Knowledge of hardware shielding cover, only by doing so can the corresponding electromagnetic waves be shielded!

In some products, due to high frequency and radio frequency, there will be external radiation, resulting in EMI problems. Such as mobile phones. I have to use a shielding cover. So how to choose a shield? How to understand the knowledge of shield? This is a learning. The function of the shielding case is to prevent the signal inside the shielding case from radiating outward, and also to shield the external radiation, so that the circuit in the shielding case is not affected. The concept of shielding: shielding is actually equivalent to a filter, placed on the propagation path of electromagnetic waves, forming high impedance to a part of the frequency band. The greater the impedance, the better the shielding performance. So what kind of shield can effectively prevent signal radiation? There are several aspects to this. That article talked about skin depth indicating the effective cross-sectional area of u200bu200bthe high-frequency alternating current flowing in the conductor. The skin depth is directly related to the frequency of the high-frequency current. As the frequency of the alternating current increases, the skin depth becomes smaller. Calculated by the following formula: temperature coefficient of material conductivity (or resistivity);: permeability of wire material (copperu003d4π×10-7 H/m);: conductivity of material (copperu003d5.8×107 S/m ) The higher the signal frequency, the shallower the skin depth and the more skin the signal. For signal transmission, the shallower the skin depth, the higher the impedance of the conductor and the smaller the utilization rate of the conductor. This is not the result we want. But for shielding, it's different. He wants to prevent signal transmission. The shallower the skin, the better the shielding performance. So for the low frequency, it is difficult to shield and this is the reason. The skin depth of 50hz is 5-15mm, so it is difficult to shield. Who makes such a thick shield. Different metals, the same signal frequency, the skin depth will be different. To make the shield, it is necessary to select metal materials with good electrical conductivity and magnetic permeability. For general metals, 0.5mm thickness can have a better shielding effect on electromagnetic waves of 1MHZ. Not to mention the electromagnetic wave of 100MHZ. Secondly, let's talk about the hole size of the shielding cover. All shielding covers will have some holes or gaps for some reasons. These leaks will cause electromagnetic waves to flow in and out. How big a hole can shield and how high frequency electromagnetic waves? Original WeChat public account: Wolonghui IT technology. It is necessary to use shielding effectiveness. Here is a formula. The shielding effectiveness is related to the maximum size of the hole and the wavelength of electromagnetic waves. The larger the hole, the smaller the shielding effectiveness, and the longer the electromagnetic wave, the greater the shielding effectiveness. The unit of this shielding effectiveness is dB. That is to say how much dB can be attenuated. For 100M electromagnetic waves. 10mm hole. The calculated shielding effectiveness is about 43.5dB. The larger the shielding effectiveness value, the better the performance. Through this article, everyone should have some understanding of shielding knowledge. What do you want to say about the shield, you are welcome to contact Xiaoshuo for mutual exchanges.