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# What are the common problems with EMC?

Posted:06:11 PM September 06, 2018 writer: G

1. Why do you want to make electromagnetic compatibility design for your product?

A: Meet the functional requirements of the product, reduce the commissioning 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. What are the aspects of 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.

3. In the field of electromagnetic compatibility, 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.

4. Why can't the spectrum analyzer observe transient disturbances 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.

5. 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.

6. Measuring biomagnetic information of the human body is a new medical diagnostic method. This biomagnetic measurement must be carried out in a magnetic field shielding room. This shielding room must be able to shield the alternating electromagnetic field from static magnetic field to 1 GHz. 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.

7. When designing the shielded enclosure, what factors are selected for 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.

8. 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.

9. What issues should you pay 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 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.

10. When designing the shield structure, there is a principle: try to keep the cables 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.

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