1. Why do electromagnetic compatibility designs for products?
Answer: Meet the functional requirements of the product, reduce the debugging time, and make the product meet the requirements of the electromagnetic compatibility standard, so that the product will not cause electromagnetic interference to other devices in the system.
2. Which aspects can be used to design products for electromagnetic compatibility?
Answer: Circuit design (including device selection), software design, circuit board design, shielding structure, signal line/power line filtering, circuit grounding design.
3. In the field of electromagnetic compatibility, why is it always described in decibels (dB)? How many dBmV is 10mV?
Answer: Because the amplitude and frequency range to be described are very wide, it is easier to express with logarithmic coordinates on the graph, and dB is the unit when expressed in logarithm, and 10mV is 20dBmV.
4. Why can't the spectrum analyzer observe transient interference such as electrostatic discharge?
Answer: Because the spectrum analyzer is a narrow-band sweep frequency receiver, it only receives energy in a certain frequency range at a certain moment. Transient interference such as electrostatic discharge is a kind of impulse interference. Its spectrum range is very wide, but the time is very short. In this way, when the transient interference occurs, the spectrum analyzer observes only a small part of its total energy, which cannot reflect the actual situation. The interference situation.
5. When diagnosing electromagnetic interference problems in the field, it is often necessary to use a near-field probe and a spectrum analyzer. How to make a simple near-field probe with a coaxial cable?
Answer: Strip the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small loop (1~3 turns) with a diameter of 1~2 cm, and weld it to the outer layer.
6. For a piece of equipment, the original electromagnetic radiation emission intensity is 300mV/m. After adding the shielding box, the radiation emission is reduced to 3mV/m. What is the shielding efficiency of this case in dB?
Answer: The shielding effectiveness of this case should be 40dB.
7. When designing a shielding case, what factors should be used to select shielding materials?
Answer: From the perspective of electromagnetic shielding, the type of electric field wave to be shielded is mainly considered. For electric field waves, plane waves or higher frequency magnetic field waves, generally metals can meet the requirements. For low frequency magnetic field waves, materials with higher magnetic permeability should be used.
8. In addition to the shielding material, the shielding effectiveness of the chassis is affected by other factors?
Answer: Affected by two factors, one is the conductive discontinuities 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.
9. What problems should be paid attention to when shielding magnetic field radiation sources?
Answer: Since the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding purpose is achieved mainly by absorbing the loss. Therefore, a shielding material with a higher magnetic permeability should be selected. In addition, when designing the structure, keep the shielding layer as far away as possible from the radiation source (to increase reflection loss), and try to avoid holes, gaps, etc., close to the radiation source.
10. When designing the shielding structure, there is a principle: try to keep the cables in the case 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 (it has nothing to do with the frequency of the magnetic field). Therefore, when the cable is very close to the gaps and holes, magnetic field leakage will occur, reducing the overall shielding effectiveness.
11. Measuring the biomagnetic information of the human body is a new medical diagnosis method. This biomagnetism measurement must be carried out in a magnetic field shielded room. This shielded room must be able to shield the alternating electromagnetic field from static magnetic field to 1GHz. Please suggest this Design scheme of shielded room.
Answer: First, consider the selection of shielding materials. Since magnetic fields with very low frequencies are to be shielded, materials with high magnetic permeability, such as Permalloy, should be used. Since the permeability of permalloy will decrease after processing, it must be heat treated. Therefore, the shielding room should be made as an assembling type, which is assembled from plates. The plates are processed in advance according to the design, and then heat treated, transported to the site, and installed very carefully. The joints of each plate should be overlapped to form a continuous magnetic path. The shielding room constructed in this way can have better shielding effectiveness against low-frequency magnetic fields, but the gap will cause high-frequency leakage. In order to make up for this shortcoming, the outer layer of the permalloy shielded room is welded with aluminum plates to form a second layer of shielding to shield the high-frequency electromagnetic field.
12. What is a cut-off waveguide plate (honeycomb plate), what occasions should it use, and what should be paid attention to when using it?
Answer: The array board composed of many cut-off waveguides is used when high shielding efficiency and ventilation is required. When using, pay attention to the use of electromagnetic sealing gaskets between the honeycomb board and the chassis for installation or welding.
13. There is an equipment with a plastic case, and the electromagnetic radiation exceeds the standard. In order to make it meet the requirements of electromagnetic compatibility standards, the developer sprayed conductive paint on the inside of the case. The result did not improve significantly. Please analyze what may be the reason.
Answer: The original plastic case has too many holes and too large holes, which will cause serious leakage, or the gap may not be tight (maybe the contact is not tight, or there may be no conductive paint sprayed on the joint), resulting in leakage. In addition, the cables (signal cables, power cables) on the original chassis generally do not have good filtering measures, and these cables cause leakage of the chassis.
14. What kinds of transparent shielding windows are there and what should be paid attention to when using them?
Answer: There are two types of shielding windows consisting of glass sandwiched with metal mesh and shielding windows coated with a thin metal film on the glass. When in use, pay attention to the metal mesh or conductive coating must be in close contact with the conductive substrate of the shielding chassis.
15. When using metal mesh sandwiched shielding glass on the screen of CRT monitor, there will be annoying streaks. How to reduce this phenomenon?
Answer: Rotate the direction of the screen to make an angle of 15-20 degrees between the weft line and the scanning line of the picture tube.
16. What are the two key characteristics of electromagnetic sealing gaskets? List as many types of electromagnetic sealing gaskets as possible, and explain the applicable occasions of various products.
Answer: The two characteristics that electromagnetic sealing gaskets must have are elasticity and conductivity. Common types of electromagnetic sealing gaskets include: finger reeds, metal mesh gaskets, conductive rubber, conductive cloth-wrapped foam rubber, spiral tubes, etc. Avoid using finger reeds except where there is tangential sliding contact. When there are environmental sealing requirements, use conductive rubber, and other occasions can use conductive cloth gaskets. When the frequency of shielding is not high, silk screen gaskets can also be used to ensure that no excessive compression can be used. Spiral pipes can be used.
17. What problems should be paid attention to when using electromagnetic sealing gasket?
Answer: The thickness of the panel is appropriate to prevent deformation under the rebound force of the gasket, resulting in larger gaps. When the thickness of the panel is thin, the interval between the fastening screws should be smaller. Set a limit structure to prevent excessive compression, select appropriate metal materials, and reduce electrochemical corrosion.
18. A metal rod must pass through a shielded case. How to deal with it so as not to damage the shielding effectiveness of the case?
Answer: The periphery of the metal rod is reliably overlapped with the shielding base through the beryllium copper reed.
19. What is the main function of the power line filter, what parameters are mainly considered when selecting the model, and what should be paid attention to when using the power line filter?
Answer: The function of the power line filter is to suppress the conduction emission current from propagating along the power line. When selecting the model, consider the insertion loss (common mode and differential mode), rated current, voltage, effective frequency range and other parameters. Pay attention to the installation method when using it. The radio frequency must be well grounded, and the input and output must be isolated to prevent the filtered wire from reappearing. Pollution.
20. Why is the high frequency filtering characteristics of power line filters important?
Answer: If the high frequency characteristics are not good, it will cause the equipment's radiation emission to exceed the standard or be sensitive to pulse interference.
21. When designing structural electromagnetic compatibility, there is a principle: The filtered power cord should be as far away as possible from various signal cables. Why?
Answer: If the power line and the signal cable are very close, the high frequency signal on the signal cable will be coupled to the power line (especially the part that has been filtered)
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