# Study on the shielding effectiveness of double-layer PCB shielding cavity (ii)

- Oct 20, 2018-

The influence of the distance between the dielectric plate and the second aperture on the shielding effectiveness

The dielectric plate size does not change to 300 mm x 120 mm x 1 mm.Here, the q is taken as 50 mm,100 mm and 290 mm respectively, and finally compared with the situation without PCB board.

FIG. 5 shows that the farther the dielectric plate is from the second hole crack, the lower the shielding effectiveness.When the plate from the second floor of the medium hole seam 50 mm, most of the coupling field reflection occurs, coupled cavity, so the second cavity center field is the smallest, shielding effectiveness is one of the biggest, with the increase of distance, cavity center field strength has increased gradually, when increased to 290 mm cavity center reach maximum field strength, and no field strength of medium plate was close, shielding effectiveness and without close when medium plate.

2.3 influence of dielectric plate number on shielding effectiveness

The dielectric plate size is 300 mm x 120 mm x 1 mm. When there is only one dielectric plate, the dielectric plate is placed 100 mm away from the second hole seam, that is, q=100 mm in FIG. 1.When there are two dielectric plates, place them 50 mm and 100 mm away from the second hole seam, that is, in FIG. 1,q= 50 mm and q=100 mm. When there are three dielectric plates, place them 50 mm,100 mm and 150 mm away from the second hole seam, that is, in FIG. 1,q= 50 mm,q=100 mm and q=150 mm.The simulation results are shown in figure 6.

As can be seen from figure 6, the shielding effectiveness at the center of the cavity increased with the increase of the number of dielectric plates.

2.4 influence of different placement modes of dielectric plates on shielding effectiveness

The dielectric plate is of the same size and is placed in the following three different ways: parallel to the second hole seam, and 100 mm away from the ground hole seam;Parallel to the side, placed perpendicular to the center of the long edge of the hole slit;Parallel to the ground, placed perpendicular to the center of the short edge of the aperture.The three placement modes are shown in figure 7.

The simulation results of the three cases are shown in figure 8.

It can be seen from figure 8 that the shielding effectiveness is the worst when the dielectric plate is placed parallel to the ground, while the other two placement modes have little influence on the shielding effectiveness.

3. Influence of integrated operational amplifier circuit board on shielding effectiveness

There is still a certain difference between the actual printed circuit board and the equivalent macroscopic dielectric board. In this case, the macroscopic dielectric board is replaced by the integrated operational amplification board, as shown in figure 9.

Contrast medium plate and the circuit board in the shielding cavity influence on shielding effectiveness, set the medium plate size is the same as the circuit board, all are 75.59 mm x 25.69 mm x 0.711 2 mm, both place the model in the center of the shielding cavity cavity after the origin of the z axis 99.288 8 mm position, the medium plate for the aforementioned conductivity for sigma = 0.22 S - m - 1 dielectric constant for epsilon r = 2.65 the macroscopic dielectric plate, printed circuit board used as shown in figure 9 loading of integrated operational amplifier circuit board.Using CST, the PCB model of the circuit board is imported into the microwave studio of CST. After simulation, the results are shown in figure 10.

It can be seen from FIG. 10 that, under the same size, thickness and placement position, the shielding effectiveness obtained by macroscopic dielectric board and printed circuit board is not different from each other, and the error of replacing printed circuit board by macroscopic dielectric board is small.

After loading the printed circuit board, the cavity shielding efficiency is mainly shown in the variation of electric field intensity and surface current on the circuit board surface. Through CST simulation, the following results are obtained.

3.1 electric field on board surface

It can be seen from FIG. 11 and FIG. 12 that the maximum field strength without shielding is 11.070 7 V·m-1 and the maximum field strength is 0.164 V·m-1 with shielding, indicating that the shielding cavity has a good shielding effect on the circuit board.If there is no shielding, the circuit board will work normally and will be damaged seriously.

3.2 circuit board surface current

In figure 13, the maximum current on the surface of the circuit board without shielding is 0.014, 93 A·m-1. In figure 14, the maximum current on the surface of the circuit board with shielding is 2.091 8e-005 A·m-1, which is significantly smaller than that without shielding, indicating that the shielding cavity has A good shielding effect on the circuit board.

4 conclusion

In this paper, the equivalent model of the transmission line is used to derive the calculation formula of the shielding effectiveness of the rectangular cavity with double-layer circuit board. The correctness of the formula is verified by simulation.The closer the dielectric plate is to the second hole crack, the higher the shielding efficiency is;The higher the number of dielectric plates, the higher the shielding effectiveness.The shielding effectiveness is worst when the dielectric plate is placed parallel to the ground.According to the above conclusions, the reasonable layout of the internal circuit board can improve the shielding efficiency of the system when designing the housing, and the response frequency of the circuit in the cavity should avoid the resonant frequency of the cavity.In this article, in order to more relevant practical applications, will be replaced by equivalent medium plate loading PCB model of integrated operational amplifier circuit, by CST simulation, verify the macro equivalent medium plate instead of the effectiveness of the printed circuit board and similarity, and verified the shielding cavity to the internal circuit board good shielding effect and the influence of shielding cavity on the circuit board function.