How to solve EMI problems Electromagnetic interference problem diagnosis four steps

The concept and prevention of electromagnetic interference has gradually gained importance in China. Although there is no strict regulation of electromagnetic interference (EMI) in electronic products in China, due to the electromagnetic interference requirements of many countries in Europe and the United States, and the widespread use of digital products, the requirement for electromagnetic interference is an urgent matter. Because of the relationship between the author and the author, I often encounter many products that have already finished the finished product design. Because I can't pass the EMI test, the design engineer spends a lot of time and energy on the EMI modification. Because it belongs to the ex post facto remedy, I often invest a lot of time and money. Even affect the timing of product launch

To solve the EMI problem on the product, if it can be considered at the beginning of product design, you can save a lot of time and money afterwards. Since the current EMI Design-in concept is not very common, and because the prior planning does not guarantee that the finished product can fully comply with the electromagnetic interference test, how to properly diagnose the EMI problem is very important for design engineers and EMI engineers. .

In fact, if we take EMI as a disease, of course, preventive maintenance is usually important. Once a disease is diagnosed correctly, it can be quickly cured, there is no correct diagnosis, and the source of the disease cannot be found. It takes less time and effort to delay things. Therefore, on the issue of EMI, we often see a product with EMI problems. We have not been able to find the key to EMI problems. We have spent a lot of time and many countermeasures, but we have not been able to solve them. There is no shortage of professional EMI engineers. In the past, EMI often emphasized countermeasures, and even considered many countermeasures or tricks. However, without proper diagnosis, adding a large number of EMI suppression components to the products will only result in worse EMI.

When the author first contacted the product for EMI measures, he would hear senior EMI engineers say that all EMI measures were removed and they could pass the test. At first thought it was a joke, but now it is a valuable experience to recall. Then I heard many EMI engineers talk about similar experience. This article will give practical examples to give readers a better understanding of EMI's countermeasures.

Generally mentioned how to solve EMI problems, most of them are case by case. Of course, from the aspect of measures, the characteristics of each product and layout of the circuit board are different, so it is impossible to solve all EMI problems with several methods. But for a long time, we have always wanted to deal with EMI issues and make appropriate countermeasures. In addition, we also provide a reference method for professional EMI engineers. Here, we have compiled some experiences of electromagnetic interference and countermeasures in order to help readers.

We propose a set of EMI diagnosis reference steps and hope to use a systematic approach to quickly identify EMI problems. We are not prepared to discuss some theoretical calculations or formula deductions that will be explained in practice.

When a product cannot pass the EMI test, it must first have a concept and find out the problems that cannot be passed. At this time, we must not have any subjective thoughts and we must take measures in those places. There are often many experienced EMI engineers. Since many related products have been modified, they are also very familiar with the places where the products may cause EMI problems. However, the habits of directly applying drugs may be very effective, but they are also occasionally difficult to meet. After the revision, the key to finding the problem was finally the place where it was considered impossible. The reason why it would be neglected was because it was too subjective. Therefore, regardless of the characteristics of the product, we must confirm it one by one and even confirm it many times. This is because the problems that cause EMI are often complicated and not caused by a single point. Therefore, it is very important to repeat the confirmation and diagnosis.

We have detailed the preliminary diagnostic steps and explained the key points. These steps seem to be very ordinary and simple, unlike the introduction of countermeasure methods and endless changes in various theoretical cheats. In fact, many senior EMI engineers repeat these steps and judgments for most of the time when dealing with their countermeasures. The writer needs to emphasize again that only the key to finding the EMI problem is the best way to solve EMI. If only relying on theoretical estimation or empirical judgment, it will sometimes take more time and effort.

â–  Step 1

Turn the desk to the maximum position of the EUT to be tested, preliminary diagnose possible causes, and turn off the EUT power supply to confirm.

Due to the EMI test, the EUT must be rotated 360 degrees and the antenna varies from 1m to 4m. The purpose is to record the maximum radiation. Similarly, when we find that we can't pass the test, we first move the antenna position to the maximum height of noise reception, and then turn the desk to the worst angle. At this time, we know that the EUT faces the antenna and the radiation is the strongest. It is possible to preliminarily speculate on possible causes, such as poor shielding or proximity to radiation sources or passing wires and cables.

Another thing to note is that you need to turn off the power of the EUT and see if noise is present to make sure that the noise is actually generated by the EUT. It has been found that the test monitor has not been able to solve the interference at a certain point. As a result, the noise is caused by the PC rather than the Monitor. There are also some points in the OPEN SITE test monitor that cannot be passed. The sound of the test receiving instrument is judged to be Monitor. As a result, the power is turned off and it is found that the noise still exists, so the step of turning off the EUT power supply is necessary and is often easily ignored.

â– Step 2

Remove the peripheral cables connected to the EUT one by one to see if the interference noise is reduced or disappeared.

If a cable is removed and the frequency of interference is reduced or even disappeared, it can be seen that this cable has become an antenna to radiate the noise in the board. In fact, careful analysis of the key to EMI can be expressed in a very simple model.

The source of any EMI must have the presence of an antenna to generate radiation. If there is only a noise source and there is no antenna condition, the amount of radiation is very small. If it is connected to the antenna, then the energy is due to the antenna effect. Radiate into space. Therefore, in addition to the EMI countermeasures, the most important task is to destroy the source of radiation. In the past, we often saw that EMI measures cannot be separated from Shielding, Filter, and Grounding. For a ground connection, a circuit board is often fixed and cannot be processed because this part is in the circuit. The layout of the board must be carefully considered. If the board is completed, the space that can be changed at this time is very small. The general method can only find out the ground where the noise is small and connect it with the thicker ground wire to reduce the common mode (Common Mode noise. The materials and costs involved in shielding are also very high. Filtering methods are often visible Bead inductors, etc. It is not very effective to use a lot of them. This is how we do it. Many times we do not solve the antenna effect of its radiation. In general, the energy of the noise does not disappear because of the addition of some countermeasure components, that is, the energy does not diminish. What we need to do is to prevent the noise from radiating into space (radiation test) or from the power supply (conduction test).

Here, we have collated several common situations that produce radiation for the reader's reference.

(1) Cables external to the machine become radiation antennas

Since the cable connected to the outside of the machine itself acts as an antenna, it radiates noise into the space. At this time, the noise is related to the length of the cable. Because the antenna effect of the cable is greatest relative to the resonance at the half-wavelength of the noise, it often causes EMI. Failed to pass the test. It is necessary to make some judgments before solving this problem, otherwise it is easy to neglect and waste time.

(a) Noise is generated by the machine's internal circuit board or grounding

In this case, removing the cable or adding a Core will reduce or eliminate the noise. One step that must be done at this time is to approach the machine (without direct connection) to see if noise is present. If the noise does not increase, it can be reliably determined by the inside of the machine. If the cable is approached and the interference noise rises immediately, , please refer to (b) for instructions.

(b) Noise is coupled to the cable inside the machine, making the cable a radiating antenna.

This point is easily overlooked by many test engineers. In this case, as mentioned in (a), as long as a cable is approached, it can be seen from the spectrum that the noise rises immediately. This means that the noise is not simply radiated from the line, but is the noise of the machine itself. The energy is quite large. Once there is an antenna, it is immediately coupled to the antenna to radiate. In the actual test, we found that many communication products have such situations. At this time, if you simply use Core or Bead to deal with it, you cannot really solve the problem.

(2) Leads inside the machine, and the connection line becomes a radiation antenna

Since many products often have internal wires connected to each other in the hall, when these wires are close to noise sources, they can easily become an antenna and radiate noise. Judging from this point of view, in the noise below 200MHz, we can add a Core to determine whether the noise is reduced, and for high-frequency noise above 200MHz, we can move the position of the line around to see if the noise will Increase or decrease.

(3) The wiring on the circuit board becomes a radiation antenna

Because the trace is too long or close to the noise source and is coupled into the transmitting antenna, in this case, when the external cable is removed, and only the circuit board is left, the noise can still be seen on the spectrum analyzer. At this time, it can be measured with the probe. The place where the circuit board noise is strongest, find the problem of radiation to solve. The tools and methods for detection will be described in detail later.

(4) Components on the circuit board become sources of radiation

Because the used IC or the CPU itself generates a large amount of radiation during operation, the EMI test cannot pass, and the noise of the pebble species often remains after the analysis of (1), (2), and (3), and is usually solved. The method is to replace a similar component and see if the EMI characteristics will be better. In addition, when the circuit board is rewired, it is placed in the position with the least impact, that is, there is no I/O Port and connection line passing nearby. Of course, if the situation permits, the entire assembly is covered with a metal shell (Shielding). A quick and effective method.

From the above analysis, we can understand that the most critical place for electromagnetic interference radiation is the problem of wires. When proper antenna conditions exist, it is easy to generate interference. In addition, the power line is often the main cause of the antenna effect. In the EMI countermeasures, it is most likely to be neglected.

â–  Step 3

The power cord cannot be removed and can be clamped on it or swing horizontally and vertically to see if the noise is reduced or changed. If the product has a battery device, you can remove the power cable to judge, such as Notebook PC.

As mentioned before, the power line is often used as a radiation antenna, especially for desktop PCs. Noise above 300 MHz is often coupled into the power line. Therefore, it is a necessary step to determine whether the power line of the product is infected. Due to the effect of the noise band, we can judge whether or not Core can be used at frequencies below 200MHz (more than one at a time). For noise above 200MHz, the power line can be placed horizontally and vertically because the role of the Core is not significant at this time. To see if the interference noise is different, if there is a significant difference between horizontal and vertical, you can see whether there is any change in the noise level on the spectrum while swinging the power cable, so that you can know whether the power line has interference.

As for how to solve the problem if it is found that the power line will produce radiation, it is generally not easy to deal with. Usually, firstly think of ways to reduce the noise in the machine so as to avoid the secondary radiation of the power line. However, the use of Shielded cables generally does not affect the radiation. Large, so for a different length of power cord, sometimes there will be very good results.

From this point, we can know that in addition to leaving the radiated noise-generating components away from the I/O ports, they must also be kept as far away as possible from the power lines and switching power supply boards so that they cannot be coupled to the power lines, making radiation and conduction impossible. Passed the test.

â–  Step 4

Check that the grounding screw on the cable connector end is tight and the ground at the outer end is good.

After looking briefly at the problem in the first three ways, we must do some more checks because through these inspections, we may pass the EMI test without any modification. For example, check whether the screws at the cable end are locked, and sometimes loosen the screws to strengthen the shielding effect of the cable. In addition, you can check to see if the connector of the external connector of the machine is well grounded. If the shell is metal and there is spray paint, you may consider scraping the paint at the connector to make the grounding effect better. In addition if Shielded cable is used, it must be checked whether the overlying metal part at the joint end is in close contact with its iron cover. Many poor shielded cables (RS232) are mostly due to the over-shielding of the wire connector and the metal end of the cable connector. The closeness of the ground makes it impossible to fully achieve the shielding effect.

Various joints such as Keyboard and Power supply often affect the interference of noise radiation due to poor adhesion between the connector plug and the socket on the machine. Check the way the connector can be unplugged to see if the noise is reduced, the reduction indicates two kinds of books, one for the radiation interference on the line itself, and the other for the contact between the connectors is not good, then plug in the connector, with the hand pin micro Swing the connector end from side to side to see if the noise will decrease or disappear. If it can be reduced, the connector of the Keyboard or Power supply can be affixed with copper foil tape to increase the degree of closeness between the connector and the machine connector. This is also measured. It is easy to be neglected, and it will misjudge the EMI of the machine why each time is good or bad, or spend a lot of time on other measures.

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