Why should dB be introduced for electromagnetic compatibility?
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Why should dB be introduced for electromagnetic compatibility?

Posted Date: 2024-01-22

1. Why should dB be introduced for electromagnetic compatibility?

RE102 limits applicable to ground in Figure 151B

Let’s first take a look at the RE102 limit in GJB151B. The ordinate is the field strength limit expressed in logarithm. Let’s do a simple calculation first to see the range of the ordinate in this picture. The ordinate is from 0dBuV/m to 100dBuV/m, 0dBuV/m =1uV/m, 100dBuV/m =100000uV/m. We can see that the numerical range of the coordinates is very wide, from 1uV/m to 100000uV/m, which is a full 100000 times. If we use linear coordinates to make this diagram, if the ordinate is an average of 10 grids, then the value of each grid is 10000uV/m, and 10000uV/m corresponds to 80dBuV/m, so basically the ordinate has a smaller value. It cannot be represented on the picture at all. In the linear coordinates, the values ​​below 80dBuV/m are all in the first grid of the ordinate. In this way, the whole image is very deformed and looks strange.

From the above we can see that the field strength range that electromagnetic compatibility often has to deal with is very wide, often from a very small value to a very large value. We use linear coordinates to represent it, so the smaller value is on the graph It doesn't show up at all. And after we introduce dB, we compress the value rows by multiples, so that the length of each 10-fold value on the ordinate is the same. In this way, we can display the value from a very small value to a very large value on a graph. range of values.

2. What is the difference between dB and dBuV?

Everyone who does electromagnetic compatibility knows that power is converted by 10 times and voltage is converted by 20 times. This is very clear. You can check relevant books for calculation methods. What I want to emphasize here is to pay attention to the difference between dB and dBuV.

dB is a dimensionless unit, and its table only represents a multiple relationship, while dBuV is an absolute value, which is only a logarithmic conversion with reference to 1uV. It is important to understand this. For example, 1dBuV+2 dBuV cannot be added directly. The two numbers must be converted into uV before adding. And 1dBuV amplified by 2dB is equal to 3 dBuV, because 1dBuV is 3 dBuV after being amplified by 2dB. This is a logarithmic unit, which satisfies the conversion relationship of logarithmic multiple increase.

3. dBMisunderstandings in electromagnetic compatibility rectification

When the electromagnetic disturbance of the equipment cannot meet the limits stipulated in the relevant EMC standards, the reasons for the excessive emission of the equipment must be analyzed and then eliminated. In this process, it is often found that many people still have not eliminated the fault after a long period of hard work. The reason for this situation is that the dB in electromagnetic compatibility is not fully understood, which leads to the diagnosis work falling into an "infinite loop".

This situation often occurs during the rectification of CE102 and RE102. Let’s illustrate with a RE102 rectification process.

Electromagnetic compatibility rectification is a common electromagnetic compatibility work. When the equipment is tested for electromagnetic compatibility, if it fails. It needs to be rectified. We use the example below to illustrate some "strange" phenomena of dB.

Cables are the main cause of radiated interference emissions. The equipment shown in the figure has three cables, and the cables do not take proper common-mode current control measures, resulting in radiated emissions. It is assumed that the radiation generated by the cable accounts for 99.99% of the total radiation. The leakage from the chassis accounts for 0.01% of the total radiation, assuming that each cable accounts for 33.33% of the radiation.

After unplugging a cable from the device, the radiated emission is reduced by 33.33%, that is, the radiated emission is 66.67% of the original value. The dB reduction in radiation is calculated as follows:

20lg(66.67/100)= 3.5dB

After unplugging another cable from the device, follow the same calculation method above to get the dB reduction in radiation:

20lg (33.34/66.67)= 6dB

When the last cable is unplugged from the device, only 0.01% of the radiation generated by the chassis remains, and the resulting radiation reduction in dB is

20lg(0.01/33.34)=70dB

Through the above calculations, we found that although the three cables contribute the same amount to the radiated disturbance emission, when the first cable is removed, the radiation is only reduced by 3.5dB. After the second cable is removed, the radiation is reduced by 6dB. When the last cable is removed, the radiation is reduced by 6dB. After the cable was removed, the radiation intensity improved by about 70dB. This will make people think that the radiated emissions from the first and second cables are not the main ones, and that the third cable is the largest radiation source.

If you think that the third cable is the largest radiation source, as long as the radiation problem of the third cable is solved, the radiation can be reduced by 70dB. That is a misunderstanding. When the first and second cables are retained, the third cable should be used. When the cable is removed, you will find that the improvement is only 3.5dB.

Mastering this characteristic of dB is very important for rectifying electromagnetic compatibility problems. After taking a rectification measure, if the effect is not obvious, you should not think that this measure has no effect and remove it, and then take another measure. Rather, we should retain this measure and take another measure. Just because the effect of a certain measure is obvious does not mean it is considered an important measure.

Therefore, the correct electromagnetic compatibility EMI diagnosis method is that after taking suppression measures against a possible disturbance source, even if there is no obvious improvement, do not remove this measure and continue to take measures against other possible disturbance sources. When the measures are taken to During a certain measure, if the amplitude of harassment is reduced a lot and can pass the test, it does not necessarily mean that this source of harassment is the main one, but it only means that this source of harassment is larger in magnitude than the following sources of harassment.

From the above content, we can see that a correct understanding of dB is very important in electromagnetic compatibility rectification.

Review Editor: Huang Fei


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