The influence of grounding performance on test results in conduction disturbance test_Century Wisdom (Suzhou) Testing Technology Co., Ltd

English

Home page

Technical column

CASES

The influence of grounding performance on test results in conduction disturbance test

Release time:

2022-04-05 00:00

Source:

　　1 Introduction

　　The power port conduction disturbance test is an important test in EMC testing, and the measurement frequency is 150 kHz~30 MHz (some standards start from 9 kHz). In the actual test arrangement, it is found that the grounding effect has a non-negligible impact on the test results of the power port conduction disturbance due to the difference in grounding wire material, grounding method and grounding position.

　　2 Types of harassment

　　When testing and assessing radio disturbance, according to the distribution of disturbance spectrum, measurement receiver bandwidth, disturbance duration, occurrence rate and disturbance influence degree, the disturbance is divided into narrowband continuous disturbance, wideband continuous disturbance and wideband discontinuous disturbance. Broadband and narrowband are primarily distinguished by the signal frequency compared to the bandwidth of the measuring receiver. In the conduction disturbance test, whether it is measuring current or voltage, the disturbance is divided into: common mode (also called asymmetric type), differential mode (also called symmetrical type), and asymmetrical.

　　3 Conducted disturbance test principle

　　The conduction disturbance test is carried out in a shielding room. The system is mainly composed of an artificial power network and a disturbance receiver with quasi-peak and average detection. The conduction disturbance test system is shown in Figure 1. EUT can be divided into table-top equipment and floor-standing equipment. For equipment designed to be placed on the table or on the ground, the test is carried out according to the table-top layout. For specific layout, please refer to EMC test standards and related product test specifications. In Figure 1, the equipment under test (EUT) is desktop equipment or floor-standing equipment, and the desktop equipment is placed on a 0.8-meter-high insulating table (the floor-standing equipment is placed on a 10cm-high insulating mat), and the artificial power network (AMN, Figure 1 The medium model is: ENV432) placed on the ground plane. The device under test is powered by the artificial power network, and the artificial power network transmits the measured noise signal to the receiver through the signal line, the receiver is connected to the computer, and the background operation is performed through the test software.

　　It can be seen from the test arrangement that the artificial power network must be placed on the ground plane. Therefore, no matter for the conducted disturbance test in the laboratory environment or in the EUT work site, the artificial power network and the ground plane need to be well grounded.

Figure 1 Conducted disturbance test system

　　Artificial mains network (AMN) is an indispensable auxiliary device in conducted disturbance testing, which is mainly used to measure the disturbance voltage emitted by the equipment under test along the power line to the power grid. The principle of the artificial power network used in the laboratory (50Ω/50μH+5Ω V-type artificial power network as an example) is shown in Figure 2. It is connected between the power grid and the EUT. Its functions mainly include:

　　(1) Isolation and coupling, the artificial power network prevents the radio frequency electromagnetic disturbance generated by the EUT from entering the power grid, and at the same time isolates the interference signal from the power grid; the radio frequency disturbance signal is connected to the measuring receiver through the coupling capacitor.

　　(2) Stabilize the impedance effect and provide a uniform impedance (50Ω), which is convenient for comparing test results under different power grids. In Figure 2, a stable impedance is provided between the test terminal of the EUT (through the coupling capacitor) and the reference ground (as shown in Figure 2, a 1kΩ resistor is connected below the coupling capacitor, which is connected in parallel with the input terminal of the measuring receiver. Since the input impedance of the measuring receiver is 50Ω, the load impedance is approximately 50Ω).

Figure 2 Schematic diagram of artificial power network

　　4 Impedance of the ground conductor

　　Copper wire is usually used as the ground wire. Select ground wires whose materials are all copper, but with different lengths and cross-sections for the test, and their specifications are shown in Table 1.

Ground wire number	length	Cross-sectional area/cm ²	cross-sectional shape
1	5cm	7.2	rectangle
2	5cm	0.8	round
3	20cm	0.8	round
4	100cm	0.8	round

Table 1 Specifications of ground wire

　　The impedance curves of the grounding wires listed in Table 1 at different frequency points as a function of frequency are shown in Figure 3.

Figure 3. Grounding wire impedance/frequency variation diagram of different specifications

　　It can be seen from Figure 3 that:

　　(1) Wires with the same frequency point, same material, and the same cross-sectional area have different lengths and different radio frequency impedances. The longer the length, the greater the impedance.

　　(2) The radio frequency performance of rectangular cross-section wires is better than that of round cross-section wires, which is why flat wires are used for grounding at high frequencies.

　　5 Influence of different grounding methods on test results

　　In the conducted disturbance test of the power port, it is found that when the tested device adopts different grounding methods, the test results have certain differences, and sometimes this difference will even affect the judgment of the final test results. The following is a comparison and analysis of two different grounding methods: one is to connect the ground of the EUT and the ground of the artificial power network (AMN) to two different positions of the ground plane, that is, to ground respectively; the other is to connect the The ground of the EUT and the ground of the artificial power network (AMN) are connected to the same position of the ground copper plate, that is, centralized grounding. The grounding impedance is analyzed below.

　　The simplified schematic diagram of the artificial power network AMN is shown in Figure 4, where Us represents the noise source emitted by the EUT; Zs represents the internal impedance of the EUT; Z represents the measured impedance of the AMN.

Figure 4 Simplified diagram of artificial power network measurement

　　In the actual test, due to the high frequency of measurement, there is a certain impedance in the cables, etc. Here, considering the impedance of the grounding wire and the grounding plane, the first multi-point grounding method is adopted, that is, the ground of the EUT and the artificial power network (AMN ) are respectively connected to two different positions of the ground plane nearby, and the above schematic diagram is simulated as shown in Figure 5. In Figure 5, Z1 is the impedance of the grounding wire of the EUT; Z2 is the impedance of the grounding wire of the AMN; Z3 is the impedance of the ground plane between the grounding end of the AMN and the grounding end of the EUT.

Figure 5 Simplified diagram of artificial power network noise measurement when multi-point grounding

　　Let Zs'=Zs+Z1, Z'=Z+Z2, the calculated voltage measured by the artificial power supply network is:

　　UAMN1= Us*( Z'+Z3)/ ( Zs'+Z'+Z3)

　　The second single-point grounding method is adopted, that is, the ground of the EUT and the ground of the AMN are connected to the same position of the ground copper plate, and the above schematic diagram is simulated as shown in Figure 6. In Figure 6, Z1 is the impedance of the grounding wire of the EUT; Z2 is the impedance of the grounding wire of the AMN.

Figure 6 Simplified diagram of artificial power network noise measurement when a single point is grounded

　　The voltage measured on the artificial mains network is:

　　UAMN2= Us* Z’/ ( Zs’+Z’)

　　Compare the size of UAMN1 and UAMN2, then:

　　UAMN1/ UAMN2 = ( Z'+Z3) ( Zs'+Z')/ ( Zs'+Z'+Z3) Z'

　　=(Z'2+Zs'Z3+ Zs' Z'+ Z' Z3)/ Zs' Z'+ Z'2+ Z' Z3

　　=1+( Zs'Z3/ Zs' Z'+ Z'2+ Z' Z3)

　　That is: UAMN1/ UAMN2>1

　　It can be seen from the above results that when multiple points are grounded, the noise voltage measured on the artificial power network is greater than that measured when the single point is grounded.

　　6 Influence of the location of the grounding point on the test results

　　It can be seen from the above that in the power port conducted disturbance test, the EUT can choose single-point grounding or multi-point grounding for the grounding method of the artificial power network (AMN). At the same time, the grounding method of the EUT has an important impact on the test results, and even affects the judgment of the results. Next, the influence of different grounding positions of the EUT on the test results is analyzed. According to the standard layout, as shown in Figure 7, take the projection distance of AMN and EUT as the total length, and take 5 points (A, B, C, D, E) at equal distances respectively. Among them, AB=BC=CD=DE=20cm, connect the 5 points A, B, C, D, and E to the grounding point of the EUT respectively, and test the disturbance voltage results of these 5 positions as shown in Table 2.

Figure 7 Layout of conducted disturbance voltage test at different grounding point positions

frequency （MHz）	Quasi-Peak (QP, dBuv)
frequency （MHz）	position A	position B	position C	position D	position E
0.15	52.2	52.5	52.5	52.9	52.9
0.5	64.3	64.6	64.6	64.9	65.1
1	47.1	47.0	47.2	47.2	47.3
5	55.8	55.7	55.9	56.0	56.1
10	46.5	46.5	46.7	46.8	46.8
30	52.6	52.4	52.8	52.9	53.5

Table 2 Conducted disturbance voltage test results at different grounding points

　　From the test results in Table 2, we can see that when the grounding point is selected at point A, the measured disturbance voltage is small, and when the ground point is selected at point E, the measured disturbance voltage is relatively large. From point A to point E, the disturbance voltage generally tends to increase. The reason is: EUT, ground wire, reference ground plane, AMN and power wire form a loop. According to the theory of electromagnetic induction, when the magnetic flux in the closed loop changes, an induced current will be generated, and the magnitude of the current is proportional to the area of the loop. When the EUT is working normally, it will generate radiation disturbance through the shell and cables. These disturbances will induce current in the above loop, thereby increasing the disturbance voltage. The test principle is shown in Figure 8.

Figure 8. Schematic diagram of the influence of the position of the grounding point on the conducted disturbance voltage test

　　When the ground point is selected at point A, the loop area formed by EUT, ground wire, reference ground plane, AMN and power line is the smallest, the induced current is the smallest, and the disturbance voltage measured at this time is the smallest. When the ground point is selected at point E, the loop area formed by EUT, ground wire, reference ground plane, AMN and power line is the largest, and the induced current generated in the loop is the largest, so the measured disturbance voltage is also large. This shows that the selection of the location of the grounding point has an important influence on the test results of the conducted disturbance voltage. The closer the ground point is to the AMN, the smaller the area of the loop formed by the ground wire and the power wire, the smaller the generated disturbance induced current, and the smaller the measured disturbance voltage.

　　7 Conclusion

　　Through the analysis of the principle of the power port conduction disturbance test, when conducting the power port conduction disturbance test, it should be ensured that there is a good lap connection between the artificial power network and the ground reference plane, and the shortest possible flat copper strip should be used to connect to the ground reference plane superior. At the same time, the ground of the equipment under test and the ground of the artificial power network should be connected to the same point of the ground plane, so as to minimize the measurement difference caused by the ground impedance. In various international and domestic standards, regarding the conduction disturbance test, only the test arrangement and configuration of the device under test are described, and the grounding method of the device under test is not specified. The laboratory strives for consistent test results, which can be repeated on-site. The operation of the system under test should be able to represent typical usage conditions. This situation (including the configuration and wiring of the instrument) may be different from the field use. The laboratory should be aware of this difference and try to simulate the field configuration of the product as much as possible, so that the test The results can reflect the real usage, so as to effectively control and reduce the measurement risk of the laboratory.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

www.300.cn nanjing SEO

Business License

全部

全部
产品管理
新闻资讯
介绍内容
企业网点
常见问题
企业视频
企业图册