Electromagnetic compatibility EMC, refers to the equipment or system meets the requirements in its electromagnetic environment to run any device and don’t produce intolerable electromagnetic interference. The following comes a common electromagnetic compatibility EMC ten questions and ten answers,will help you know more about the electromagnetic compatibility EMC.
1.when the electromagnetic interference problem diagnosis, often need to use the near-field probe and spectrum analyzer, how to made a simple coaxial cable of the near-field probe?
Answer: strip away the coaxial cable of the outer layer (shield),exposing conductor, make the conductor around into a small ring 1 ~ 2 cm in diameter (1 ~ 3) coils, welded on the outer layer.
2.Measure the body's biological magnetic information is a new method of medical diagnosis, this kind of biological
magnetic measurement must be conducted in the magnetic field shielding room, the shield room must be shielded
from the static magnetic field to 1 GHZ alternating electromagnetic field, please put forward design scheme of the
shielding room.
A: first of all considering the selection of shielding material, due to the shielding magnetic field frequency is very low, so require to use the high permeability of the material, such as permalloy.
Because the permalloy after processing has permeability decreases, which must deal with heat treatment. Therefore, shielding room need to be made of assembling type, made from a panel. Prior to each piece of plate according to the design process, and heat treatment, delivery to the site, be very careful with installation. Junction to overlap of each plank, to the formation of a continuous magnetic path.
This form to the low frequency magnetic shield room has good shielding effectiveness, but the gap will generate high frequency leakage. In order to make up for the shortage, the outer permalloy shielding chamber with aluminum plate welded into the second block making the high frequency electromagnetic shielding effection.
3. Why can't a spectrum analyzer observe transient disturbance such as electrostatic discharge?
Answer:because a spectrum analyzer is a kind of narrow-band sweep frequency receiver, it receive only a certain frequency range of energy at some point. And transient disturbance
such as electrostatic discharge is a kind of impulse noise, the spectrum range is very wide, but time is very short, such a spectrum analyzer in the observed in the transient disturbance is onlysmall part of the total energy.
4. what factors have to fix according to what factors when design shielding box?
A: stand at the view point of electromagnetic shielding, mainly want to consider the shielding electric wave types.
For the electric field and plane wave or high frequency magnetic field wave, general metal can meet the
requirements, for low frequency wave magnetic field, need to use the high permeability of the material.
5. In the field of electromagnetic compatibility EMC , why always described in decibels (dB) of the unit?
A: to describe the amplitude and frequency of the range is very wide, on the graphics with logarithmic coordinates
are more likely to describe, and when the dB is expressed in logarithmic units.
6. What factors except the influence of shielding materials on the chassis of the shielding?
Answer: by two factors, one is conductive discrete point on the chassis, such as holes, cracks, etc.; Another is the
conductor through the shielding box, such as signal cable, power cord, etc.
7. What kinds of aspects are able to design the products EMC?
Answer: circuit design (including component selection), software design, circuit board design, block structure,
signal/power line filter, grounding method of the circuit design. situation.
8.What problem should pay attention to shielding the magnetic field source?
Answer: because the wave impedance of the magnetic field is very low, so the reflection loss is very small, but
mainly depends on absorption loss to achieve the objective of the block. Hence, have to choose a shielding
materials of high permeability. In addition, when applied to the design of the structure, to shield from radiation
source (to increase the reflection loss) as far as possible , try to avoid holes and cracks near the source.
9. What’s the reason during the design of shielding structure, there is a principle is: try to make the cable
inside the chassis from cracks and holes?
Answer: because the cable nearby, there is always a magnetic field and magnetic field easily from the hole leakage
(has nothing to do with the frequency of the magnetic field). Therefore, when the distance cable cracks and holes
is very close, magnetic leakage occurs, lower overall shielding effectiveness.
10.Why have to design the EMC for the products?
A: meet the requirements of product function, reduce debugging time, make the products meet the requirements
of electromagnetic compatibility EMC standard, make the products not on electromagnetic interference with
other devices in the system.