Detailed explanation of the differences and characteristics between electromagnetic environment and electromagnetic compatibility

1. Introduction

With the development of science and technology, the quality of human life is improving day by day. Informatization and automation are rapidly entering ordinary families, and more and more electronic and electrical equipment are used in families, resulting in the worsening of the electromagnetic environment . The so-called electromagnetic environment ( Electromagnetic Environment) refers to the transmission medium, which generally refers to various types of transmission lines, cables and space transmission media. Electromagnetic fields or electrical signals include various types of electrical signals and electromagnetic waves. The frequency ranges from near DC, low frequency to microwave, millimeter wave, and submillimeter wave; the signal forms are various, including pulsed and linked waves. Some are also modulated by various modulation methods. These electromagnetic waves and electrical signals are generated by thousands or even millions of signal sources. There are many types of radiation sources, and they are complex and changeable. The signal density can exceed one million pulses per second. These electromagnetic signals can directly affect the human body and produce the so-called electromagnetic wave biological effect. It can also affect the operation of various electrical and electronic equipment, reduce their working performance, or even destroy their normal operation.

2. Spectrum distribution of electromagnetic interference

The electromagnetic environment of the information society is extremely complex and is becoming more and more complex. Electromagnetic interference is distributed throughout the entire electromagnetic wave spectrum. If divided according to the spectrum of the most common interference, it can be roughly divided into the following frequency bands:

1. Power frequency interference: The frequency is about 50-60Hz, mainly electromagnetic field radiation generated by the power transmission and distribution system and the electric traction system;

2. Very low frequency interference: interference radiation below 30KHz, lightning, nuclear explosions and electromagnetic pulses generated by earthquakes, their energy is mainly distributed in this frequency band;

3. Long-wave signal interference: frequency range 10KHz ~ 300KHz. Including high-voltage DC transmission harmonic interference, AC transmission harmonic interference and AC electric railway harmonic interference, etc.;

4. Radio frequency and video interference: The spectrum is between 300KHz and 300MHz. Industrial medical equipment (ISM), transmission line corona discharge, spark discharge of high-voltage equipment and electric traction system, as well as internal combustion engines, electric motors, household appliances, lighting appliances, etc. are all within this range;

5. Microwave interference: frequency ranges from 300MHz to 300KHz, including high frequency, ultra-high frequency and extremely high frequency interference;

6. Nuclear electromagnetic pulse interference: The frequency range is very wide, from KHz to close to DC.

3. Intentional Electromagnetic Interference (IEMI)

In recent years, the term intentional electromagnetic interference (IEMI) has emerged. It refers to various types of electromagnetic interference released by terrorists, criminals, and hackers to disrupt the normal operation of electronic and electrical equipment. The activities of hackers and hackers may be slightly different, but the consequences are the same. Recently, the scientific and technological community has attached great importance to studying and evaluating the threat of intentional electromagnetic interference to human life.

In February 1999, a special seminar on IEMI was held at the Zurich Electromagnetic Compatibility Conference. It was agreed that IEMI would be defined as: the malicious creation of electromagnetic energy for terrorist and criminal purposes, which generates noise and signals to electrical and electronic systems and equipment, thereby disrupting, interrupting, or destroying these systems and equipment. Hackers are not explicitly mentioned in this definition, but in most countries, attacks on "entertainment" systems for commercial gain are illegal.

The threat of IEMI is real. Terrorist activities are increasing in the world, and IEMI is a new means of terrorism. It can attack covertly and through many physical barriers (such as walls, borders, etc.). Satellite TV broadcasts in my country have been disrupted and interrupted several times, which is caused by IEMI.

4. General characteristics of IEMI

From the perspective of signal type, electromagnetic interference in the electromagnetic environment can be divided into two categories, one is wideband and the other is narrowband. From the perspective of energy transmission, there are also two methods, one is radiation and the other is conduction. The waveform of the narrowband attack signal is almost a single frequency (generally the bandwidth relative to the center frequency is less than 1%), and it is radiated within a certain time interval (generally in the order of microseconds). The frequency of the equipment most likely to be affected is roughly between 0.3 and 3GHz. Of course, the working performance of equipment outside this frequency range can also be affected, especially for resonant systems. This type of electromagnetic radiation may also be modulated. This type of radiation is generally called high-power microwave radiation (HPM). This term also includes radiation other than microwaves.

Broadband emission is generally a pulse in the time domain and is repetitive. The energy of broadband radiation is distributed over a very wide frequency band. For example, ultra-wideband pulses (UWB) generally have a rise time of 0.1 nanoseconds and a fall time of about 1 nanosecond. Therefore, the energy is distributed over a very wide spectrum.

The energy of narrowband interference signals is concentrated in a single frequency, which can easily generate a field strength of several hundred kilovolts per meter. This can cause permanent damage to the equipment. In contrast, the energy of broadband electromagnetic interference is distributed in various frequencies, so the field strength is relatively weak. It is precisely because its energy is distributed in many frequencies that many frequencies may be affected for a system, and this interference is mostly repetitive, lasting for several seconds or even minutes, increasing the possibility of equipment damage.

The above interference, like other interferences dealt with by electromagnetic compatibility , can enter electronic equipment through radiation or through wires and cables. For radiated interference, it seems that the radiation with a frequency higher than 100MHz is the most concerned. This radiation can easily penetrate unprotected walls, enter the interior of buildings, and couple to machines and equipment. Moreover, the antennas in this frequency band can be made very accurate. Tests conducted in accordance with IEC standard 61000-4-3 show that general commercial equipment is easily affected when the field strength is 3~10V/m (80MHz~2.5GHz). Of course, the degree of interference varies depending on the program of the equipment.

For broadband radiation, IEC uses the electrostatic discharge test (61000-4-2), which generates a peak voltage of up to 1KV/m near the arc of electrostatic discharge. The rise time of this peak is 0.7 nanoseconds and the fall time is about 30 nanoseconds. This can simulate the situation of electromagnetic interference radiation. 5. Naturally generated electromagnetic interference

In principle, all electrical and electronic equipment may generate electromagnetic interference. However, some are serious, while others are relatively weak. Some of the main equipment and devices that generate electromagnetic interference signals are as follows.

1. Transmission line corona clutter. There are many measured data on transmission lines. Based on these data, a practical formula for calculating corona clutter can be obtained. However, the generation mechanism, emission and propagation characteristics of this clutter are not completely clear, and the theory in this area still needs to be further explored.

2. Car noise. Car noise is the main cause of urban noise in the VHF to UHF bands. According to the results of the measurement of its intensity and characteristics, corresponding measures can be taken to ensure that the quality of radio and television is basically unaffected. However, due to the recent widespread use of electronic equipment for automobile control and mobile communication equipment, this issue has been raised again. Stanford Research Institute (SRI) has improved the main components of the ignition system that emit noise, such as ignition plugs and distributor contacts, to reduce the noise in the 30MHz to 500MHz frequency band by 13 to 20 decibels. In addition, someone has calculated the pulse noise amplitude distribution of each ignition plug of a 6-engine engine. In the case of the distributor, if the electrode gap is from 0.27mm to 2.39mm, the noise can be reduced by 10 decibels. If silver contacts are added to the load electrode or covered with polymetallic alloys, the noise can also be reduced. Automobile electrical devices other than the ignition system can also emit noise, and their characteristics are being tested and studied.

3. Contact noise. It can be roughly divided into contactor noise and noise caused by discharge when the conductor is opened and closed. Discharge noise generated by the opening and closing of relay and motor contacts and commutator brushes accounts for a considerable proportion of artificial noise.

4. Electric locomotive noise. When the electric locomotive is running, the discharge between the conductor, the pantograph and the contact wire is also artificial noise.

If the current path of the pantograph is surrounded by filtering materials and some auxiliary measures are taken, the clutter can be reduced by 20 decibels, but an absolutely effective method to prevent clutter has not yet been found.

5. Industrial, scientific and medical radio frequency equipment (ISM) clutter. ISM equipment is a frequency conversion device that converts 50Hz AC into radio frequency through radio frequency oscillation. It is used for industrial induction and dielectric heating, medical electrothermal method and surgical tools, ultrasonic generators, microwave ovens, etc. Although the ISM equipment itself is shielded, there are gaps, oil holes, pipeline in and out, and poor grounding, which will still cause electromagnetic field leakage to form interference.

6. Urban clutter. Since urban clutter is closely related to social activities, it always changes with the times. In Japan, urban clutter tests are conducted regularly every year. Many scholars and experts in Europe and the United States also collect clutter test data. This work has also begun in my country. The source, degree and characteristics of urban clutter are changing at any time, and its test methods and statistical processing methods need to be further explored.

7. Others. The above mainly introduces the current status and existing problems of several types of man-made noise. In addition, electrostatic discharge and abnormal operation of radio stations are sometimes harmful. Moreover, there are various mixed waves generated by transients on the power lines of almost all machines and equipment, which will cause the machine to malfunction. With the widespread adoption of digital circuits, the problem is even more serious. In addition, it is also found that there is interference of unknown reasons. The micro switch of the automatic boiler igniter in the food industry also has poor contact, so it is also necessary to accelerate research to find out the cause of these noises.

6. Nuclear Electromagnetic Pulse

Everyone knows that there are three major effects of a nuclear explosion: shock waves, thermal radiation (light radiation) and radioactive contamination. In fact, nuclear weapons have a fourth effect - electromagnetic pulse (EMP). If a hydrogen bomb is exploded at a high altitude outside the atmosphere, there will be no shock waves or thermal radiation due to the lack of air, and the radioactive dust will weaken with the square of the distance and be absorbed by the atmosphere, so it is very weak when it reaches the ground and is harmless to humans. However, a nuclear explosion at an altitude of more than 100 kilometers can produce a very strong electromagnetic pulse (50~100kV/m) over an area of ​​millions of square kilometers. One of the third-generation nuclear weapons currently being developed by the United States, Russia and other countries is the nuclear electromagnetic pulse bomb. Here the electromagnetic pulse effect of a nuclear explosion is highlighted. If the energy released by ordinary nuclear weapons in the form of electromagnetic pulses only accounts for 3/1010 to 3/105 of the total energy released by the nuclear bomb, the nuclear electromagnetic pulse bomb can increase this value to 40%. The consequence of EMP is to destroy electrical and electronic equipment without harming people, which is exactly the opposite of a neutron bomb. EMP can destroy and paralyze the enemy's command, control, communication and intelligence (C3I) systems, disconnect the power grid, and affect metal pipelines and underground cable communication networks, leaving the enemy in a world without power, communication, or computers. Because it does not kill or injure people, nuclear weapons are "conventionalized", which increases the danger of nuclear war.

VII. Summary

Since the 1980s, electronic equipment has undergone fundamental changes. Integrated circuits have replaced transistors, which has greatly reduced their ability to withstand damage (high voltage and high current breakdown and burnout). The current of an integrated circuit is one thousandth of that of a transistor, and one millionth or even one thousandth of that of an electron tube. Nuclear electromagnetic pulses have become a major threat to electronic equipment. The higher the level of microelectronics technology, the worse the ability of electronic equipment to withstand damage. It is no wonder that some Russian military aircraft and radio stations still use ultra-small electron tubes instead of transistors and integrated circuits. Many countries' field radio stations are still assembled with discrete components.