At present, people have gradually realized that the relationship between electromagnetic compatibility of electronic products is getting closer and closer. Whether there are electromagnetic interferences in color TVs, computers and communication products, and the electromagnetic compatibility status of these products, the state has issued many relevant standards, electromagnetic compatibility. Has become an important parameter to assess the performance 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 the device are not caused by electromagnetic radiation of other devices in the same electromagnetic environment, or suffer from impermissible performance degradation. For example, in the electromagnetic environment where the computer and color TV, VCD and mobile phone are housed in the same room, it can work normally, that is, the coexistence state of the respective functions can be completed in a common electromagnetic environment.
2 Electromagnetic compatibility problems in electronic measurement
When we conduct electromagnetic compatibility testing of 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 performance and accuracy of test systems and instruments are the most important factors to measure the accuracy of system and instrument testing, and less consideration is given to the electromagnetic compatibility of the test system and the instrument itself. Incorrect measurement results caused by poor compatibility are often overlooked, and 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 with each other. Especially when the electromagnetic compatibility of the measured object is poor, the influence between the test system and the test system is very serious, and may even lead to erroneous measurement. result. I participated in the design of the 581 radar crystal liquid electromagnetic compatibility measurement, using a German imported emission (signal generator) source instrument for testing, because the instrument has a slight leakage, causing the IF circuit in the tested product to self-excitation Interference, turn off the test instrument, and eliminate self-excitation. For example, I conducted the RSB and TS-3 signal sources to check the XB-35 color TV signal generator. Especially in the case of small signal measurement (such as sensitivity), the interference is particularly serious, affecting the test. Therefore, technicians engaged in quality inspection should understand and master the principle of electromagnetic compatibility, and apply relevant technologies in practical work to solve the electromagnetic compatibility problems encountered in the measurement process. Distinguish whether it is a product issue or a 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 inspection device. Sensitive features. In order to avoid such electromagnetic damage, the electromagnetic environment must be analyzed. The electromagnetic environment is often dominated by a large number of interference sources with different characteristics. Many factors are determined and randomly changed. 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 All kinds of interference, and take appropriate protective measures. Learn to distinguish the essential parts of electromagnetic interference.
3 See the essence through the phenomenon
When we carry out the measurement, some electronic measuring devices sometimes have some abnormal phenomena during the work, such as the pointer type instrument will appear jitter, sudden jump phenomenon; the digital instrument digital appears irregular jumping phenomenon, etc. The reason for these phenomena may be due to the irrational circuit structure of the instrument itself, imperfect working principle, poor component quality, and defects in the manufacturing process. This phenomenon often occurs in domestic measuring instruments, and does not exclude mooring. There are also problems in this product; 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, changes in ambient temperature, and other electrical equipment, especially when the signal being detected is very When it is weak, the effect becomes more serious and prominent. This kind of external and internal useless signals that affect the measurement results of the electronic measuring device interfere with each other, in order to eliminate or weaken various interferences. All necessary measures must be taken to influence the operation of the electronic measuring device. Therefore, technicians engaged in electronic measurement should not simply think that there is a problem with the instrument or the sample being tested when encountering the above-mentioned similar phenomenon and the measurement result of abnormal measurement or unreliable service. Instead, first check whether there is interference and find the interference source. And should try to eliminate all kinds of interference to make the measurement more accurate.
The existing problems should be analyzed. In the electronic measuring device, there are various connections, which can be divided into internal and external contacts. External contacts such as input signals, output signals, power supplies, and external environmental conditions (including temperature, humidity, pressure, and various field strengths). These external factors are not bad for the electronic measuring device and instrument table when in normal conditions. Some of the effects are necessary and useful, but when these external factors change, they will affect the electronic measuring device and become a harmful link, which becomes a source of external interference. The internal parts of the instrumentation and the electronic measuring device are also interconnected. For example, the forward transmission of the signal is a useful connection, and the parasitic coupling between the parts is a harmful connection. Therefore, it is necessary to find a way. Cut or weaken those harmful connections without affecting or damaging the connections needed to properly measure and work.
For external interference, it can be solved by appropriate anti-interference measures. Internal disturbances from electronic measuring devices and instrumentation can be eliminated by proper design and proper layout of the device. Practice has proved that different measurement principles and measurement methods are affected by interference. At the same time, the influence of interference on the operation of electronic measuring devices and instrumentation is through its internal causes.
In view of the above, the essence of the transmission phenomenon is to study the anti-interference problem of electronic measuring devices and instrumentation, which cannot be completely attributed to the problem of protective measures. Instead, it should be studied together with working principles and test methods to achieve specific problems.
4 Generation and analysis of electromagnetic interference
In the detection, we encounter electromagnetic interference. The most common one is electrical noise, that is, the disturbed signal transmission superimposed on the useful signal, and the electrical physical quantity that causes the original useful signal to be distorted, referred to as noise. When the instrument is in operation, the noise is always superimposed on the useful signal, affecting the measurement result, and sometimes even completely drowning the useful signal, making the measurement work impossible. In the measurement process, the signal-to-noise ratio should be improved as much as possible to suppress the useful signal. Noise interference.
4.1 Noise generation
There are many types of noise, and the methods of generation, transmission, and suppression are also different, and are classified according to the cause, and there are internal noise and external noise.
Internal noise refers to the noise generated inside the instrument and the device or the device itself. Common types are: thermal noise, shot noise, contact noise, induced noise, AC noise, oscillation noise, reflected noise, and others.
External noise refers to the noise that invades the instrumentation and devices from the outside. There are mainly two types of natural noise and artificial noise. Natural noise refers to atmospheric noise, solar noise, and cosmic noise. Man-made noise has discharge noise, high-frequency noise, power frequency noise, radiation noise, etc. Among them, power frequency noise is a serious problem. Power frequency noise is power transmission and distribution line, power frequency power supply due to power frequency induction, static induction, electromagnetic induction. Noise generated by earth leakage current, etc., is the most influential interference to the instrumentation; and radiation noise is noise generated by high-power transmitting and receiving devices, which causes great interference to the electronic measuring device through radiation or through the power line. .
4.2 Propagation of noise
The propagation of noise is derived from the noise source. Different noises must have different noise sources. The noise source must be transmitted through a certain coupling path to send the noise to the instrument and device to affect its normal operation and form interference. Therefore, the noise must form the following channels: a, noise source; b, noise-sensitive receiving circuit or device; c, noise channel from the noise source to the receiving circuit.
The noise may be coupled to and integrated into the receiving circuit through common wires (such as utility power, public wiring, etc.), mutual sense space radiation of adjacent wires between the devices, and conductor paths in the alternating electromagnetic field. The coupling mode mainly conducts. Coupling and radiative coupling, some of which 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 the detection instrument and the parasitic capacitance in the device. The interference voltage is proportional to the angular frequency of the noise source, the distributed capacitance, and the input impedance of the receiving circuit.
Inductive coupling is caused by the alternating magnetic field formed by the alternating current in the noise source and the surrounding circuit, which is formed by induction in a highly sensitive receiving loop. The interference voltage is proportional to the angular frequency of the noise source, the mutual inductance and the noise source. Current.
The common impedance coupling is due to the common impedance of the two circuits. When a current flows through one circuit, the common impedance generates an interference voltage in the other circuit to form a common impedance coupling interference. The interference voltage is proportional to the common impedance and noise. Source current. Common impedance coupling is a common type of interference in instrumentation. It generally has the following forms:
The common impedance coupling interference formed by the power supply bone resistance, when the same power supply is used to simultaneously supply power to multiple meters, if the output current of a high level circuit flows through the power supply, this current will generate a voltage drop on the internal resistance of the power supply. Interference voltage is formed, causing interference to other low-level circuits.
The signal output circuits interfere with each other. When the signal output circuit of the electronic measuring device has multiple loads, if any load changes, the changer will affect the other output circuits through the 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 flows through the grounding wire, common impedance coupling interference will be generated through the grounding wire impedance.
Leakage current coupling is noise interference caused by leakage current of the current through the insulation resistance when the insulation is poor.
Electromagnetic radiation coupling means that the interference source transmits interference to the receiving circuit through space radiation, and the degree of interference of the receiving circuit is proportional to the interference intensity of the location.
4.3 Ways of electromagnetic interference
The noise generated by various noise sources must enter the electronic measuring device through various twisting channels, causing interference to them, causing measurement errors. According to the way the noise enters the measuring circuit and the relationship with the useful signal, the noise interference can be divided. For differential mode interference and common mode interference.
The differential mode interference is the interference generated by the signal input end of the detecting instrument changing with respect to the potential of the other signal input terminal, that is, the interference signal and the useful signal are superimposed and directly act on the input end, so that it directly Affect the measurement results.
Common mode interference is relative to a common potential base point (usually the ground point). Simultaneous interference on the two input terminals of the instrumentation, although this interference does not directly affect the measurement results, but when the signal input circuit parameters are asymmetrical, this common mode interference will be converted into differential mode interference, The measurement results have an influence. In the actual measurement, the voltage value of the common mode interference is generally large, and the coupling mechanism and its coupling circuit are also complicated, and the elimination is more difficult. Therefore, the common mode interference ratio differential mode interference measurement The impact is even more serious.
5 Elimination of electromagnetic interference
The degree of influence of electromagnetic interference on the measurement result is relative to the signal. The high level signal allows for greater interference, while the signal level signal allows for larger interference, and the lower the signal level, the limitation of interference. The more stringent, the frequency of interference is usually very wide, but for an electronic instrument, not all frequencies of interference have the same result, for the DC measuring instrument, because the instrument itself has low-pass filtering characteristics Therefore, it is not sensitive to high frequency AC interference; for low frequency measuring instruments, if the input is equipped with a filter, the interference outside the normal can be filtered out; however, for power frequency interference, the filter will be useful for 50Hz. The signal is filtered out, so power frequency interference is the most serious and difficult to remove interference for low frequency electronic meters. For wideband electronic meters, various interferences in the working frequency band will work. The suppression of interference should focus on the three elements of noise formation, and the corresponding measures should be adopted according to the specific situation. There are five methods commonly used:
5.1 Grounding
In the electronic measurement, grounding is one of the main methods to suppress interference. The grounding or grounding surface of the equipment is connected to the ground with low impedance. The main purpose of grounding is to:
(1) Give the zero potential reference of the device (unified reference potential point);
(2) Preventing the accumulation of electric charge on the outer casing or shielding layer of the device, causing unsafe conditions of the human body and the instrument, or causing spark discharge;
(3) Ground the equipment 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 cable
In the electronic measuring device and the electronic product to be tested, many connecting wires are required, and the connecting wires are an important cause of interference, and it should be considered to correctly arrange these connecting wires to reduce various parasitic couplings. The lead inductance of the wire has no major influence on the low frequency, but the influence on the high frequency cannot be ignored. The lead inductance must be minimized. In order to suppress the induced interference, the coaxial cable or the shielded twisted pair should be used at high frequencies. And the wire should be as short as possible; in the test system, there are connecting wires for different purposes, such as power lines, RF lines, audio lines, control lines, etc., to be classified, so that the different types of wires are as far away as possible, and not arranged in parallel In order to avoid radiation coupling, it is better to use shielded wires for the connecting wires. In addition, the thickness of the wires is related to noise. Choosing the appropriate connecting wires is preparation 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 applied to suppress external interference. For interference in the test system, It is not possible 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 opposite to shield grounding. The purpose of shield grounding is to divert the interference current from the signal circuit, that is, to prevent the interference current from passing through the signal line, but to let the interference current flow through the casing or shield to the ground, and the floating is blocked. The path of the interference current, after the measurement system is floated, increases the impedance between the common line of the test system and the earth, greatly reduces the common mode interference current, and can improve the common mode interference suppression capability.
However, floating is not absolute. Although the impedance between the common line of the test system and the earth is large (insulation resistance level), the resistive leakage current interference can be greatly reduced, but there is still parasitic capacitance between them, that is, capacitive leakage. The current still exists.
5.5 filtering
Filtering is an important measure to suppress and prevent interference. Whether it is to suppress the interference source and eliminate the coupling or improve the anti-interference ability of the system, filtering technology can be adopted. Any electronic device using AC power, the noise will be conductively coupled through the power line. In the circuit, interference is formed. In order to suppress such interference, it is necessary to use a filter in the input end of the AC power supply of the test system, and a commonly used anti-interference method.
In the measurement process, you will encounter various problems. It is necessary for the measurement technician to seriously analyze and solve the problem. Do not easily judge the unqualified products under the electronic product. It is necessary to distinguish the measurement instrument problem, the measurement method problem, and the measurement environment problem. When it is not relevant, you can conclude the product under test. Especially in the automatic test process, the surveyor should be good at analyzing the measurement results, eliminating various interferences, improving the accuracy of the measurement and reliable data. Surveyers must have good business qualities and the ability to analyze and solve problems.
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