Basic concepts of EMI

Date: 2017-07-28

Basic concepts
a. EMC, EMI and EMS
Electromagnetic Compatibility (EMC) means the compatibility of products that are in the same electromagnetic environment (electric field and magnetic field). It has 2 meanings:
lThe Electromagnetic Interference (EMI) level of the product is less than the limitation. So that it doesn’t interfere with the other products in the same environment.
lThe product is able to resist the electromagnetic interference that comes from another product in the same environment which is lower than the limitation. We call such capability Electromagnetic Susceptibility (EMS).
The subject of electromagnetic compatibility is illustrated based on the following 3 factors.
- Interfering source
- Object being interfered with
- Spreading or coupling path
b. Emission, Radiated Emission and Conducted Emission
Emission: The product (source) gives out energy by means of electromagnetic wave.
Based on different spreading path, it is divided into radiated emission and conducted emission.
Radiated emission (RE): The product (source) gives out electromagnetic wave over space.
The radiation is measured by means of electric field strength. Its unit is µV/m.
Conducted Emission (CE): The product (source) gives out electromagnetic wave along the conductors such as power cord and signal line. According to the frequency it is measured by voltage and power. The units are µV and pW respectively.
When the product and the conductor are shorter than the wavelength (relatively low frequency end), conducted emission is the major emission. When their size is longer than the wavelength (relatively high frequency end), radiated emission is the major emission.
c. Peak value, Quasi-Peak value and Average Value
Peak value (P value) is the largest value of time domain variable in a specific time range.
Quasi-Peak value (QP value) is the time domain value that positively correlates to the P value
and its frequency of occurrence.
Average value (AV value) is the arithmetic average value of the time domain variable in a specific time interval.
d. Radio Frequency and Power Frequency
Radio Frequency (RF): When the alternated current flows through the conductor, there is alternating electromagnetic field or electromagnetic wave generated around the conductor. Electromagnetic wave with frequency less than 100KHz is absorbed by the earth. Electromagnetic wave with frequency higher than 100KHz transmits in the air. It can be reflected by ionosphere therefore travel for a long distance. We call such high frequency electromagnetic wave with ability of long distance traveling radio frequency. Its frequency usually ranges from 100KHz to 30GHz.
Power Frequency is the frequency of the industrial alternating current power supply. It is usually 50Hz or 60Hz.

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