What are EMI and Conducted Emissions

I.What is EMI?
Electromagnetic interference is the “decline of equipment, transmission channel or system performance caused by electromagnetic crosstalk”. Electromagnetic crosstalk is only an electromagnetic phenomenon, i.e. an objective physical phenomenon, which may lead to the reduction or damage of equipment performance, but not necessarily the consequences. Electromagnetic interference is the consequence caused by electromagnetic crosstalk. In the past, physical phenomena and their consequences were not clearly divided in terms of terminology, and were generally referred to as interference.

LISUN EMI receiver system for EMI (Electromagnetic Interference) radiation conduction or conducted emissions testing. The EMI-9KB EMI receiver is produced by the full closure structure and strong electro-conductibility material, which has high shielding effect. Due to the new technology for the EMI Test System, it solved the instrument self-EMI problem. The test results are according to the international format test report. The EMI Test System EMI-9KB fully meets CISPR15:2018, CISPR16-1, GB17743, FCC, EN55015 and EN55022.

II.Classification of sources of electromagnetic interference
Electromagnetic interference (EMI) can be classified in many ways, such as by transmission path, such as conduction interference and radiation interference, where in the transmission paths for conduction interference include capacitive coupling, inductive coupling and common impedance coupling; by the transmission characteristics of radiation interference, such as near-field induction coupling and far-field radiation coupling; by the frequency band, such as narrowband interference and broadband interference; by the range of interference frequency, such as power frequency and audio interference, ultra-low frequency interference, carrier interference, radio frequency and video interference and microwave interference; by the subjective intention of the interferer, such as intentional interference sources and unintentional interference sources; and by the nature of the source, such as natural interference and artificial interference, etc.

III.Three Elements of Electromagnetic Interference
All electromagnetic interference is composed of three basic elements, namely: electromagnetic interference sources, a medium for transmitting the electromagnetic interference energy to the sensitive equipment, i.e. the transmission channels or coupling paths; and sensitive equipment which responds to to the electromagnetic interference or is affected by to it.

IV.Time, Space and Spectrum Characteristics of Electromagnetic Interference Sources
The distribution of interference energy over time is related to the operational time of the interference source and the probability of interference occurrence, which can be divided into three types: periodic interference, non-periodic interference and random interference. Periodic interference is interference that can be repeated at a certain time interval; non-periodic interference can not be repeated in certain periodic, but its occurrence time is fixed and predictable; and random interference varies in an unpredictable manner, and its characteristics are also irregular, so random interference can not be analyzed by time distribution function, but should be analyzed by the amplitude spectrum rate characteristics.

According to the principle of electromagnetic interference, the working modes of electromagnetic interference can be divided into two categories:radiated interference and conducted interference. Radiated interference refers to interference absorption in the form of electromagnetic wave from distant interference sources. Conducted interference refers to interference entering the disturbed equipment through coupling capacitors, inductors and common impedance from near interference sources. In the low-frequency range, conducted interference is the main form of electromagnetic interference.A useful signal in one channel, if entering another channel, will become the undesired signal accompanied by a small spark generated by power conversion.

All kinds of electronic and electrical equipment may become a line-conducted interference source. Conducted interference source can be divided into two categories: non-functional and functional. Non-functional conducted interference source are usually related to mechanical and electrical equipment, such as converters, heating circuits and data processing machines. The mechanism of these interferences is usually caused by arc discharge or sudden change of current. When the normal operation of a part directly affects the work of another part, functional interference is generated, such as pulse generators, clock computers and other periodic generators. Functional interference sources are generally easier to deal with than zero-functional interference sources, because their frequency and power are determined by design. Conducted interference will only occur when there is source, transmission path and receiver simultaneously, so corresponding design rules must be applied to each.

V.Conducted Emissions

Measuring the power lines shows that the conductive emissions frequency of most electronic and electrical equipment extends from several hundred kHz to more than 100MHz. Of course, when the frequency is very high, due to conductor loss and the effect of distributed inductors and capacitors, the conductive current is greatly attenuated. At the output end of direct current power supply, AC sound and other interference may occur at different degrees. In the frequency domain after AC rectification, some waveform disturbances may also be observed due to the components of power filter, parasitic capacitor and the undesirable parallel inductance resonance. It may cause damping oscillation at the output end of the power supply. Conducted interference generated by AC generators and electromagnetic coils have transient and steady-state properties. When the load is disconnected, the current supplied by the generator will drop sharply, thus generating the peak voltage transient with an amplitude of 125V. When the EM interference of the AC generator is disconnected, it will generate transient interference with an amplitude of 100V~-40V. If the electromagnetic coil inductive switch and the decay transient of the AC generator appear at the same time, it may cause 600V transient interference voltage. Data processing machines generate a lot of broadband noise. The interference sources include motors, converters, cam contacts, solenoid magnets, transistors, relays, amplifiers, triggers, gate circuits, power lines and driven armathures, etc. These devices often bring serious interference into the power lines and data lines. For example, the conduction spectrum range of computer logic components is 0.05MHz~20MHz, instruction program device power line conduction spectrum range is 1MHz~25MHz, and instruction program device signal line conduction spectrum range is 0.1MHz~25MHz.

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