1. Definition of electromagnetic interference
(1) Electromagnetic disturbance (EMD)
Electromagnetic disturbance is "any electromagnetic phenomenon that may cause degradation of the performance of a device, equipment, or system, or that may have an effect on animate or inanimate matter. Electromagnetic harassment may be electromagnetic noise, unwanted signals, or changes in the propagation medium itself."
(2) Electromagnetic Interference (EMI)
Electromagnetic interference is "a degradation of the performance of equipment, a transmission channel, or a system caused by electromagnetic harassment". Electromagnetic harassment is only electromagnetic phenomenon, that is, the objective existence of a physical phenomenon, it may cause the degradation of equipment performance or damage, but not necessarily have formed the consequences. And electromagnetic interference is the consequences caused by electromagnetic harassment. In the past, the terminology is not a clear division of the physical phenomenon and its consequences, collectively referred to as interference. But for convenience, usually people in the analysis of electromagnetic interference is often associated with electromagnetic harassment discussed together, or collectively referred to as electromagnetic interference.
2. Electromagnetic interference (harassment) source classification
The classification of electromagnetic interference can be many kinds of points, for example, according to the propagation path, there are conducted interference and radiation interference, which conducts the nature of the transmission of interference with electrical coupling, magnetic coupling and electromagnetic coupling; according to the nature of the transmission of radiated interference, there are near-field induced coupling and far-field radiated coupling; according to the frequency band, there are narrow-band interference and wide-band interference; according to the frequency range of the interference, there are narrow-band interference and wide-band interference; according to the frequency range of the interference, there is a wide range of interference. Broadband interference; according to the interference frequency range, can be subdivided into five kinds; according to the implementation of the subjective intention of the interference points, can be divided into intentional and unintentional sources of interference; according to the nature of the source of the interference points, there are natural interference and man-made interference, and so on. Later we will explain in detail the natural and man-made interference.
Frequency range classification of electromagnetic interference
Classification of electromagnetic interference according to frequency range
Classification of electromagnetic interference sources
3. Electromagnetic interference of the three elements
All electromagnetic interference is generated by a combination of the three basic elements, which are: the source of electromagnetic interference; the interference energy-sensitive equipment; the source of electromagnetic interference will be transmitted to the sensitive equipment, i.e., the medium of the transmission channel. Sensitive equipment medium, that is, the transmission channel or coupling pathway. Accordingly, the method of suppressing all electromagnetic interference should also be addressed by these three elements.
(1) electromagnetic interference source: refers to any component, device, equipment, system or natural phenomenon that generates electromagnetic interference.
(2) coupling via (or transmission channel): refers to the electromagnetic interference energy transfer to the disturbed equipment channel or medium.
(3) Sensitive equipment:Refers to equipment that is affected by, or responds to, electromagnetic interference.
4. Natural interference (noise)
Natural sources of electromagnetic interference exist on Earth and in the universe, and natural electromagnetic phenomena produce electromagnetic noise. Natural interference is mainly divided into cosmic interference, atmospheric interference, lightning interference and thermal noise.
(1) Cosmic interference
Cosmic interference is electromagnetic harassment from the solar system, the Milky Way and extragalactic star systems, mainly including the space background noise and the sun, the moon, Jupiter and other radio noise emissions. Solar radio noise, on the other hand, with the sun's activity changes significantly, solar activity in high years radio noise increases significantly. The sun's interference frequency from 10MHz to tens of GHz. the Milky Way's peak interference occurs in the frequency band 100 ~ 200MHz. cosmic interference has the greatest impact on the frequency band is 20 ~ 500MHz.
(2) thunder and lightning interference
Thunder and lightning interference is mainly generated by local lightning in the summer and lightning discharges in tropical areas in the winter. On average, about 100 lightning discharges occur every second on Earth. Lightning is a series of disturbing pulses, whose electromagnetic emissions can propagate thousands of kilometers away with the help of ionospheric transmission. The spectrum of lightning interference is distributed up to 50 MHz, with the main energy distribution around 100 xi Hz, which has a greater impact on radio communications on Earth up to 20 MHz. Other natural phenomena in the atmosphere (e.g., sandstorms, rain, fog, etc.) also form stronger sources of electromagnetic noise.
(3) atmospheric interference
Atmospheric interference is that in addition to lightning discharges in the atmosphere, dust, rain, snow, hail and other particles in the high-speed through the aircraft, airships, the surface of the relative friction movement due to the migration of charge and thus the deposition of electrostatic forces, when the potential to rise to 1MV, a spark discharge, corona discharge occurs. This discharge produces a broadband RF noise spectrum distribution in the range of a few hertz to tens of megahertz, which will seriously affect the high-frequency, very high-frequency band radio communications and navigation.
(4) Thermal noise
Thermal noise is the irregular electrical undulation that occurs in a conductor that is in a certain thermodynamic state, which is caused by the irregular movement of free electrons in the conductor, such as resistive thermal noise, gas discharge noise, and scattering noise of active devices.
4. Man-made interference (noise)
Man-made interference comes from intentional and unintentional emission interference sources, respectively.
(1) Intentional emission interference sources
Intentional emission interference sources are dedicated to the radiation of electromagnetic energy equipment, such as radio, television, communications, radar, navigation and other transmitting equipment, is through the launch of useful signals to the space electromagnetic energy to work, they do not need these signals of the electronic system or equipment will constitute a functional interference, but also an important source of contamination of the electromagnetic environment. After analysis, it is not difficult to see that this type of interference sources have the following characteristics:
①In order to ensure a certain role in the distance, these devices have a high power transmitter, a large amount of electromagnetic energy into space. For example, medium-wave broadcasting output power up to megawatts, short-wave broadcasting output power up to several hundred kilowatts, at present, China's television broadcasting 1 to 12 channels of the output power is generally 10 xi W, 13 channels above the transmitter power of 30 xi W. Long-range radar pulse transmitter power up to more than 10 MW.
②These radio transmitting equipment, are in accordance with the relevant provisions of the radio management, working in the designated frequency band to suppress mutual interference between the wireless system, the transmitter power and operating frequency of these devices can be artificially defined and limited, and the spatial distribution of radiated energy is determined by the directionality of the transmitting antenna.
3 broadcasting (including FM radio) and television transmitters, the number of transmitters, transmit power and transmit antennae high, the electromagnetic energy emitted to cover a wide area, radio and television broadcasting and the electromagnetic pollution caused by the environment than the same power of the other industrial sources of interference to be much larger. Because the former is a useful signal can not impose electromagnetic shielding, and the latter produces useless interference signals, available shielding and other technical measures to be suppressed, and radio and television transmission towers are built near the city, so the radio and television transmission is the main source of pollution of the electromagnetic environment of the city interference.
(2) unintentional emission sources of interference
There are many devices that unintentionally emit electromagnetic energy, such as automobile ignition systems, a variety of different electrical devices and devices with electric motors, lighting fixtures, neon advertisements, high-voltage power lines, industrial, scientific, and medical equipment, as well as receivers, such as the native oscillation of the receiver radiation, unintentionally emit electromagnetic energy. This emission may be to the space of the radiation, may also be along the conductive emission, the electromagnetic energy emitted is random or regular, generally occupy a very wide frequency band or discrete spectrum, the power emitted can be from the micro-watt to megawatt order of magnitude. Unintentional emission of interference sources are mainly the following kinds:
1) for industrial, scientific, medical and life in the high-power equipment such as industrial heating equipment (induction heaters and dielectric heaters, etc.), radio frequency arc welding, medical heating equipment (microwave therapy machine) microwave surgical equipment, ultrasonic generators and microwave ovens. This type of equipment is characterized by high power, the number of general output power of up to kilowatts or even megawatts, and its number is rapidly increasing year by year. Electromagnetic leakage from their work can cause strong interference. The International Special Committee on Radio Interference (CISPR) has stipulated interference limit values for such equipment.
②Ignition systems, generators, fans, windshield wiper motors, etc. in automobiles and other motorized vehicles cause interference by radiating electromagnetic energy outward. Usually, the ignition system is the strongest source of broadband interference, when the ignition produces a very steep waveform front arc, the spectrum is a low-frequency fundamental component plus many harmonics, as well as occupying a wide section of the spectrum of the transient (transient process), so that the noise in the range of 10 to 100MHz has a very large field strength. General observations show that the noise of a car is about 103B smaller than that of a truck, and the noise of a motorcycle is about the same as that of a truck, which is due to the fact that although motorcycles are much less powerful than cars and trucks, little or no shielding measures are taken. For example, the metal shell of a small car provides about 153B of shielding. Automotive interference is generally vertically polarized (especially in the frequency range up to 100 MHz), the amplitude of interference generated by automobiles is generally normally distributed, and the peak amplitude of the interference pulse is related to the type of ignition system in the automobile, the speed of the automobile, the mechanical loads under which it is normally operated, and the degree of aging and wear and tear on the automobile. With the development of the economy, the number of motorized vehicles, such as automobiles, in personal possession is growing at a rate of more than 12% per year. Statistics show that when the traffic volume doubles, its interference power spectral intensity increases by 3 to 63B, so cars and other motorized vehicles are one of the important sources of interference.
③Some other unintentional transmitting equipment
Electric motors, lighting equipment (fluorescent lamps, fluorescent lamps, etc.), electric power transmission lines, electrified railroads, public **** power supply.
④Electrostatic discharge interference
Electrostatic discharge is also a harmful source of electromagnetic nuisance. When two materials with different dielectric constants come into contact, especially when rubbing each other, the transfer of electric charge between the two will occur, and each becomes an object with a different charge. When the charge accumulates to a certain extent, it will produce high voltage. At this time, charged objects and other objects close to the corona discharge or spark discharge, the formation of electrostatic harassment. The most dangerous consequence of electrostatic harassment is that it may cause a fire, resulting in flammable and explosive materials detonation; secondly, it may lead to measurement, control system failure or malfunction, may also lead to computer program errors, integrated circuit chip damage.
5 nuclear explosion electromagnetic pulse
nuclear explosion will produce a very strong electromagnetic pulse, the intensity of which can be up to 1000kV/垃 or more, the distribution of the scope of the very wide. High-altitude nuclear explosions can affect the radius of thousands of kilometers. Nuclear electromagnetic pulse for weapons, space vehicles, ships, ground radio command system, industrial control systems, power electronic equipment will cause serious interference and damage.
6. Electromagnetic interference (nuisance) source of time, space, spectral characteristics
(1) the spatial distribution of interference energy
For the intentional radiation interference source, the spatial distribution of its radiation interference is relatively easy to calculate, mainly depends on the directionality of the transmitting antenna and transmission path loss.
For the unintentional radiation source, it is impossible to calculate theoretically, but some mathematical models and empirical data about the distribution of interference field of unintentional radiation source can be obtained by statistical measurement.
For random interference, because the future value can not be determined, its interference level can not be expressed in terms of a definite value, need to be expressed in terms of the probability of the occurrence of its specified value.
(2) time distribution of interference energy
The distribution of interference energy over time is related to the operating time of the interference source and the probability of occurrence of the interference, according to the probability of occurrence of the interference time can be divided into three types of periodic interference, non-periodic interference and random interference. Periodic interference refers to interference that can be repeated at a defined time interval. Non-periodic interference can not be repeated in a determined period, but the time of its appearance is determined, and can be predicted; random interference, on the other hand, can not be predicted in a way to change, and the characteristics of its change is not regular, so random interference can not be analyzed by the time distribution function, but should be applied to analyze the amplitude of the spectral rate characteristics.
(3) Frequency characteristics of interference
In accordance with the frequency distribution characteristics of interference energy can determine the spectral width of the interference, according to the spectral width of its interference, can be divided into narrow-band interference and broadband interference. Generally speaking, the bandwidth of narrowband interference is only a few tens of hertz, the widest only a few hundred kilohertz. And broadband
Interference in the energy distribution in tens to hundreds of megahertz, or even wider range. In the field of electromagnetic compatibility disciplines, the bandwidth is relative to the receiver's bandwidth, according to the definition of the national military standard GJB72-85, narrow-band interference refers to the main energy spectrum falls within the passband of the measurement receiver, while wide-band interference refers to the energy spectrum is quite wide, when the measurement of the receiver in the Shi2 pulse width tuning, it is not more than 3dB on the receiver output response.
Intentional Transmission The frequency distribution of the source interference energy can be derived from the transmitter's operating band and out-of-band emission and other characteristics, while the unintentional emission sources, the statistical law to derive empirical formulas and mathematical models.
Reference: www.pcbhf.com/