Some basic theories and explanations of EMC
Time:2023-01-03
Views:1390
1. Why should we design products for electromagnetic compatibility?
Answer: Meet the functional requirements of the product, reduce the debugging time, make the product meet the requirements of electromagnetic compatibility standards, and make the product not produce electromagnetic interference to other equipment in the system.
2. What aspects can we carry out EMC design for products?
Answer: Circuit design (including component selection), software design, circuit board design, shielding structure, signal line/power line filtering, and circuit grounding method design.
3. In the field of electromagnetic compatibility, why is it always described in decibels (dB)?
Answer: Because the range of amplitude and frequency to be described is very wide, it is easier to express it in logarithmic coordinates on the graph, and dB is the unit when expressed in logarithm.
4. Why can‘t the spectrum analyzer observe the transient interference such as electrostatic discharge?
Answer: Because the spectrum analyzer is a narrowband sweep receiver, it only receives energy within a certain frequency range at a certain time. However, 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. Thus, the spectrum analyzer can only observe a small part of its total energy when the transient interference occurs, and cannot reflect the actual interference.
5. When diagnosing electromagnetic interference problems on site, it is often necessary to use a near-field probe and a 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. Wind the core wire into a small ring (1~3 turns) with a diameter of 1~2cm and weld it on the outer layer.
6. Measuring the bio magnetic information of human body is a new medical diagnosis method. This kind of bio magnetic measurement must be carried out in a magnetic field shielding room, which 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.
A: First of all, consider the selection of shielding materials. Because the magnetic field with very low frequency should be shielded, materials with high permeability, such as permalloy, should be used. The permalloy must be heat treated because its magnetic permeability will decrease after processing. Therefore, the shielding room shall be assembled by plates. Each plate shall be processed according to the design in advance, then heat treated, transported to the site, and installed with great care. The joint of each plate shall be overlapped to form a continuous magnetic path. The shielding chamber formed in this way can have a good shielding effect on low-frequency magnetic field, but the gap will produce high-frequency leakage. In order to make up for this deficiency, the outer layer of permalloy shielding room is welded with aluminum plates to form a second layer of shielding, which plays a shielding role in high-frequency electromagnetic fields.
7. When designing the shielding case, what factors should be considered to select shielding materials?
Answer: From the perspective of electromagnetic shielding, the type of electric field wave to be shielded shall be mainly considered. For electric field wave, plane wave or magnetic field wave with high frequency, general metals can meet the requirements. For low-frequency magnetic field wave, materials with high permeability should be used.
8. In addition to the shielding materials, what other factors affect the shielding effectiveness of the chassis?
Answer: It is affected by two factors. One is the conductive discontinuities on the chassis, such as holes and gaps; The other is the wire passing through the shielding box, such as signal cable, power cord, etc.
9. What should be paid attention to when shielding magnetic field radiation sources?
Answer: Because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding is mainly achieved by absorption loss. Therefore, shielding materials with high permeability should be selected. In addition, during the structural design, the shielding layer shall be kept away from the radiation source as far as possible (to increase the reflection loss), and holes, gaps, etc. shall be kept away from the radiation source as far as possible.
10. When designing the shielding structure, one principle is to keep the cables in the chassis away from gaps and holes as much as possible. 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 the cable is close to the gap and hole, magnetic field leakage will occur, reducing the overall shielding effectiveness.
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