Basic knowledge of CE certification EMC test:
(1) in the field of electromagnetic compatibility (EMC), why is decibel (dB) always used?
Answer: because of the wide range of amplitudes and frequencies to be described, it is easier to represent them in logarithmic coordinates on a graph, and dB is the unit in logarithmic terms.
(2) in addition to shielding materials, what other factors affect the shielding efficiency of the chassis?
Answer: affected by two factors, one is the electrical discontinuity on the chassis, such as holes, gaps, etc.; The other is the wire through the shield box, such as the signal cable, the power cord, etc.
(3) in the field of electromagnetic interference diagnosis, it is often necessary to use the near-field probe and spectrum analyzer, how to make a simple near-field probe with coaxial cable?
Answer: strip the outer layer of coaxial cable (shield layer), expose core wire, wind core wire into a diameter of 1~2 cm small ring (1~3 turns), welding on the outer layer.
(4) in the design of shielding structure, there is a principle: try to make the cable in the chassis away from gaps and holes, why?
A: since there is always a magnetic field near the cable, the magnetic field can easily leak through the hole (regardless of the frequency of the field). Thus, when cables are very close to gaps and holes, magnetic leakage occurs, reducing the overall shielding effectiveness.
(5) what factors should be used to select shielding materials when designing shielding chassis?
A: from the perspective of electromagnetic shielding, we should mainly consider the types of electric field waves 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, materials with high magnetic conductivity should be used.
(6) why do electromagnetic compatibility (EMC) designs for products?
Answer: meet the functional requirements of the product, reduce the debugging time, make the product meet the requirements of ce certification electromagnetic compatibility (EMC) standard, so that the product will not produce electromagnetic interference to other equipment in the system.
(7) why can't spectrum analyzer observe transient interference such as electrostatic discharge?
A: because spectrum analyzer is a narrowband sweep receiver, it only receives energy in a certain frequency range at a certain time. However, transient interference such as electrostatic discharge is a kind of pulse interference, whose spectrum range is very wide, but the time is very short, so the spectrum analyzer observed when transient interference occurs is only a small part of its total energy, can not reflect the actual interference situation.
(8) what should be paid attention to when shielding the radiation source of magnetic field?
A: because the wave impedance of the magnetic field is very low, the reflection loss is very small. Therefore, choose the shielding material with high magnetic permeability. In addition, in the structural design, the shielding layer should be kept away from the radiation source as far as possible (to increase reflection loss), and avoid holes, gaps and other close to the radiation source.
(9) from what aspects can electromagnetic compatibility (EMC) design of products be carried out?
A: circuit design (including device selection), software design, circuit board design, shielding structure, signal line/power line filtering, circuit grounding design.
(10) the measurement of biological magnetic information of human body is a new medical diagnosis method. The measurement of biological magnetic information must be carried out in the magnetic field shielding chamber, which must be able to shield the alternating electromagnetic field from static magnetic field to 1GHz. Please propose the design scheme of this shielding chamber.
A: first, consider the selection of shielding materials. Since magnetic fields with very low frequencies need to be shielded, materials with high permeability, such as permalloy, should be used. Because permo alloy after processing, magnetic conductivity will be reduced, must be heat treatment. Accordingly, screen room wants to make assemble type, assemble by plank and become. Each plate is processed in accordance with the design in advance, then heat treated and transported to the site for installation with great care. The junctions of each plate are superimposed to form a continuous magnetic path. The shielding chamber formed in this way can shield the low-frequency magnetic field effectively, but the gap will generate high-frequency leakage. To make up for this deficiency, the outer layer of permalloy shielding chamber is welded with aluminum plate to form a second layer of shielding, which plays a role in shielding high-frequency electromagnetic fields.
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