70. When designing a shielding case, what factors are used to select the shielding material?
Answer: From the perspective of electromagnetic shielding, the type of electric field wave to be shielded should mainly be considered. For electric field waves, plane waves or higher frequency
magnetic field waves, general metals can meet the requirements. For low frequency magnetic field waves, materials with higher magnetic permeability should be used.
71. In addition to being affected by the shielding material, the shielding effectiveness of the chassis is also affected by what factors?
Answer: It is affected by two factors, one is the conductive discontinuity on the chassis, such as holes, gaps, etc.; the other is the wires passing through the shielding box, such as signal cables, power lines, etc.
72. What should be paid attention to when shielding the magnetic field radiation source?
Answer: Because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding purpose is mainly achieved by the absorption loss.
Therefore, a shielding material with a higher magnetic permeability should be selected . In addition, when doing structural design, keep the shielding layer as far away from the radiation source as possible (to increase reflection loss), and try to avoid holes, gaps, etc. close to the radiation source.
73. When designing the shielding structure, there is a principle: try to keep the cables in the chassis away from gaps and holes, why?
Answer: Since there is always a magnetic field near the cable, the magnetic field can easily leak from the hole (independent of the frequency of the magnetic field). Therefore, when cables are placed in close proximity to gaps and holes, magnetic field leakage occurs, reducing overall shielding effectiveness.
74. Why do we use the grounding method instead of the power supply method in order to suppress noise signals in many cases? The ground and power supply are one of the layers on the multi-layer PCB. According to the relativity of the voltage zero point, even the power supply Can't the layer also be used as a voltage zero point?
Answer 1: Grounding can also be said to be connected to a reference point. Since it is a reference point, it must be able to serve as a reference. It is considered to be ground, that is to say, at least it is considered to be zero, without any impedance (in fact, it depends on the layout). If the output impedance of the power supply is zero, of course it can also be used as a reference point, and it can also be used as a bypass channel for noise signals.
Answer 2: The ground of the signal has several meanings:
1. Absolute ground - EARTH, the earth
2. Relative ground - GROUND, reference ground
3. No ground - sometimes in order to reduce interference, the 0/1 of the signal are intentionally each other Relative value, not relative to ground, that is, signal 0 is not taken as ground. Such as the signal in CAN. One of the difficulties in hardware design is how to solve the grounding problem, from IC chips to a large system.
Answer 3: Using the ground clip on the oscilloscope probe to clamp the power supply may burn the oscilloscope. The ground clip on the oscilloscope probe is connected to the ground of the oscilloscope power cord (if the probe is not isolated). Using it to clamp the power supply will directly short the power supply to ground. Using the ground clips of different probes to clamp on points of different potentials will also short-circuit. Therefore, the recommended method is to connect the power supply of the oscilloscope to the mains through an isolation transformer. Or, as we usually do, unplug the ground pin on the oscilloscope power cord to avoid future troubles. Grounding and isolation are issues we should always pay attention to during design and testing.
Answer 4: Although both the power supply plane and the ground plane have voltage zero points on AC, the ground plane is relatively clean, so it is usually grounded instead of power supply.
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