1. Definition of electromagnetic interference
(1) electromagnetic interference (EMD)
Electromagnetic interference refers to "any electromagnetic phenomena that may reduce the performance of devices, equipment or systems or have an impact on living or inanimate substances. Electromagnetic disturbance may be electromagnetic noise, useless signals or changes in the medium itself. "
(2) Electromagnetic interference
Electromagnetic interference (EMI) is "the decline of equipment, transmission channel or system performance caused by electromagnetic interference". Electromagnetic disturbance is just a kind of electromagnetic phenomena, that is, an objective physical phenomenon, which may degrade or damage the performance of equipment, but it may not necessarily have consequences. Electromagnetic interference is the result of electromagnetic disturbance. In the past, there was no clear term definition of physical phenomena and their consequences, which were collectively called interference. However, for the sake of convenience, when people analyze electromagnetic interference, they are often discussed together with electromagnetic harassment, or collectively referred to as electromagnetic interference.
2. Classification of electromagnetic interference (disturbance) sources
There are many ways to classify electromagnetic interference, for example, according to the propagation path, there are conducted interference and radiation interference, among which the transmission characteristics of conducted interference are electrical coupling, magnetic coupling and electromagnetic coupling; According to the transmission characteristics of radiation interference, there are near-field inductive coupling and far-field radiation coupling; According to the frequency band, there are narrowband interference and broadband interference; According to the interference frequency range, it can be subdivided into five types; According to the subjective intention of implementing jammers, it can be divided into intentional interference sources and unintentional interference sources; According to the nature of interference sources, there are natural interference and human interference, and so on. Natural interference and man-made interference will be discussed in detail later.
Frequency range classification of electromagnetic interference
According to the classification of electromagnetic interference in frequency range
Classification of electromagnetic interference sources
3. Three elements of electromagnetic interference
All electromagnetic interference is produced by the combination of three basic elements, which are: electromagnetic interference source; Equipment sensitive to interference energy; The medium that transmits the electromagnetic interference source to sensitive equipment, that is, transmission channel or coupling channel. Accordingly, the methods to suppress all electromagnetic interference should also be solved by these three elements.
(1) Electromagnetic interference source: refers to any component, device, equipment, system or natural phenomenon that produces electromagnetic interference.
(2) Coupling channel (or transmission channel): refers to the channel or medium that transmits electromagnetic interference energy to the interfered equipment.
(3) Sensitive equipment: refers to the equipment that is affected by or reacts to electromagnetic interference.
4. Natural interference (noise)
Natural electromagnetic interference sources exist in the earth and the universe, and natural electromagnetic phenomena will 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 interference from solar system, Milky Way and extragalactic galaxies, mainly including space background noise and radio noise emitted by the sun, moon and Jupiter. Solar radio noise changes obviously with solar activity, and increases significantly in high solar activity years. The interference frequency of the sun ranges from 10MHz to tens of GHz. The interference peak of the Milky Way appears in the frequency band 100 ~ 200 MHz. The frequency band with the greatest influence of cosmic interference is 20 ~ 500 MHz.
(2) Lightning interference
Lightning interference is mainly local lightning in summer and lightning discharge in tropical areas in winter. On average, there are about 100 lightning discharges per second on the earth. Lightning is a series of interference pulses, and its electromagnetic emission can travel thousands of kilometers through ionospheric transmission. The spectrum distribution of lightning interference is below 50MHz, and the main energy distribution is around 100ⅸHz, which has a great influence on the radio communication below 20MHz on the earth. Other natural phenomena in the atmosphere (such as sandstorm, rain and fog, etc.). ) will also form a strong electromagnetic noise source.
(3) Atmospheric interference
Atmospheric interference refers to dust, raindrops, snowflakes, hail and other particles in the atmosphere, except lightning discharge, moving at high speed on the surface of aircraft and spacecraft, resulting in charge migration and electrostatic deposition. When the potential rises to 1MV, spark discharge and corona discharge occur. The frequency spectrum of broadband radio frequency noise generated by this discharge is in the range of several hertz to several tens of megahertz, which will seriously affect radio communication and navigation in high and very high frequency bands.
(4) Thermal noise
Thermal noise refers to the irregular electrical fluctuation in a conductor under a certain thermodynamic state, which is caused by the irregular movement of free electrons in the conductor, such as resistance thermal noise, gas discharge noise, shot noise of active devices and so on.
4. Man-made interference (noise)
Man-made interference comes from intentional emission interference sources and unintentional emission interference sources respectively.
(1) intentionally emits interference sources.
Intentional jamming sources are devices that radiate electromagnetic energy, such as radio, television, communication, radar, navigation and other transmitting devices, which work by emitting electromagnetic energy of useful signals into space. They will cause functional interference to electronic systems or equipment that do not need these signals, and are important pollution sources of electromagnetic environment. After analysis, it is not difficult to see that this kind of interference source has the following characteristics:
In order to ensure a certain range, these devices all have high-power transmitters to emit a large amount of electromagnetic energy into space. For example, the output power of medium-wave broadcasting can reach MW, and the output power of short-wave broadcasting can reach several hundred kilowatts. At present, the output power of TV broadcasting channels in China is generally 10ⅸW, and the transmission power of channels above 13 is 30 ⅸ w. The pulse transmission power of long-range radar can reach above 10MW.
(2) These radio transmitting devices all work in the designated frequency band according to the relevant regulations of radio management, so as to suppress the mutual interference between wireless systems. The transmission power and working frequency of these devices can be specified and limited artificially, and the spatial distribution of radiation energy is determined by the directivity of the transmitting antenna.
(3) Broadcasting (including FM broadcasting) and TV transmitting stations are numerous, with high transmitting power, high transmitting antenna and wide coverage of emitted electromagnetic energy. The electromagnetic pollution caused by radio and television transmission to the environment is far greater than other industrial interference sources with the same power. Because the former emits useful signals and cannot be shielded, the latter produces useless interference signals, which can be suppressed by shielding and other technical measures, while radio and television transmission towers are mostly built near cities, so radio and television transmission is the main interference source that pollutes the electromagnetic environment of cities.
(2) Unintentional emission of interference sources
Many devices emit electromagnetic energy unintentionally, such as automobile ignition system, various electrical equipment and equipment with motors, lighting equipment, neon advertising, high-voltage power lines, industrial, scientific and medical equipment, local oscillation radiation of receivers, etc. This kind of emission can be radiation into space or transmission along a conductor. The emitted electromagnetic energy is random or regular, usually occupying a wide frequency band or discrete frequency spectrum, and the emitted power can range from picowatts to megawatts. Unintentional emission interference sources mainly include the following:
① High-power equipment used in industry, science, medical care and life. Such equipment includes industrial heating equipment (induction heaters and medium heaters, etc.). ), radio frequency arc welding, medical heating equipment (microwave physiotherapy machine), microwave surgical equipment, ultrasonic generator and microwave oven, etc. This kind of equipment is characterized by high power and a large number, and the output power can generally reach kilowatt or even megawatt, and the number is increasing rapidly year by year. The electromagnetic leakage when they work will cause strong interference. The International Special Committee on Radio Interference (CISPR) has stipulated the interference limits for such equipment.
(2) Ignition system, generator, fan, wiper motor, etc. Of motor vehicles such as automobiles. Interference caused by the outward radiation of electromagnetic energy. Generally speaking, the ignition system is the strongest broadband interference source. When it is ignited, it produces an arc with steep waveform front, and its spectrum is low-frequency fundamental component plus many harmonics, as well as transients (transient processes) occupying a wide spectrum. This noise has great field strength in the range of 10 ~ 100 MHz. Generally speaking, the noise of cars is about 103B lower than that of trucks, while the noise of motorcycles is similar to that of trucks. This is because although the power of motorcycles is much lower than that of cars and trucks, little or no shielding measures are taken. For example, the metal shell of an automobile can provide a shielding effect of about 153B. Automobile interference is generally vertically polarized (especially in the frequency range of 100MHz), and the amplitude of automobile interference is generally normal distribution. The peak amplitude of interference pulse is related to the type of automobile ignition system, the speed of automobile, the mechanical load of normal work and the aging and wear degree of automobile. With the development of economy, the number of motor vehicles owned by individuals is increasing at a rate of more than 12% per year. Statistics show that when the traffic volume doubles, the spectrum intensity of interference power will increase by 3 ~ 63b, so automobiles and other motor vehicles are one of the important interference sources.
③ Other unintentional launching equipment.
Motor, lighting equipment (fluorescent lamp, fluorescent lamp, etc.). ), transmission lines, electrified railways, public power supply.
④ Electrostatic discharge interference
Electrostatic discharge is also a harmful source of electromagnetic interference. When two substances with different dielectric constants come into contact, especially when they rub against each other, charge transfer will occur between them, making them objects with different charges. When the charge accumulates to a certain extent, a high voltage will be generated. At this time, when the charged object is close to other objects, corona discharge or spark discharge will occur, resulting in electrostatic interference. The most dangerous consequence of electrostatic interference is that it may cause fire and lead to the explosion of flammable and explosive materials; Secondly, it may lead to the failure or malfunction of the measurement and control system, and may also lead to computer program errors and integrated circuit chip damage.
⑤ Nuclear explosion electromagnetic pulse
A nuclear explosion will produce a strong electromagnetic pulse, the intensity of which can reach more than 1000kV/ L, and it is widely distributed. The radius of high-altitude nuclear explosion can reach thousands of kilometers. Nuclear electromagnetic pulse will cause serious interference and damage to weapons, spacecraft, ships, ground radio command systems, industrial control systems, power electronic equipment and so on.
6. Time, space and spectrum characteristics of electromagnetic interference (disturbance) sources
Spatial distribution of (1) interference energy
For intentional radiation interference sources, the spatial distribution of radiation interference is easy to calculate, which mainly depends on the directivity of the transmitting antenna and the transmission path loss.
For unintentional radiation sources, it is theoretically impossible to calculate strictly. Through statistical measurement, some mathematical models and empirical data about the interference field distribution of unintentional radiation sources can be obtained.
For random interference, because the future value cannot be determined, its interference level cannot be expressed by a certain value, but by the probability of its specified value.
(2) Time distribution of interference energy
The distribution of interference energy with time is related to the working time of interference source and the probability of interference. According to the time probability of interference, it can be divided into periodic interference, aperiodic interference and random interference. Periodic interference refers to interference that can be repeated within a certain time interval. Although aperiodic interference cannot be repeated in a certain period, its occurrence time is certain and predictable. The change of random interference is unpredictable and its change characteristics are irregular, so random interference can not be analyzed by time distribution function, but by the spectral rate characteristics of amplitude.
(3) Frequency characteristics of interference
According to the frequency distribution characteristics of interference energy, the spectrum width of interference can be determined, which can be divided into narrowband interference and broadband interference. Generally speaking, the bandwidth of narrowband interference is only a few tens of hertz, and the widest is only a few hundred kilohertz. And broadband
The energy of interference is distributed in the range of tens to hundreds of megahertz or even wider. In the field of electromagnetic compatibility, the bandwidth is relative to the bandwidth of the receiver. According to the definition of GJB72-85, narrowband interference means that the main energy spectrum falls within the passband of the measurement receiver, and broadband interference means that the energy spectrum is quite wide. When the measuring receiver is tuned within 12 pulse widths, its influence on the output response of the receiver is less than 3dB.
The frequency distribution of interference energy of intentional transmitter can be obtained according to the working frequency band and out-of-band emission characteristics of transmitter, while for unintentional transmitter, empirical formula and mathematical model can be obtained through statistical laws.
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