Introduction to CDNE test method_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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Introduction to CDNE test method

Release time:

2022-09-16 00:00

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　　Compared with the previous CISPR 15:2015 (GB/T 17743-2017), the current standard CISPR 15:2018 (GB/T 17743-2021) has been revised and supplemented in terms of test items, test methods, and test limits. The CDNE method replaces the previous CDN method, and this article focuses on the CDNE method.

　　1. Terms and Definitions

　　1.1 CDNE-X measuring emission coupling/decoupling network

　　Coupling/decoupling network for measuring emissions in the frequency range of 30MHz~300MHz; for unshielded AC power, DC or control ports with two wires, the suffix X is M2, for unshielded AC power with three wires, For DC or control ports, the suffix X is M3, and for shielded cables with x internal conductors, the suffix X is Sx.

　　1.2 RGP reference ground plane (reference ground plane)

　　The potential is used as a common reference potential and has a flat conductive surface with defined parasitic capacitance to the EUT and surrounding objects.

　　2. Equipment

　　2.1 Overview

　　CDNE couples the asymmetric (common mode) disturbance voltage generated by the EUT to the measurement receiver through the cable, and at the same time decouples the disturbance from the test environment on the wire. The symmetrical (differential mode) interference generated by the EUT is loaded on the 100Ω symmetrical impedance of the EUT power port (ie CDNE-M2 or CDNE-M3), or loaded on the symmetrical load (CDNE-Sx) of the AE port. CDNE cannot be used to measure power lines carrying useful symmetrical (differential mode) signals.

　　RGP serves as the reference for the measured asymmetric (common mode) disturbance voltage.

　　2.2 Scope of application

　　CDNE is used to measure the asymmetrical disturbance voltage of small-sized EUT with one or two connecting cables, and the measurement frequency range is 30MHz~300MHz. CDNE can also be placed between EUT and AE to decouple the asymmetric disturbance generated by AE and stabilize the impedance.

　　2.3 Disturbance voltage measurement

　　CDNE is used to measure the asymmetric disturbance voltage Vdis generated by the EUT on the connecting cable. This measurement considers the reading Vmeas of the measuring receiver and the voltage division factor FCDNE. The voltage division factor FCDNE(dB) also includes the meas of at least 6dB attenuation of the internal attenuator used to reduce the uncertainty introduced by the mismatch between CDNE and the measurement receiver. The measurement results are calculated with the following formula:

　　Vdis=Vmeas+FCDNE(dBuv)

　　2.4 Interruption impedance of CDNE

　　The internal impedance of the EUT at the connection is generally unknown, so the measurement results and their reproducibility are highly dependent on the termination impedance of the CDNE. The asymmetrical (common mode) terminal impedance is selected as 150Ω. Compared with the specified test method, the measurement uncertainty introduced by this terminal impedance is the smallest.

　　The CDNE-M2 and CDNE-M3 provide a symmetrical impedance of 100Ω to the power port of the EUT, which represents the symmetrical impedance of most AC power networks.

　　2.5 Decoupling attenuation of CDNE

　　Both the unknown impedance of the AE and the interference generated by the AE will affect the measurement results. This effect can be reduced by using asymmetric attenuation between the EUT port and the AE port. This asymmetrical attenuation is usually achieved with common mode chokes.

　　3. Technical requirements of CDNE-X

　　3.1 Mechanical and electrical parameters

　　The CDNE is contained in a metal housing, the center of the EUT port of the CDNE is located 30mm (+10mm) above the RGP, this height takes into account the typical impedance of the connecting cable of 150Ω.

　　Figures 1 to 3 are the structural diagrams of CDNE, and Figure 4 is the electrical parameters of CDNE-X.

　　Figure 1 CDN-M2 with minimum internal attenuation meas of 6dB

　　Figure 2 CDN-M3 with minimum internal attenuation meas of 6dB

　　Figure 3 CDN-Sx for shielded cables with x internal conductors and internal attenuation meas minimum 6dB

　　Figure 4 Electrical parameters of CDNE-X

　　The level of the asymmetrical voltage is generally greater than the level of the unintentional symmetrical voltage. Therefore, an LCL with a minimum value of 20dB is sufficient to prevent the symmetrical voltage from affecting the measurement results. In contrast, for measurements on EUTs communicating in differential mode using unshielded cables, CDNE-specific LCL values (minimum and maximum) need to be specified.

　　The attenuation meas at the receiver port is used to reduce the uncertainty introduced by the mismatch between the CDNE and the measurement receiver. This attenuator is preferably integrated in the CDNE. If an external attenuator is used, it should be directly connected to the receiver port of CDNE, and the attenuator should be kept during calibration and measurement. The minimum value of attenuation meas should be 6dB.

　　4. Limit value

　　The limits of the CDNE method specified in GB/T 17743-2021 are shown in Table 1.

Frequency range ( MHz )	Quasi-peak limit ( d B u V )
3 0~100	6 4~54
1 00~200	5 4
2 00~300	5 4~51

　　Table 1 Limits of the CDNE method specified in GB/T 17743-2021

　　5. Precautions when using CDNE for test arrangement and measurement

　　5.1 Overview

　　When the radiation is mainly generated by connecting cables, the CDNE method can be used for disturbance measurement.

　　According to CISPR16-2-1, this measurement method is not applicable to the EUT under the following conditions:

　　a) The maximum size of the EUT housing is greater than 1/4 of the wavelength corresponding to the highest frequency measured, unless otherwise specified by the Product Technical Committee;

　　b) The rated voltage exceeds 600V;

　　c) More than 2 cables.

　　However, GB/T 17743-2021 clearly stipulates that when using the CDNE method, the EUT must meet the following requirements:

　　a) The maximum size of the EUT is 3m╳1m╳1m (length╳width╳height);

　　b) The clock frequency of the EUT is less than or equal to 30MHz.

　　5.2 Test arrangement

　　The test arrangement is located on the RGP, and for the safety of personnel and equipment, the RGP is connected to the protection ground. The distance between the EUT and any other metal body shall not be less than 0.8m. When the distance is short, it should not be less than 0.4m. At this time, the uncertainty should be increased by 0.2dB. The conductive floor of the shielded room can be used as RGP.

　　CDNE is connected to RGP through a metal casing. RF grounding can be improved by applying additional stress on the CDNE case. For safety reasons, it needs to be reliably connected to the protective ground by screws or similar means. CDNE should be placed at least 200mm from the edge of the ground plane.

　　The EUT is located at 100mm±2mm on the RGP and is supported by a non-conductive object with a relative permittivity εr as small as possible (eg εr<1.05, foam polystyrene). The edge of the RGP shall extend at least 200mm beyond the border of the EUT.

　　The distance between CDNE and EUT is 200mm±20mm. The CDNE is placed on one side of the EUT cable under test, so that the length of the cable is the shortest, and the cable cannot be folded or bundled.

　　The connecting cable of the EUT goes down vertically, about 30mm away from the RGP, and connects horizontally to the EUT port of the CDNE (see Figure 5).

　　The AE power port of the CDNE is connected to the auxiliary equipment, namely CDNE-M2, CDNE-M3 are connected to the power supply, and the CDNE-SX is connected to the control unit. The receiver port of the CDNE is connected to the input port of the measurement receiver.

　　The EUT test arrangement with two connecting cables is shown in Figure 6. The actual arrangement depends on which side of the EUT the cable is connected to. Figure 6 illustrates an arrangement where a cable is connected to adjacent sides of the EUT.

　　When two cables are connected to the same side of the EUT, the two CDNEs shall be placed on the side immediately adjacent to the EUT (see Figure 7). The distance between two CDNEs is 20mm±10mm. For connecting non-test cables, the CDNE receiver port is terminated with a 50Ω load.

　　Figure 5 Test arrangement of EUT with single cable

　　Figure 6 Test arrangement with two cables connected to adjacent sides of the EUT

　　Figure 7 Test arrangement with two cables connected to the same side of the EUT

　　5.2 Precautions during measurement

　　a) select the operating conditions of the EUT as described by the manufacturer;

　　b) Select and monitor the necessary warm-up time;

　　c) In order to confirm the correctness of the test results, the environmental disturbance should be checked to ensure that the ratio of the disturbance level to the environmental disturbance is greater than 20dB;

　　d) The detector and measurement time of the measuring receiver are determined by the pre-test and final test. For pretesting, at least a peak detector should be used. In the final measurement, use the detector specified in the product standard to measure the asymmetrical disturbance voltage Vdis;

　　e) For the measurement of the disturbance voltage Vdis, the CDNE voltage division coefficient FCDNE should be added to the measured reading value Vmeas (dBμV), namely: Vdis=Vmeas+FCDNE(dBμV);

　　f) For the EUT with two connecting cables, each cable shall be measured separately, and the maximum reading value among the two cables shall be the measurement result Vdis.

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