An electronic measurement issue that cannot be ignored: electromagnetic compatibility

1 Overview

At present, people have gradually realized that the relationship between the electromagnetic compatibility of electronic products and life is becoming more and more close. Whether color TVs, computers and communication products have electromagnetic interference, and how are the electromagnetic compatibility of these products? The state has issued many relevant standards. Electromagnetic compatibility has become an important parameter for assessing the performance and quality of electronic products. Electromagnetic compatibility is one of the main performance indicators of any electronic engineering and system equipment. The so-called electromagnetic compatibility means that the device and equipment will not cause or suffer unacceptable performance degradation due to the electromagnetic emission of other devices and equipment in the same electromagnetic environment. For example, in the electromagnetic environment where computers, color TVs, VCDs and mobile phones are located in the same room, they can all work normally, that is, they can complete their respective functions in a common electromagnetic environment.

2 Electromagnetic compatibility issues in electronic measurement

When we carry out electromagnetic compatibility testing of electronic products, a problem that is easily overlooked is that the electronic measuring instrument itself also has electromagnetic compatibility issues.

Because, in electronic measurement, the performance indicators such as the function and accuracy of the test system and instrument equipment are the most important factors to measure the accuracy of the system and instrument test, and the electromagnetic compatibility performance of the test system and instrument equipment itself is less considered. The incorrect measurement results caused by poor electromagnetic compatibility are often ignored, and the conclusions are often given to the electronic products under test. Many test systems are composed of many different instruments, and there are different degrees of interference between them. Especially when the electromagnetic compatibility of the object being tested is poor, the impact on the test system is very serious, and may even lead to erroneous measurement results. When I participated in the design of the electromagnetic compatibility measurement of the 581 radar crystal liquid, I used a German imported transmission (signal generator) source instrument for testing. Because the instrument had a slight leakage, it caused the self-excitation and interference of the intermediate frequency circuit in the product being tested. Turn off the test instrument and the self-excitation is eliminated. For example, when I used RS-2 and TS-3 signal sources to calibrate the XB-35 color TV signal generator, especially in the case of small signal measurement (such as sensitivity), the interference was particularly serious, affecting the test. Therefore, technicians engaged in quality inspection should understand and master the principles of electromagnetic compatibility, and use relevant technologies in actual work to solve electromagnetic compatibility problems encountered in the measurement process. Distinguish whether it is a product problem or a problem with the test itself.

In fact, any inspection device must work in a certain electromagnetic environment. Unexpected electromagnetic energy in the electromagnetic environment will reduce the technical performance of the inspection device or cause permanent damage. This electromagnetic effect mainly depends on the sensitive characteristics of the inspection device. Characteristics of electromagnetic environment. In order to avoid this electromagnetic damage, the electromagnetic environment must be analyzed. The electromagnetic environment is often caused by a large number of interference sources with different characteristics. There are many factors that determine it, and it changes randomly. These interferences will affect the reliability and usability of the test system and equipment. In order to control electromagnetic interference, it is necessary to identify various types of interference and take corresponding protection measures. Learn to distinguish the essential parts of electromagnetic interference.

3 See the essence through the phenomenon

When we carry out measurements, some electronic measuring devices sometimes have some abnormal phenomena in their work, such as jitter and sudden jump of pointer instruments; irregular jumping of the digital numbers of digital instruments, etc. The reasons for these phenomena may be, on the one hand, due to the unreasonable circuit structure of the instrument itself, imperfect working principle, poor quality of components, defects in manufacturing process, etc. This phenomenon often occurs in domestic measuring instruments, and it is not ruled out that foreign products also have this problem; on the other hand, it may be due to changes in the working environment (conditions) of the instrument, such as power supply voltage, frequency fluctuations, ambient temperature changes, and the influence of other electrical equipment, especially when the detected signal is very weak, this influence becomes more serious and prominent, and the various external and internal useless signals that affect the measurement results of the electronic measuring device interfere. Through the phenomenon analysis of the essence, in order to eliminate or weaken the influence of various interferences on the work of the electronic measuring device, various necessary measures must be taken. Therefore, technicians engaged in electronic measurement, when encountering similar phenomena as mentioned above, abnormal measurements or unconvincing measurement results, should not simply think that there is a problem with the instrument or the sample being measured, but should first check whether there is interference, find the source of interference, and try to eliminate various interferences to make the measurement more accurate.

The existing problems should be analyzed. In electronic measuring devices, there are various connections, which can be divided into internal and external connections. External connections such as input signals, output signals, power supplies, and external environmental conditions (including temperature, humidity, pressure, and various field strengths). When these external factors are in normal conditions, they have no bad effects on electronic measuring devices and instrument tables. Some are still necessary and useful, but when these external factors change, they will affect the electronic measuring devices and become harmful connections and become the source of external interference. The internal parts of instruments and electronic measuring devices are also interconnected. For example, the forward transmission of signals is a useful connection, while the parasitic coupling between the parts is a harmful connection. Therefore, we must find ways to cut off or weaken those harmful connections, while not affecting or damaging those connections required for normal measurement and work.

For external interference, it can be solved by appropriate anti-interference measures. For internal interference from electronic measuring devices and instruments, it can be eliminated and weakened by the correct design and reasonable layout of the device. Practice has proved that different measurement principles and measurement methods are affected differently by interference. At the same time, the influence of interference on the operation of electronic measuring devices and instruments is through its internal causes.

In view of the above, looking through the phenomenon to see the essence is to study the anti-interference problem of electronic measuring devices and instruments. It cannot be completely attributed to the problem of protection measures, but should be studied in combination with the working principle and test method to treat specific problems differently.



4 Generation and analysis of electromagnetic interference

In the detection, we encounter electromagnetic interference. The most common one is electrical noise, that is, the electrical physical quantity superimposed on the useful signal that disturbs the signal transmission and distorts the original useful signal, referred to as noise. When the detection instrument is working, the noise is always superimposed on the useful signal, affecting the measurement result, and sometimes even completely submerging the useful signal, making the measurement impossible. In the measurement process, the signal-to-noise ratio should be improved as much as possible so that the useful signal can suppress the interference of noise.

4.1 Generation of Noise

There are many types of noise, and the methods of generating, transmitting and suppressing them are also different. According to the causes of generation, there are internal noise and external noise.

Internal noise refers to the noise generated inside the detection instrument and device or the device itself. Common noises include: thermal noise, shot noise, contact noise, induction noise, AC noise, oscillation noise, reflection noise and others.

External noise refers to the noise that invades the detection instrument and device from the outside. There are mainly two types of natural noise and man-made noise. Natural noise refers to atmospheric noise, solar noise and cosmic noise. Man-made noise includes discharge noise, high-frequency noise, power frequency noise, radiation noise, etc. Among them, the power frequency noise has a more serious impact. Power frequency noise is the noise generated by power transmission and distribution lines and power frequency power supply due to power frequency induction, electrostatic induction, electromagnetic induction, earth leakage current, etc. It is the most influential interference to the detection instrument; while radiation noise is the noise generated by high-power transmitting and receiving devices, etc., which will cause great interference to electronic measuring devices through radiation or power lines.

4.2 Noise propagation

Noise propagation comes from noise sources. Different noises must have their own noise sources. Noise sources must propagate through certain coupling paths to send noise to detection instruments and devices, affecting their normal operation and causing interference. Therefore, noise interference must have the following paths: a. Noise source; b. Noise-sensitive receiving circuits or devices; c. Noise channels between noise sources and receiving circuits. Noise

may couple and combine noise sources into receiving circuits through common wires (such as public power supplies, public connections, etc.), mutual inductance spatial radiation of adjacent wires between equipment capacitors, and wire paths in alternating electromagnetic fields. The coupling methods mainly include conduction coupling and radiation coupling. Some noise can be transmitted through both conduction and radiation.

Conductive coupling is divided into capacitive coupling, inductive coupling, common impedance coupling and leakage current coupling.

Capacitive coupling is a coupling formed by parasitic capacitance in the detection instrument and the device, and its interference voltage is proportional to the angular frequency of the noise source, distributed capacitance, and input impedance of the receiving circuit.

Inductive coupling is formed by the alternating magnetic field formed by the alternating current in the noise source interlinking with the surrounding loop, generating induction in the highly sensitive receiving loop, and its interference voltage is proportional to the angular frequency of the noise source, mutual inductance coefficient and noise source current.

Common impedance coupling is due to the existence of common impedance between two circuits. When current passes through one circuit, interference voltage is generated in the other circuit through the common impedance, forming common impedance coupling interference, and its interference voltage is proportional to the common impedance and noise source current. Common impedance coupling is a common interference in detection instruments, which generally has the following forms:

Common impedance coupling interference formed by power supply bone resistance. When the same power supply is used to power multiple instruments at the same time, if the output current of the high-level circuit flows through the power supply, this current will produce a voltage drop on the internal resistance of the power supply, forming an interference voltage, causing interference to other low-level circuits.

Signal output circuits interfere with each other. When the signal output circuit of an electronic measuring device carries multiple loads, if any load changes, the change will affect other output circuits through common coupling of output impedance. Common coupling interference

formed by ground line impedance. If the common line of an electronic measuring device is grounded, if a large current passes through the ground line, common impedance coupling interference will be generated through the ground line impedance.

Leakage current coupling is noise interference caused by leakage current through insulation resistance when the insulation is poor.

Electromagnetic radiation coupling refers to the interference source transmitting interference to the receiving circuit through space radiation. The degree of interference to the receiving circuit is proportional to the interference intensity at the location.

4.3 Modes of electromagnetic interference The

noise generated by various noise sources must enter the electronic measuring device through various coupling channels, interfere with it, and cause measurement errors. According to the different ways in which noise enters the measuring circuit and its relationship with the useful signal, noise interference can be divided into differential mode interference and common mode interference. Differential

mode interference is interference caused by a change in the potential of one signal input terminal of the detection instrument relative to another signal input terminal, that is, the interference signal and the useful signal are superimposed together and directly act on the input terminal. Therefore, it directly affects the measurement result.

Common-mode interference is interference that appears simultaneously on the two input terminals of the test instrument relative to a common potential base point (usually the grounding point). Although this interference does not directly affect the measurement results, when the signal input circuit parameters are asymmetric, this common-mode interference will be converted into differential-mode interference, affecting the measurement results. In actual measurements, since the voltage value of common-mode interference is generally large, and its coupling mechanism and coupling circuit are also relatively complex, it is difficult to eliminate it. Therefore, common-mode interference has a more serious impact on measurement than differential-mode interference.

5 Elimination of electromagnetic interference

The influence of electromagnetic interference on the measurement results is relative to the signal. High-level signals are allowed to have greater interference, while low-level signals are allowed to have greater interference. The lower the signal level, the stricter the restriction on interference. Usually, the frequency range of interference is very wide, but for an electronic instrument, not all frequency interferences have the same results. For DC measuring instruments, since the instrument itself has low-pass filtering characteristics, it is not sensitive to high-frequency AC interference; for low-frequency measuring instruments, if a filter is installed at the input end, interference other than the usual can be filtered out; however, for power frequency interference, the filter will filter out the useful signal of 50Hz, so power frequency interference is the most serious and difficult to remove interference for low-frequency electronic instruments. For wide-band electronic instruments, all kinds of interference within the working frequency band will play a role. Interference suppression should focus on the three elements of noise formation, and take corresponding measures in a targeted manner according to the specific situation. There are five commonly used methods:

5.1 Grounding

When conducting electronic measurements, grounding is one of the main methods to suppress interference, that is, to connect the ground wire or grounding surface of the equipment to the earth with a low impedance. The main purpose of grounding is:

(1) to provide a zero potential reference (uniform reference potential point) for the equipment;
(2) to prevent the accumulation of charge and voltage rise on the equipment housing or shielding layer, which may cause personal and instrument insecurity or spark discharge;
(3) to ground the equipment housing or shielding layer to form a low impedance path for high-frequency interference voltage to prevent it from interfering with electronic equipment.

5.2 Connection wires

In electronic measuring devices and electronic products under test, many connection wires are required. Connection wires are an important cause of interference. Consideration should be given to properly arranging these connection wires to reduce various parasitic couplings. The lead inductance of the wire has no great influence on low frequency, but its influence on high frequency cannot be ignored. The lead inductance must be minimized. In order to suppress inductive interference, coaxial cable or shielded twisted pair wire should be used at high frequency, and the wire should be as short as possible; in the test system, there are connecting wires for different purposes, such as power lines, radio frequency lines, audio lines, control lines, etc., which should be classified so that wires of different categories are as far away as possible and not arranged in parallel. In order to avoid radiation coupling, it is best to use shielded wires for connecting wires. In addition, the thickness of the wire is related to noise. It is necessary to select appropriate connecting wires before measurement.

5.3 Shielding

In order to suppress electromagnetic interference, whether it is external interference or internal interference, the interference source or receiver must be shielded. However, in electronic measurement, this method can only be used to suppress external interference. For interference within the test system, it is unlikely to use shielding.

5.4 Floating

Floating means that the common line (signal ground line) of the electronic measuring device is not connected to the ground. Floating is opposite to shielded grounding. The purpose of shielded grounding is to divert interference current from the signal circuit, that is, not to let the interference current flow through the signal line, but to let the interference current flow through the casing or shielding layer to the ground. Floating is to block the path of interference current. After the measurement system is floated, the impedance between the common line of the test system and the ground is increased, which greatly reduces the common mode interference current and improves the common mode interference suppression capability.

However, floating is not absolute. Although the impedance between the common line of the test system and the ground is large (insulation resistance level), which can greatly reduce the resistive leakage current interference, there is still parasitic capacitance between them, that is, capacitive leakage current still exists.

5.5 Filtering

Filtering is an important measure to suppress and prevent interference. Whether it is to suppress interference sources and eliminate coupling or improve the anti-interference ability of the system, filtering technology can be used. For any electronic equipment using AC power, noise will be coupled into the circuit through the power line to form interference. In order to suppress this interference, it is very necessary to use a filter at the AC power input end of the test system. It is also a commonly used anti-interference method.

During the measurement process, various problems will be encountered, which require measurement technicians to carefully analyze and solve. Do not easily conclude that the electronic products being tested are unqualified. It is necessary to distinguish whether it is a problem with the measuring instrument, the measuring method, or the measuring environment. Only when it is irrelevant can you draw a conclusion about the product being tested. Especially in the automatic testing process, measurement personnel must be good at analyzing measurement results, eliminating various interferences, and improving the accuracy of measurement and reliable data. Measurement personnel must have good business qualities and the ability to analyze and solve problems.