Often found that hardware engineers know little about electromagnetic compatibility (EMC) basics, and Xiaobian borrows flowers and summarizes some of the electromagnetic compatibility (EMC) knowledge points.

(1) In the field of electromagnetic compatibility (EMC), why is it always described in decibels (dB)?
A: Because the range and frequency range to be described are wide, it is easier to represent the logarithmic coordinates on the graph, and dB is the unit in logarithmic representation.

(2) chassis shielding effectiveness in addition to the impact of shielding materials, but also by what factors?
A: by the impact of two factors, one is the conductive barrier on the chassis, such as holes, crevices, etc .; the other is through the shield box wires, such as signal cables, power lines and so on.

(3) in the field of electromagnetic interference problem diagnosis, often need to use near-field probe and spectrum analyzer, how to use coaxial cable to produce a simple near-field probe?
A: The coaxial cable outer layer (shielding layer) peeled, so that the core exposed, the core wire into a diameter of 1 to 2 cm small ring (1 to 3 turns), welding on the outer layer.

(4) in the design of shielding structure, there is a principle is: try to make the chassis of the cable away from the cracks and holes, why?
A: Because there is always a magnetic field near the cable, and the magnetic field is easy to leak from the hole (regardless of the frequency of the magnetic field). Therefore, when the cable from the gap and the hole is close, it will occur magnetic field leakage, reduce the overall shielding effectiveness.

(5) design shielding chassis, depending on what factors choose shielding materials?
A: From the perspective of electromagnetic shielding, the main consideration should be taken to shield the type of electric field waves. For the electric field wave, plane wave or high frequency magnetic field wave, the general metal can meet the requirements, for low frequency magnetic field wave, to use a higher permeability of the material.

(6) why do the product to do electromagnetic compatibility (EMC) design?
A: Meet the functional requirements of the product, reduce the debugging time, so that products meet the requirements of electromagnetic compatibility (EMC) standards, so that the product does not produce electromagnetic interference to other devices in the system.

(7) why the spectrum analyzer can not observe the electrostatic discharge and other transient interference?
A: Because the spectrum analyzer is a narrowband swept receiver, it only at a time to receive a frequency range of energy. And electrostatic discharge and other transient interference is a kind of pulse interference, the spectrum range is very wide, but the time is very short, so the spectrum analyzer in the transient interference occurs when only a small part of its total energy, can not reflect the actual Of the interference situation.

(8) shielding magnetic field radiation source to pay attention to what the problem?
A: Because the wave impedance of the magnetic field wave is very low, so the reflection loss is very small, and mainly rely on the absorption loss to achieve the purpose of shielding. So to choose a higher permeability of the shielding material. In addition, in the structural design, to make the shield as far as possible away from the radiation source (to increase the reflection loss), try to avoid holes, cracks and other close to the radiation source.

(9) to do the electromagnetic compatibility of the product (EMC) design from which aspects can be carried out?
A: circuit design (including device selection), software design, circuit board design, shielding structure, signal line / power line filter, circuit grounding design.

(10) measurement of the body's bio-magnetic information is a new medical diagnostic method, this bio-magnetic measurement must be carried out in the magnetic field shielding room, the shielding room must be able to shield from the static magnetic field to 1GHz alternating electromagnetic field, The design of this shielded room.
A: First consider the choice of shielding materials, due to shielding the frequency is very low magnetic field, so the use of high permeability materials, such as permalloy. As permalloy after processing, the permeability will be reduced, must be heat treatment. Therefore, the shielding room to be made of assembled, made of sheet metal assembled. Prior to the plate in accordance with the design and processing, and then heat treatment, transport to the scene, very careful to install. The joint of each plate is overlapped so as to form a continuous magnetic path. This constitutes a shielded room can have a good low-frequency magnetic field shielding effectiveness, but the gap will produce high-frequency leakage. In order to make up for this deficiency, in the outer layer of the permalloy shielded room with aluminum plate welded into a second layer of shielding, play a role in shielding high-frequency electromagnetic field.

The above is some of the basic knowledge of electromagnetic compatibility (EMC), I hope the engineers in the electromagnetic compatibility (EMC) design help!