EMC ten questions and ten answers, take you to understand the "previous life and life" of electromagnetic compatibility

Electromagnetic compatibility EMC refers to the ability of a device or system to operate in its electromagnetic environment and to produce unacceptable electromagnetic interference to any device in its environment. The following are the common EMC EMC questions and answers. Through the following questions and answers, you can learn more about EMC EMC.

1. When performing electromagnetic interference diagnosis on site, it is often necessary to use near-field probes and spectrum analyzers. How to make a simple near-field probe with coaxial cable?

Answer: Peel the outer layer (shield layer) of the coaxial cable to expose the core wire, and wind the core wire into a small ring (1~3匝) with a diameter of 1~2 cm and solder it on the outer layer.

2, measuring the biomagnetic information of the human body is a new medical diagnosis method, this biomagnetic measurement 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 this The design of the shielded room.

A: First of all, consider the choice of shielding materials. Because it is necessary to shield magnetic fields with low frequency, it is necessary to use materials with high magnetic permeability, such as permalloy. Since the permalloy is processed, the magnetic permeability is lowered and heat treatment must be performed. Therefore, the shielded room should be assembled and assembled from plates. The plates are processed in advance according to the design, then heat treated, transported to the site, and carefully installed. The joints of each sheet are overlapped to form a continuous magnetic path. The shielded chamber thus constructed has a good shielding effect on the low frequency magnetic field, but the gap generates high frequency leakage. In order to make up for this deficiency, the outer layer of the permalloy shielded chamber is welded with a second layer of aluminum plate to shield the high-frequency electromagnetic field.

3. Why can't the spectrum analyzer observe transient interference such as electrostatic discharge?

A: Because the spectrum analyzer is a narrowband swept receiver, it only receives energy in a certain frequency range at a certain time. Transient interference such as electrostatic discharge is a kind of pulse interference, and its spectrum range is very wide, but the time is very short, so that the spectrum analyzer observes only a small part of its total energy when transient interference occurs, which does not reflect the actual Interference situation.

4. When designing the shielded chassis, according to which factors choose the shielding material?

Answer: From the perspective of electromagnetic shielding, it is mainly necessary to consider the type of electric wave that is shielded. For electric field waves, plane waves or magnetic field waves with high frequencies, general metals can meet the requirements. For low-frequency magnetic field waves, materials with high magnetic permeability should be used.

5. In the electromagnetic compatibility EMC field, why is it always described in decibels (dB)?

A: Because the range of amplitudes and frequency to be described is very wide, it is easier to represent the graph with logarithmic coordinates, and dB is the unit in logarithm.

6. What is the influence of the shielding effectiveness of the chassis in addition to the shielding materials?

A: Affected by two factors, one is the conductive discontinuity on the chassis, such as holes, gaps, etc.; the other is the wires that pass through the shielding box, such as signal cables, power lines, and so on.

7. What are the aspects of electromagnetic compatibility EMC design for products?

A: Circuit design (including device selection), software design, circuit board design, shielding structure, signal line / power line filtering, circuit grounding design.

8. What problems should I pay attention to when shielding the magnetic field radiation source?

Answer: Since the wave impedance of the magnetic field wave is very low, the reflection loss is small, and the absorption loss is mainly achieved by the absorption loss. Therefore, it is necessary to select a shielding material with a high magnetic permeability. In addition, when doing structural design, the shielding layer should be kept away from the radiation source as much as possible (to increase the reflection loss), and holes, gaps, etc. should be avoided as close as possible to the radiation source.

9. When designing the shielding structure, there is a principle: try to make the cable inside the chassis away from the gaps and holes, why?

A: Because there is always a magnetic field near the cable, 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 the hole, magnetic field leakage occurs, reducing the overall shielding effectiveness.

10. Why do you need EMC design for the product?

A: Meet the functional requirements of the product, reduce the commissioning time, and make the product meet the EMC requirements of the EMC, so that the product will not cause electromagnetic interference to other devices in the system.

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