Electromagnetic compatibility EMC refers to the ability of a device or system to meet the requirements in its electromagnetic environment and not produce intolerable electromagnetic interference to any device in its environment. The following are ten questions and ten answers to common electromagnetic compatibility.
1. 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.
2. Measuring the biomagnetism 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 shielding room is welded with aluminum plates to form a second layer of shielding to shield high-frequency electromagnetic fields.
3. 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.
4. 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.
5. 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 on the graph with logarithmic coordinates, and dB is the unit when expressed in logarithm.
6. 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.
7. From which aspects can the EMC design of products be carried out?
Answer: Circuit design (including device selection), software design, circuit board design, shielding structure, signal line/power line filtering, circuit grounding design.
8. What problems should be paid 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 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 (in order to increase the reflection loss), and try to avoid holes and gaps close to the radiation source.
9. 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.
10. Why do electromagnetic compatibility and EMC design for products?
Answer: Meet the functional requirements of the product, reduce the debugging time, make the product meet the requirements of the electromagnetic compatibility EMC standard, so that the product will not cause electromagnetic interference to other equipment in the system
