In recent years, the media has often mentioned the term "public hazard", such as water pollution, air pollution, noise problems, etc. Once upon a time, environmental pollution has become our biggest headache.
With the rapid development of electronic technology, various electronic devices have appeared in our surroundings. Some of these devices are operated separately, and some are combined with other devices to form the whole system. The circuit works of these electronic devices are somewhat analogous. Some are digital, or both analog and digital. In these increasingly electronic devices and large energy facilities, it also causes extremely serious electromagnetic signal pollution, namely Electro-MagneTIc Interference EMI. For example, the engine ignition of some steam locomotives may cause interference to the TV set and appear on the TV screen in stripes. Or it may cause interference to the radio and appear on the horn in the form of fried beans or background noise. When listening to the sound, it may be lifted or high-frequency. The noise generated by the equipment is inserted into the audio equipment to cause unpleasant sound output, or the wrong operation of the computer caused by the startup of the refrigerator when using the home computer.
Electromagnetic interference is ubiquitousSome electronic products that have never appeared in the past are now one of the main sources of electromagnetic interference, such as hair dryers, washing machines, air conditioners, refrigerators, elevators, fluorescent lamps, electric sewing machines, video recorders, electric welders and high-frequency equipment. In addition, the power line will also generate a lot of interference problems. In addition to man-made noise, nature also has many interference problems, such as static electricity, lightning strikes and noise from outer space. In short, our living circle is full of electromagnetic interference, and it seems that the higher the standard of living, the problem is The more serious it is.
Perhaps for many people, they think that the noise is only to make the TV screen look bad, or the audio equipment emits some noise, and it will not cause any danger; but if a heart patient is using ECG equipment Also, if it is interfered by electromagnetic noise, or when the passenger plane makes an emergency landing in a storm, and its electronic navigation system is disabled by electromagnetic interference, what should be done? Only ask for yourself! Fortunately, most people have never entered such a dangerous environment, but these dangers do exist and must not be ignored.
Electromagnetic interference was already present when Marconi designed the first radio transmitter. Electromagnetic interference, also known as radio frequency interference (RFI), was called noise interference earlier. The biggest question for Marconi's design of radio transmitters was the transmitter interference problem of the two sets of spark-tuned circuits, but if Marconi is still alive, he will be surprised at the seriousness of today's electromagnetic interference problems and their complexity. Degree.
For decades, engineers have known that almost every electronic device emits some electromagnetic waves when it is working. Therefore, it is impossible for electronic devices to interfere with other electronic devices when using them. However, due to the wide variety of such noises, The causes and methods of transmission are all-encompassing, neither predictable nor cost-effective, and often the time and effort spent on solving noise interference is often more than the cost of circuit design. In recent years, many organizations have conducted research on Electromagnetic Compatibility (Electro Magne TIc CompaTIbility EMC). The so-called electromagnetic compatibility refers to the ability of the device to work normally in a possible electromagnetic interference environment.
Some sophisticated and expensive military equipment, instruments, large computers, aviation equipment, etc., in order to eliminate the problem of electromagnetic interference, many electromagnetic compatibility improvement methods are also adopted, but some of the consumer equipments that are of the nature of home appliances are not the cost concerns of manufacturers. It believes that only a small part of their products will enter the electromagnetic interference environment, so even if the circuit designed to resist electromagnetic interference is not able to achieve the desired results in these products. Coupled with the competition in the market, it is simply impossible for manufacturers to research and improve and add some circuits that do not directly contribute to their sales. When users encounter interference problems, they have to find a way!
Electromagnetic noise solutionElectromagnetic noise can be divided into Radiated Noise and Conducted Noise according to its propagation method. The three main factors that constitute noise interference are "noise generating source" and "receptive body affected by noise". And the "noise propagation path", if the three are missing, it cannot constitute a noise problem. The interference may come from your own home, from the neighbor's home, or from a nearby power transmission line. Noise can also enter through the antenna. In many cases, the noise is likely to be picked up directly by the wiring inside the house or the wires of the equipment.
There are three ways to solve electromagnetic interference: one is that the interference generated by the "noise generating source" must be suppressed to the minimum, and the other is that the "receptive body" (such as televisions, computers, audio equipment, etc.) must be protected, with a view to The ability to resist noise interference, the third is that the "noise propagation path" is completely blocked or minimize the chance of noise transmission.
In general, in addition to the interference caused by electromagnetic radiation directly, electromagnetic interference introduced through the power line is the most common; for some high-fidelity audio equipment, the noise caused by the power line will be in many cases. Cleverly moving the original sound, and has a dramatic effect on the credibility of the fax, and at this time you can not see the change of the audio waveform with the oscilloscope; similarly, the radiated electromagnetic interference will also affect the signal line. , speaker cable, etc. constitute an influence.
For many people, they believe that the power delivered from the power facility to the user is similar to the sine wave produced by their signal generator. The only difference is power. In fact, if you use an oscilloscope to make observations, you will find that what you see is not a "clean" 60Hz sine wave, but a variety of distortions, distortions, surges, depressions, etc. This phenomenon also changes with time and place; at this point, if you are deeply aware of the seriousness of this problem, then the best way is to buy a bunch of batteries to use, or to buy a generator back, just to generate electricity The source of the machine's power must not be a gasoline engine, because the spark plug of the gasoline engine is also one of the sources of electromagnetic interference. To be honest, these two solutions are really the only way to cure the problem, and military units, telecommunications units and radio stations have indeed used a room of lead-acid batteries and generators to supply the power needed for their equipment. However, the current workaround for us is to get it "beauty and plastic" before the power goes into the equipment.
Isolation transformerIt has been thought of using an isolation transformer to block the propagation path of noise, because transformers designed for power supply are usually designed to operate at a fixed low frequency, and their frequency response is also quite poor. Therefore, magnetic lines of force at high frequencies Cross-linking is poor, and as a result, multiple types of noise are limited. This concept is good, but in order to reduce the cost and reduce the magnetic loss, the general transformer and the secondary are close together, then there is enough capacitance between them to pass the noise. There is a way to reduce the capacitance between the primary and secondary rings. Even if the primary and secondary rings are separated, the more efficient the separation, the lower the efficiency of the transformer. The better method is in the primary and secondary. The layers are separated by a layer of copper and grounded, that is, Faraday Shield. If the ground is grounded with Zs 0, the electrostatic capacitances C1 and C2 between A and B are no longer connected in series. It only works by Cs. At this time, the electrostatic isolation material used, such as the material of the copper sheet, the thickness of the copper sheet, etc., will affect the effect. In fact, even if the thickness of the isolation material is very thick, there will be some Zs. Therefore, the influence of C1 and C2 cannot be completely eliminated. If the number of spacers is increased, the effect will be larger. In addition to the series connection to reduce the equivalent capacitance value, the isolation of electromagnetic waves is somewhat effective. In short, in order to achieve good electrostatic isolation, the Zs value should be reduced as much as possible, and the Cs value should be reduced.
To truly achieve a transformer with a high noise cut-off effect, it is necessary to achieve "box shielding", that is, to completely separate the primary ring from the secondary ring and to perform electrostatic isolation, and to configure the primary and secondary rings. Wiring is minimized so that the capacitance between the primary and secondary rings will be less than 0.005 pF. It's just that the manufacture of such a transformer is not as simple as the traditional EI model, and the price is not low. If you really love your equipment, it is worth it. However, if you can find such a transformer of the same specifications in your equipment, you may wish to replace it directly, which will be better than when you use the isolation transformer.
Recently, there is a power socket box with a radio frequency filter in the market, which is simpler and more economical than the method of using an isolation transformer, but it is still the same as when using a transformer for noise isolation, and its filter (or transformer) itself to the equipment. The shielded power line often forms an antenna, especially in the VHF band; most filters have good attenuation characteristics between 1MHz and 10MHz, and frequencies above 10MHz are getting worse. That is, it is not able to suppress high frequency noise above 100MHz into your equipment.
Some manufacturers have found that their products have interference problems, the Metropolis will directly install the filter in the equipment in order to reduce the energy of these noises so as not to interfere with the normal operation of the circuit; but the expected result is all The electronic products can all avoid the problem of electromagnetic interference. Then, only when the design is fully considered to be electromagnetic compatibility, otherwise it will be found after the product is introduced, then it may take a higher price. The cost and spirit can solve some problems, and sometimes it is impossible to solve the interference problems.
postscript:This article only briefly introduces some sources of electromagnetic interference and their simple suppression methods. In fact, these are not enough. As for the detailed solution, it must be started from the time of product design. Readers who are interested can refer to this. In fact, you will have a deeper understanding of electronic equipment.
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