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Influence of grounding performance of artificial power network on conduction disturbance test

Release time:

2013-09-16 00:00

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introduction

In the conducted emission test, the equipment under test (EUT) couples the conducted disturbance voltage of the EUT to the EMI receiver through the artificial mains network (AMN). The test schematic diagram is shown in Figure 1.

Figure 1 Schematic diagram of conducted disturbance voltage test

AMN is a very important auxiliary device in the conduction disturbance emission test. It provides a specified impedance for the EUT terminal for conduction disturbance voltage test within a given radio frequency range (such as 9kHz~30MHz), and couples the electromagnetic disturbance of the EUT to the EMI receiver. machine while isolating the test equipment from unwanted signals on the power supply. The performance of AMN depends on the following three important parameters: 1) The impedance of AMN output terminal (EUT terminal); 2) The isolation between power supply terminal and RF terminal; 3) The voltage division coefficient reflecting the electromagnetic emission coupling of EUT to the test instrument terminal ( or called insertion loss). GB/T6113.102 has strict regulations on the impedance characteristics, isolation and voltage division coefficient of AMN. If the absolute value of the voltage division coefficient (that is, insertion loss) is greater than 0.5dB, the test result should be included in the measurement result of the disturbance voltage and corrected. For the conducted disturbance test in the shielded room, because the shielded room provides a good grounding condition for the normal operation of the AMN, its isolation and voltage division coefficient are not much different from the values given by the measurement. Usually, the isolation is generally Greater than 40dB, meeting the test requirements, the measurement value of the voltage division coefficient can also be directly included in the test results of the EMI receiver and corrected. Some large electromechanical equipment, either because of their large size and heavy weight, cannot be moved into the EMC laboratory for conduction disturbance testing, or because the shielding room cannot provide the working conditions for EUT to work normally, they can only be tested at the working site of the equipment . In most cases, the working site of these devices does not have a good grounding test environment like a shielded room. How will the isolation and voltage division coefficient of AMN change at this time? Can it meet the test requirements? This paper will study the above problems and explore the trend of AMN's isolation and voltage division coefficient changing with grounding resistance under different grounding impedances.

1. Impedance of grounding wire

The grounding wires whose materials are all copper but have different lengths and cross-sections are selected for testing, and their specifications are shown in Table 1.

Table 1 Specifications of different ground wires

The impedance curves of the grounding wires listed in Table 1 at different frequency points as a function of frequency are shown in Figure 2.

Fig. 2 The curve diagram of the impedance of ground wires of different specifications changing with frequency

It can be seen from Figure 2 that,

(1) Conductors with the same frequency, the 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) For the same ground wire, as the frequency increases, the skin depth will decrease, the radio frequency resistance will increase, and the inductive reactance and impedance of the wire will increase.

(3) As the length of the wire increases, the radio frequency resistance increases, and the inductive reactance and impedance increase.

(4) As the wire diameter increases, the radio frequency resistance increases. In the case of the same cross-sectional area of the wire, the equivalent diameter of the rectangular cross-section is smaller than the diameter of the circular cross-section, so in the case of the same cross-sectional area and length, the radio frequency impedance of the rectangular cross-section wire is smaller than that of the circular cross-section wire , that is to say, the radio frequency performance of the rectangular cross-section wire is better than that of the circular cross-section wire, which is why flat wires are used for grounding at high frequencies.

2. AMN isolation _

Isolation refers to the degree of isolation between the power supply end of the AMN and the RF receiving end under the condition that the EUT end of the AMN is connected to a given 50Ω impedance. The isolation test is based on the provisions of Appendix H of CISPR16-1-2:2006 (GB/T6113.102-2008). The first step is to measure the output U1 when the signal source is connected to a 50Ω load. The test schematic diagram of the receiver whose output port of the signal source is connected with an impedance of 50Ω is shown in FIG. 3 .

Figure 3 Test schematic diagram of signal source connected to 50Ω load

The second step is to connect the relevant end of the EUT end of the artificial power network (such as L-PE, N-PE) to a matching impedance of 50Ω. The output power setting of the signal source is consistent with the first step, and the output port of the signal source is connected to the power supply. Between the relevant terminal of the port and the reference ground, use the receiver to measure the output value U2 at the RF port. The test schematic diagram is shown in Figure 4.

Figure 4 Schematic diagram of isolation test

Isolation FD=U1-U2, the unit is dB; U1 is the reference voltage of the power supply terminal, the unit is dBμV; U2 is the output voltage of the receiver port, the unit is dBμV.

In order to understand the impact of different grounding conditions on the performance of the artificial power network, the isolation of the artificial power network is tested in the case of AMN grounding (ground wires of different specifications and impedances shown in Table 1) and non-grounding conditions. The actual layout of the test is shown in Figure 5 shows.

Figure 5 The actual layout of the isolation test

Test the isolation of AMN in the frequency range of 150kHz to 30MHz, and the test results are shown in Figure 6.

Figure 6 is a graph of isolation versus frequency for different grounding conditions

(1) Within the tested frequency range, the isolation is different when the AMN is not grounded and grounded; when the AMN is not grounded, the isolation is the smallest, and the minimum value is about 35dB. When the 1m copper wire is grounded, after 22MHz, The isolation is less than 40dB, which does not meet the requirement of 40dB isolation. That is, when the artificial power network is not grounded, or the grounding wire is long, the AMN isolation will be reduced, and the electromagnetic disturbance at the power supply end may be connected to the RF output end of the receiver, causing uncertainty in the test results and affecting the repetition of the test. precision and precision.

(2) As the frequency increases, because the impedance of the grounding wire increases, its isolation decreases, that is, in the case of high frequency, the isolation performance of AMN decreases, and the test results are more easily affected.

(3) When the AMN is grounded with different grounding wires, due to the different impedances of the grounding wires, the isolation is different. The smaller the impedance, the better the isolation performance. When using 5cm copper tape and 5cm copper wire for grounding, the isolation performance is the best .

(4) For copper strips and copper wires with the same length and different cross-sections, since the impedance of copper strips is smaller than that of copper wires with circular cross-sections, their grounding performance is slightly better than that of copper wires in most frequency ranges.

(5) When AMN is grounded through copper wires with the same cross section and lengths of 5cm, 20cm, and 100cm respectively, the 5cm copper wire has the highest isolation degree, and the isolation degree increases with the increase of the length of the ground wire and the increase of impedance. Reduced, that is, the smaller the grounding impedance, the greater the isolation, and the better the performance of the artificial power network. Therefore, the length of the AMN ground wire should be reduced as much as possible to improve the isolation performance of the AMN.

3. The voltage division coefficient of AMN

The disturbance voltage at the EUT port passes through the L, R, and C components in the artificial power network to the RF test port, which will produce a certain voltage drop. This voltage drop is represented by a voltage division coefficient. The size of the voltage division coefficient directly affects the accuracy of the test results. . The voltage division coefficient can be tested by a network analyzer, or by a combination of a signal generator and a receiver. The test principle is similar, as shown in Figure 7. The following takes the combination of a signal generator and a receiver as an example to illustrate.

Figure 7 Schematic diagram of the test of the partial pressure coefficient

When testing, connect the T-adapter to the EUT end of the AMN, and:

(1) Feed the signal generated by the signal generator into the B terminal of the T-type adapter, and connect the EMI receiver to the A terminal of the T-type adapter to receive the signal, denoted as V1.

(2) The signal fed to the signal generator at the B-end remains unchanged, connect the A-end of the T-type adapter with a 50Ω matcher, connect the EMI receiver to the RF test end of the AMN, and obtain the test value V2.

(3) If the unit of V1 and V2 is dBμV, then the voltage division coefficient Fv=V1-V2.

Use the 5cm copper strip, 5cm copper wire, 20cm copper wire and 1m copper wire listed in Table 1 to ground the artificial power network respectively, and test its voltage division coefficient. The actual test layout is shown in Figure 8, and the test results are shown in Figure 8. 9 and Figure 10.

Figure 8 The actual test layout of the partial pressure coefficient

Figure 9 The voltage division coefficient curves for different grounding wires

Figure 10. The voltage division coefficient curve when the ground wire is 1m

As can be seen from Figure 9,

(1) Ground the AMN with different ground wires to test the voltage division coefficient, and the value of the voltage division coefficient is very different. The grounding performance of 5cm copper strip and 5cm copper wire is similar, and its voltage division coefficient is less than 1dB, which is better than 20cm copper wire and 1m copper wire; the grounding performance of 1m copper wire is poor, and the voltage division coefficient is above 0.5dB . In the actual test of the disturbance voltage, the length of the ground wire is different, and the voltage division coefficient of the artificial power network is different, which will have different effects on the test results. The longer the ground wire, the greater the voltage division coefficient, and the ground wire should be minimized. The length of the cable improves the coupling degree between the EUT end and the RF measurement end.

(2) The voltage division coefficient increases with the increase of the frequency. In comparison, the 5cm copper strip, 5cm copper wire, and 20cm copper wire change slightly with the frequency change, but the 1m ground wire has a significant effect on the AMN voltage division coefficient. The impact increases dramatically with frequency. That is, the longer the ground wire, the higher the frequency, and the greater the ground impedance, the greater its impact on the voltage division coefficient.

(3) The voltage division coefficient curve of the 1m ground wire changing with the frequency is shown in Figure 10. It can be seen that as the frequency increases, the voltage division coefficient gradually increases. At 30MHz, the voltage division coefficient is close to 20dB, that is, when When the artificial power network is grounded with a long grounding wire, due to the increased grounding impedance, the voltage division coefficient is large, and the coupling performance between the EUT end and the RF test end is poor, and because the voltage division coefficient is a function of frequency, it is Correction brings difficulties.

4. Conclusion

When conducting the conducted disturbance test at the EUT power supply terminal, it should be ensured that there is a good lap connection between the artificial power supply network and the ground reference plane. If the grounding is not good, the grounding impedance of the artificial power network will increase, and with the increase of the length of the ground wire and the increase of the frequency, the isolation performance and coupling performance (voltage division coefficient) of the artificial power network will be significantly worse, and may also lead to failure Meet the isolation requirement of 40dB in CIS-PR16-1-2. It brings great difficulties to the correction of the test results, and the accuracy of the test results cannot be guaranteed. For the situation where the test must be carried out at the working site of the EUT, due to the complex electromagnetic field in the environment, it is impossible to provide a grounding reference like the laboratory, and the increase in the grounding impedance will also seriously affect the isolation performance and coupling performance of the AMN, and the test repeatability Neither the precision nor the accuracy of the test results can be well guaranteed. It is recommended to use a voltage probe to test the disturbance voltage at the equipment work site.

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