EMC ten questions and ten answers, take you to understand the electromagnetic compatibility
Time:2020-03-04
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EMC is the ability of the equipment or system to operate in its electromagnetic environment and does not produce intolerable electromagnetic interference with any equipment in its environment. The following is the common EMC ten questions and ten answers, through the following questions and answers, can help you understand more about the knowledge of EMEMC.
Electromagnetic compatibility EMC means the ability of the equipment or system to operate in its electromagnetic environment without causing intolerable electromagnetic interference with any equipment in its environment. The following is the common EMC ten questions and ten answers, through the following questions and answers, can help you understand more about the knowledge of EMEMC.
1. When diagnose electromagnetic interference on the field, it often need to use near field probe and spectrum analyzer. How to make a simple near field probe with coaxial cable?
Answer: Peel the outer layer (shielding layer) of the coaxial cable to expose the core wire, and the core wire is wound into a small ring of 1~2 cm (1~3 turns) and welded on the outer layer.
2. the measurement of the biological magnetic information of the human body is a new medical diagnosis method, the measurement of the biological magnetic field must be carried out in the magnetic field shielding room, this shielding room must be able to shield the alternating electromagnetic field from the static magnetic field to 1GHz, please propose the design scheme of this shielding room.
Answer: the first consideration is the choice of shielding materials, because of the magnetic field with a very low shielding frequency, so the use of high magnetic permeability materials, such as Po Momo alloy. After the alloy is processed, the magnetic conductivity will be reduced and heat treatment must be performed. Therefore, the shielding room should be made into the assembly type, assembled by the plate. In advance each plate according to the design, and then heat treatment, transport to the site, very careful installation.
The joint of each plate should overlap in order to form a continuous magnetic flux path. The shielding chamber formed in this way can have a better shielding efficiency for the low frequency magnetic field, but the gap will produce high frequency leakage. In order to make up for this deficiency, the outer layer of the slope Mo alloy shielding chamber is welded into a second layer of shielding with aluminum plate, which plays a shielding role for the high-frequency electromagnetic field.
3. Why can‘t the spectrum analyzer observe the transient interference such as electrostatic discharge?
A: Because a spectrum analyzer is a narrow band sweep receiver, it receives energy within a certain frequency range at a certain time. The transient interference such as electrostatic discharge is a kind of pulse interference, its spectrum range is very wide, but the time is very short, so the spectrum analyzer observed when the transient interference occurs is only a small part of its total energy, and cannot reflect the actual interference situation.
4. When designing the shielding case, according to what factors to choose the shielding material?
Answer: from the Angle of electromagnetic shielding, mainly should consider the type of electric field wave. For electric field waves, plane waves or high frequency magnetic field waves, generally the metal can meet the requirements, for low frequency magnetic field waves, to use materials with high magnetic permeability.
5. In the EMEMC field, why always use the unit description of decibel (dB)?
A: Because the amplitude and frequency range to describe are very wide, it is easier to express with logarithmic coordinates on the graph, and dB is the unit of logarithmic representation.
6. the shielding efficiency of the chassis is not only affected by the shielding materials, but also affected by what factors?
Answer: affected by two factors, one is the conductive discontinuity point on the chassis, such as holes, gaps, etc.; the other is the wire through the shielding box, such as signal cable, power cord, etc.
7. From what aspects can EMC design for the product?
Answer: Circuit design (including device selection), software design, circuit board design, shielding structure, signal line / power line filter, and grounding mode design of the circuit.
8. What problems should we pay attention to when shielding the source of magnetic field radiation?
Answer: because the wave impedance of the magnetic field wave is very low, so the reflection loss is very small, and mainly rely on the absorption loss to achieve the purpose of shielding. Therefore, the shielding material with higher magnetic permeability should be selected. In addition, when doing the structural design, to make the shielding layer as far away from the radiation source (to increase the reflection loss), try to avoid holes, gaps and other close to the radiation source.
9, in the design of the shielding structure, there is a principle is: try to make the cable in the chassis away from the cracks and holes, why?
A: Because there is always a magnetic field near the cable, and the magnetic field can easily leak from the hole (regardless of the frequency of the magnetic field). Therefore, when the cable is close to the gap and holes, magnetic field leakage will occur, reducing the overall shielding efficiency.
10. Why do EMEMC design for the product?
Answer: meet the product function requirements, reduce the debugging time, so that the product meets the requirements of the electromagnetic compatibility EMC standard, so that the product will not produce electromagnetic interference to other equipment in the system.
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