Abstract: in order to improve the resistance of electronic components in equipment to electromagnetic interference from external and internal components, according to the transmission line theory, the double-layer PCB shielding cavity model is converted into a circuit diagram, and the equivalent formula of the central shielding effectiveness of the cavity is derived by using the circuit diagram.Using Matlab to generate the shield efficiency curve of transmission line method, and through the simulation software CST modeling simulation, the simulation results are in good agreement with the Matlab output curve, which verifies the correctness of the formula.CST was used to study the influence of some factors such as circuit board size, quantity, placement method and distance from the aperture to the shielding effectiveness.In order to be more practical, the cavity shielding effectiveness and the effect of the cavity on the function of the circuit board were studied by using the printed circuit board of loading integrated operation amplifier.
Electronic devices usually use chassis to shield the interference of external electromagnetic field, casing external usually opening to provide ventilation, visibility, and the hole can make external electromagnetic field coupling through hole slit to the device inside the casing, and inside the casing of the equipment or printed circuit boards induction current and voltage, reduce the performance of the equipment or components, severe damages to the internal equipment.Therefore, it is of great practical significance and value to study the electromagnetic shielding effectiveness for electromagnetic interference.From the previous studies, there are many ways to improve the shielding effectiveness of such as area, under the same hole array of shielding effectiveness of shielding effectiveness is better than that of single span and double layer shielding effectiveness of shielding effectiveness is better than that of single hole, also studies the influence of many factors on the shielding effectiveness, such as the size of the hole, shape, hole spacing, such as electromagnetic wave polarization direction.In this paper, the formula of the shielding effectiveness of the double-layer PCB shielding cavity is derived and verified by CST simulation.
The model of the double-layer shield cavity with rectangular hole loading circuit board is shown in figure 1.In general, the energy that is coupled into the shielding cavity through the aperture is more than that through the wall of the cavity. Therefore, the coupling energy is only considered.
In this paper, a double-layer shielding cavity model made of copper is adopted.A and b are the width and height of the shielding cavity, the length of the front cavity is d1, and the length of the back cavity is d2.W and l are the length and width of the cavity opening.P is the central observation point of the posterior chamber;Q is the distance from the inner hole to the PCB plate.The thickness of PCB plate is t ';The cavity thickness is t.
According to the transmission line theory proposed by M.P.R. obinson, the hole crack is equivalent to the coplanar strip line with short circuit at both ends, and the rectangular housing is equivalent to the wave guide with short circuit at the end.The equivalent circuit diagram of the model is shown in figure 2.
In figure 2, where V0 as equivalent source, Z0 for air wave impedance, which is about 377 Ω, Zos characteristic impedance for seam hole, the equivalent of rectangular waveguide characteristic impedance and propagation constant for Zgmn and Kgmn.
The effective width is obtained by Gupta's formula  :
After the transmission line between AB and AB, the equivalent voltage V2 and the equivalent impedance Z2 of point B can be obtained by the davenan theorem:
Circuit board is a complex whole, it includes the metal plates, wire, electronic components and the lossy medium and so on, in this using a conductivity of sigma = 0.22 S - m - 1, the dielectric constant of epsilon r = 2.65 the macroscopic dielectric plate to replace the PCB (7-8], the characteristic impedance and propagation constant for mid-december and Kg. The one when the board of the medium size with rectangular cross section size is infinite close to cavity, the cavity of electromagnetic field on the surface of the medium plate, reflection can be ignored,According to the transmission line theory, the voltage at the left end of the dielectric plate passes through the dielectric plate with thickness of t 'to the right of the plate, and the voltage V5 and impedance Z5 are [6,8]:
The voltage of each mode wave at point P is:
2 analysis of simulation results
In order to verify the correctness of the theoretical results, the electric field intensity of 1 V - m - 1 plane electromagnetic wave radiation thickness to 1 mm rectangular shielding case, cavity size is 300 mm * 120 mm * 120 mm, among them former cavity length and 300 mm, the cavity after 300 mm long, hole slot size is 80 mm x 20 mm, medium plate size is 300 mm * 120 mm * 1 mm, installed in 100 mm distance from the second layer seam hole, the simulation frequency is 200 MHz ~ 1 GHz.
Medium plate with hole center and the observation points on a straight line, when shielding cavity with medium plate, the incident wave coupling into the cavity, in case of medium plate, dielectric loss, electromagnetic wave energy is mainly divided into three parts: one part through the further spread of medium plate, part of reflection, partly through medium plate and cavity aperture diffraction, the medium plate also absorb energy.Because of the transmission and diffraction of electromagnetic wave, electromagnetic field still exists in the space behind the dielectric plate.
Figure 3 is a comparison between the shielding effectiveness of the equivalent transmission line method and the CST simulation method in the center of the rear cavity. It can be seen that the results of the two methods are partially different in the low frequency, but they are more consistent after 300 MHz.The cavity resonates at 707mhz.
2.1 influence of changing dielectric plate size on shielding effectiveness
In figure 4, the inner hole to load the PCB distance q = 100 mm, using three different size of medium plate, respectively for 500 mm x 10 mm, 100 mm x 40 mm and 200 mm x 80 mm. It can be seen that at a given frequency range, medium plate, the greater the cavity shielding effectiveness is higher, this is because the greater the medium plate, the dielectric loss, the greater the resonance point of field strength is smaller, the greater the shielding effectiveness of the shield.