EMC testing and certification of electromagnetic compatibility

Date：2022-12-12 16:17:00 Views：891

1、 In the field of electromagnetic compatibility (EMC), 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.

2、 In addition to the shielding data, what other factors affect the shielding effectiveness of the chassis?

Answer: It is affected by two factors. One is the conductive disconnection points 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.

3、 When diagnosing electromagnetic disturbance problems on site, it is often required to use near-field probes and spectrum analyzers. How to make a simple near-field probe with coaxial cable?

Answer: peel off 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.

4、 When planning the shielding structure, there is a principle: try to keep the cables in the chassis away from gaps and holes. Why?

Answer: Because there is always a magnetic field near the cable, and the magnetic field can easily leak out of the hole (independent of the frequency of the magnetic field). Therefore, when the gap between the cable and the hole is very close, the magnetic field leakage will occur, reducing the overall shielding effectiveness.

5、 When planning shielding enclosures, which elements should be used to select shielding data?

Answer: From the perspective of electromagnetic shielding, we should mainly consider the type of electric field wave shielded. 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, data with high permeability should be used.

6、 Why EMC planning for products?

Answer: Satisfy the functional requirements of the product, cut down the debugging time, make the product satisfy the requirements of the ce certification electromagnetic compatibility (EMC) standard, so that the product will not cause electromagnetic interference to other equipment in the system.

7、 Why can't spectrum analyzer observe transient disturbances such as electrostatic discharge?

Answer: Because the spectrum analyzer is a narrowband sweep receiver, it only accepts energy within a certain frequency range at a certain time. However, the transient disturbance such as electrostatic discharge is a kind of pulse disturbance. Its spectrum range is very wide, but the time is very short. In this way, the spectrum analyzer can only observe a small part of its total energy when the transient disturbance occurs, which cannot reflect the practical disturbance situation.

8、 What should be paid attention to when shielding magnetic field radiation sources?

Answer: Because the wave impedance of magnetic field wave is very low, the reflection loss is very small, and the purpose of shielding is mainly achieved by absorption loss. Therefore, shielding materials with high permeability should be selected. In addition, when making structural planning, the shielding layer shall be kept away from the radiation source as far as possible (to add reflection loss), and holes, gaps, etc. shall be prevented from approaching the radiation source as far as possible.

9、 What aspects can EMC planning be carried out?

Answer: Circuit planning (including component selection), software planning, circuit board planning, shielding structure, signal line/power line filtering, and circuit grounding method planning.

10、 Measuring the bio magnetic information of the human body is a new method for medical diagnosis. It is necessary to measure this kind of bio magnetic information in a magnetic field shielding room, which is necessary to shield the alternating electromagnetic field from the static magnetic field to 1GHz. Please put forward the plan for this shielding room.

A: First of all, consider the selection of shielding materials. Because it is necessary to shield the magnetic field with very low frequency, it is necessary to use materials with high permeability, such as permalloy. Because permalloy permeability will decrease after processing, it is necessary to conduct heat treatment. Therefore, the shielding chamber shall be assembled by plates. Each plate shall be processed according to the plan in advance, then heat treated, transported to the site, and installed with great care. The joint of each plate shall be stacked to form a continuous magnetic path. The shielding room formed in this way can have a good shielding effect on low-frequency magnetic field, but high-frequency leakage will occur in the gap. In order to compensate for this lack, 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.