Research on the mechanism of electromagnetic coupling

With the continuous development of science and technology and the continuous expansion of human activities, microelectronics technology and computer technology have been applied to all aspects of social life. A large number of electrical and electronic products with high technical content and complex internal structures have been widely used, making them increasingly informatized and Electromagnetic sensitization. The impact of complex electromagnetic environment on human beings has attracted more and more attention. Therefore, it is of great significance to study the electromagnetic coupling mechanism.

1. Concepts related to electromagnetic environment

The electromagnetic environment refers to the sum of electromagnetic phenomena existing in space. Various man-made electromagnetic radiation and natural electromagnetic radiation constitute a complex electromagnetic environment. Man-made electromagnetic radiation includes mobile phones, radio talkies, radio and television transmitters, satellites, radars, etc., lightning, static electricity, geomagnetic field, sunspots, etc. Activities, cosmic rays, etc. constitute natural electromagnetic radiation sources.

The formation of electromagnetic radiation requires the following three elements at the same time:

Electromagnetic wave source refers to the components, equipment, systems or natural phenomena that generate electromagnetic waves;

Coupling channel refers to the channel or medium that couples or propagates energy from a wave source to a sensitive device and causes the device to respond;

Sensitive equipment refers to equipment that responds to electromagnetic waves.

These three elements are usually called the three elements of electromagnetic coupling.

2. Coupling method of electromagnetic radiation

(1) Antenna coupling

All metal conductors exposed to electromagnetic fields can be considered as antennas. "Front-door" coupling refers to the coupling of electromagnetic pulse or microwave energy into the electronic system through the antenna on the target. Therefore, the coupling strength can be calculated based on the design characteristics of the antenna. When the electromagnetic wave frequency is equal to the antenna design frequency, the coupling reaches its peak value.

(2) Hole-slit coupling

Typically, electronic devices are packaged in containers made of conductive materials. Due to the needs of equipment for heat dissipation, ventilation, gaps, cracks, power feeding, and signal transmission, the container cannot be sealed and there are different types of holes and seams, which provide a coupling path for electromagnetic radiation. "Back-door" coupling means that high-energy electromagnetic pulse energy couples into the system through holes and gaps in the target, interfering with or destroying electronic equipment. When the wavelength is smaller than the hole size, the electromagnetic wave will enter the shield without any obstruction; when the wavelength is larger than the hole size, the electromagnetic wave will be blocked; when there is a hole with a size comparable to the electromagnetic wavelength, the coupling of electromagnetic waves will be serious. Resonance will occur.

(3) Coupling of power lines and transmission lines

The power cord is a long line exposed outside the system and is vulnerable to electromagnetic energy attacks. It can both receive and transmit interference energy. If there is a power line or signal transmission line connected from the shielded case to the inside of the system, the received and induced current will propagate along the line and enter the shield. Generally, microwave pulse current is propagated. Even if the current is not introduced from the lead of the core wire of the transmission cable, but is induced on the outer shielding layer, it will be coupled to the core wire through the transfer impedance and directly enter the electronic system. For microwaves, the transfer impedance of shielded cables is also much larger than that of radio frequencies, so microwaves can enter the core wires through the braided shielding of power lines and transmission lines.

(4) Skin effect of metal shell

The penetration of electromagnetic waves into metal shells is achieved through the skin effect. The skin depth of electromagnetic waves in the material is

In the formula, f is the frequency of electromagnetic waves, and are the magnetic permeability and electrical conductivity of the metal shell material respectively. For a 2GHz microwave signal, its skin depths in copper (=5.5×107S/m) and aluminum (=3.2×107S/m) are 1.52 and 2.82 respectively. For higher frequency microwave signals, the value of skin depth is smaller.

3. The mechanism of action of electromagnetic energy

The impact of electromagnetic hazard sources on high-tech equipment mainly occurs through conduction coupling and radiation coupling of energy. Its mechanism of action can be summarized as the following four aspects:

(1) Thermal effect

The thermal effect generated by electrostatic discharge and high-power electromagnetic pulse is generally completed in nanoseconds or microseconds, and is an adiabatic process. As an ignition source and detonation source, it can instantly cause the explosion of flammable and explosive gases or electrical pyrotechnics. It can also overheat microelectronic devices and electromagnetic sensitive circuits in the system, causing local thermal damage, circuit performance deterioration or failure, and even Causing inventory materials to burn and explode.

(2) Radio frequency interference and "surge" effect

Radio frequency interference caused by electromagnetic radiation causes electrical noise and electromagnetic interference to information equipment, causing it to malfunction or malfunction. Strong electromagnetic pulses and their "surge" effects can also cause hard damage to the system, which can not only degrade or invalidate the performance parameters of devices or circuits, but can also form a cumulative effect, burying potential hazards and affecting the usability of circuits or equipment. reduce.

(3) Strong electric field effect

The strong electric field (especially the electrostatic field) generated by electromagnetic hazards can not only cause the breakdown of the gate oxide layer of the MOS circuit or the dielectric breakdown between metal lines, causing circuit failure, but also cause potential damage to system self-test instruments and sensitive devices. impact on work.

(4) Magnetic effect

Strong currents caused by electrostatic discharge, lightning strikes, and similar electromagnetic pulses can produce strong magnetic fields, causing electromagnetic energy to be directly coupled into the system and interfering with the normal operation of electronic equipment.

References:

[1] Liu Shanghe, Wu Zhancheng: Electromagnetic environment and high-tech weapons and equipment [J]. Modern Military, 2001.

[2] Wu Xiong: Environmental issues of electric fields, magnetic fields and electromagnetic fields [J]. Electric Power Environmental Protection, 2007, (4).

[3] Gao Yan, Yu Bo: Characteristics of complex electromagnetic environment [J]. Journal of Sichuan Ordnance Industry, 2008, (1).

[4] Gao Bin, Tang Xiaobin: Preliminary study on the effects of complex electromagnetic environment [J]. Journal of China Electronics Research Institute, 2008, (4).

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