EMC Issues in the Use of Electronic Measuring Instruments

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 or 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 coexist in a common electromagnetic environment to complete their respective functions.

2 Electromagnetic compatibility issues in electronic measurement

When we conduct electromagnetic compatibility testing on electronic products, a problem that is easily overlooked is that the electronic measuring instruments themselves also have electromagnetic compatibility problems.

Because, in electronic measurement, the function, accuracy and other performance indicators of the test system and instrument equipment are the most important factors to measure the accuracy of the system and instrument test, while the electromagnetic compatibility of the test system and the 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 under test is poor, the impact on the test system is very serious, and may even lead to incorrect 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 under test. 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 apply relevant technologies in actual work to solve electromagnetic compatibility problems encountered during the measurement process, and distinguish whether it is a product problem or a problem with the test itself.

In fact, any test device must work in a certain electromagnetic environment. Unexpected electromagnetic energy in the electromagnetic environment will reduce the technical performance of the test device or cause permanent damage. This electromagnetic effect mainly depends on the sensitive characteristics of the test device. 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 methods. Learn to distinguish the essential parts of electromagnetic interference.

3. See the essence through the phenomenon

When we are carrying out measurements, some electronic measuring devices may sometimes have some abnormal phenomena during work, such as jitter and sudden jump of pointer instruments; irregular jumping of the 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 and other problems. This phenomenon often occurs in domestic measuring instruments, and it is not ruled out that foreign products also have such problems; 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 signal to be tested 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. By analyzing the essence of the phenomenon, various necessary measures must be taken to eliminate or weaken the influence of various interferences on the work of the electronic measuring device. Therefore, when technicians engaged in electronic measurement encounter similar phenomena as mentioned above, measurement anomalies or unconvincing measurement results, they should not simply assume 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 include: 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 meters. Some of them are still necessary and useful. However, when these external factors change, they will affect the electronic measuring devices and become harmful connections, becoming a 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.

External interference can be solved by appropriate anti-interference measures. Internal interference from electronic measuring devices and instruments can be eliminated and weakened through 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, seeing the essence through the phenomenon is to study the anti-interference problem of electronic measuring devices and instruments. It cannot be completely attributed to the problem of protective measures, but should be studied in combination with the working principles and test methods to treat specific problems differently.

4 Generation and Analysis of Electromagnetic Interference

During the detection, we encounter electromagnetic interference, the most common of which is electrical noise, which is an electrical physical quantity superimposed on the useful signal, disrupting signal transmission and distorting the original useful signal, referred to as noise. When the detection instrument is working, noise is always superimposed on the useful signal, affecting the measurement results, and sometimes even completely drowning out the useful signal, making the measurement impossible. During 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 Noise Generation

There are many types of noise, and the methods of generating, transmitting and suppressing them are also different. According to the causes of their 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 ones 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 instruments and devices from the outside, which mainly includes 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 most serious one is power frequency noise. 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., which has the greatest impact on detection instruments; 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 through power lines.

4.2 Noise Propagation

The propagation of noise comes from the noise source. Different noises must have their own different noise sources. The noise source must be propagated through a certain coupling path to send the noise to the detection instrument and device, affecting its normal operation and causing interference. Therefore, the noise interference must have the following paths: a. Noise source; b. Noise-sensitive receiving circuit or device; c. Noise channel between the noise source and the receiving circuit.

Noise may be coupled and combined into the receiving circuit through common wires (such as public power supply, public connections, etc.), the mutual inductance spatial radiation of adjacent wires between devices, and the wire paths in the alternating electromagnetic field. 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 formed by parasitic capacitance within 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 caused 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. Its interference voltage is proportional to the angular frequency, mutual inductance and current of the noise source.

Common impedance coupling is caused by the existence of common impedance in two circuits. When current flows through one circuit, interference voltage is generated in the other circuit through the common impedance, forming common impedance coupling interference. The interference voltage is proportional to the common impedance and the noise source current. Common impedance coupling is a common interference in detection instruments, which generally takes the following forms:

The common impedance coupling interference formed by the 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 the output impedance.

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

Leakage current coupling is the noise interference caused by leakage current through the 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 its location.

4.3 Methods of electromagnetic interference

The noise generated by various noise sources will inevitably 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 the interference caused by the change in the potential of one signal input terminal of the detection instrument relative to the other signal input terminal. That is, the interference signal and the useful signal are superimposed on each other 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 higher-frequency AC interference; for low-frequency measuring instruments, if a filter is installed at the input, 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, various interferences 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 making electronic measurements, grounding is one of the main methods to suppress interference, that is, to connect the ground wire or ground plane of the equipment to the earth with low impedance. The main purpose of grounding is:

(1) Provide the zero potential reference (uniform reference potential point) of the equipment;

(2) Prevent the accumulation of charge and voltage rise on the equipment casing or shielding layer, which may cause personal and equipment insecurity or spark discharge;

(3) Ground the device casing 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 line

In electronic measuring devices and electronic products under test, many connecting wires are needed. Connecting wires are an important cause of interference. The correct layout of these connecting wires should be considered to reduce various parasitic couplings. The lead inductance of the wire has no great influence on low frequencies, but its influence on high frequencies cannot be ignored. The lead inductance must be minimized. In order to suppress inductive interference, coaxial cables or shielded twisted pair wires should be used at high frequencies, and the wires 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. They 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 measurements, this method can only be applied 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) of the electronic measuring device is not connected to the ground. Floating is the opposite of shield grounding. The purpose of shield 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 very large (insulation resistance level), which can greatly reduce the interference of resistive leakage current, 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, eliminate coupling or improve the system's anti-interference ability, filtering technology can be used. For any electronic device using AC power, noise will be coupled into the circuit through the power line, causing interference. In order to suppress this interference, it is 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.