle of PCB stack design is to arrange high-speed signal layer and power layer near the ground plane. The following figure shows the stacked design of 4-layer and 6-layer boards. In Figure 4, a refers to three common 6-layer PCB designs for high-speed signal layers b, 4c, and 4d.

Stacking design diagram of PCB

In the design of a three-layer and 6-layer PCB, Design b is the worst, Layer S should be a high-speed signal layer, and Layer S2 in Design C and d should be a high-speed signal layer. Design C is best because each signal layer is closely adjacent to the ground plane to ensure the shortest signal return path, and layers S2 and P are shielded by GND1 and GND2. Compared to design, S3 is far away from the GND layer in design d and P can only cause a single side effect of the design, not a double side effect.

PCB medium equivalent antenna

The basic function of an antenna is to radiate and receive radio waves. During the radiation process, high-frequency currents can be converted into electromagnetic waves; During reception, electromagnetic waves are converted into high-frequency currents. EMC in field radiation mainly refers to far field radiation. The formation of an antenna depends on two basic conditions: an RF signal source and a signal source with a certain length connected to the RF conductor. In the engineering field, it is believed that when the conductor length conforms to the formula= λ/ At 20, when the antenna effect occurs, l=（ λ/ 4) "At n, the antenna effect is maximum, and n is a natural number.".

When the signal is transmitted inside the PCB, the inner ring has the same effect as the ring antenna. The larger the loop area, the greater the antenna effect, and the more stringent the effect. PCB circuit control can effectively prevent mode difference interference, which is feasible in practice. However, increasing the length of the printed line can lead to significant rod antenna effects. Therefore, during PCB layout, the length of interconnecting signals should be minimized. If the internal printed line of the PCB cannot meet the antenna radiation requirements due to length limitations. In this case, the I/O cable can be seen as an extension of the printed line to meet radiation requirements. Even if it does not exist, it does not exist, even if it does not exist. I/O stable direct connections should also stop cable crosstalk coupling at I/O.