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Relationship between anechoic chamber method and CDN Method for radiation disturbance Test of lighting equipment

Release time:

2014-08-29 00:00

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introduction

      The man-made electromagnetic energy in space is increasing rapidly, and the electromagnetic environment is deteriorating day by day. Countries and relevant organizations in the world have attached great importance to the issues related to electromagnetic compatibility. In the field of lighting equipment, with the rapid development of electric lighting and related equipment technology, the operating frequency of lighting equipment is getting higher and higher. How to reduce its electromagnetic interference has become the focus of attention.

      At present, according to the requirements of CISPR 15 and GB 17743-2007 standards, the anechoic chamber method or CDN method can be used to test and determine the EMC radiation items of lighting equipment . For the two test methods, the test results are not comparable, what is the relationship between the two methods, and where is the difference?

      This paper introduces the test site and test equipment configuration of the two test methods, points out the difference between the two test methods, and tries to find out the relationship between the two test methods through model research. 

1. Standards for radiation disturbance testing of lighting equipment

      The main basis for the radiation disturbance test of lighting equipment is:

National / International Standard GB 17743/CISPR 15 : Limits and methods of measurement of radio disturbance characteristics of electrical lighting and similar equipment. 

2. Main test sites, instruments and test methods for radiation disturbance of lighting equipment

      The open test field refers to an elliptical area that is open in the air and surrounded by open space and no reflective objects. The ground is a flat metal grounding surface (the actual size exceeds the horizontal metal grounding plate of the elliptical area). The open test field is mainly used for electromagnetic radiation disturbance testing in the frequency range of 30MHz-1GHz .

      Anechoic chambers are generally divided into semi-anechoic chambers and full anechoic chambers. The full anechoic chamber is equipped with absorbing materials on the walls, ceiling and floor, while the semi-anechoic chamber is only equipped with absorbing materials on the walls and ceiling, the floor is galvanized steel, no absorbing materials are installed, and the semi-anechoic chamber is The ideal measurement environment for measuring electromagnetic compatibility, the anechoic chamber method mentioned below in this article refers to the semi-anechoic chamber. The shielding body is generally assembled from galvanized steel modules, which has a good external shielding effect and can well shield external magnetic fields, plane waves and microwaves.

On the test instrument, the new EMC scanner is combined with the spectrum analyzer to realize the visualization of electromagnetic radiation. At the same time, the instrument performance should meet the requirements of CISPR 16-1 quasi-peak, average or RMS-AV detector. In the frequency range specified by the standard, the test receiver generally needs to cover two different frequency bands, which are 9kHz-30MHz and 30MHz-1GHz respectively . The receiver cooperates with the receiving antenna and is tested in the anechoic chamber, which is the anechoic chamber method.

      CDN coupling decoupling network. The main function is signal coupling or decoupling. It is commonly used in EMC conduction immunity test to supply power to equipment and auxiliary equipment, to isolate power grid noise, and to couple interference signals to equipment at the same time. It is stipulated in CISPR 15 that CDN needs to comply with IEC 61000-4-6 , the impedance parameter |Zce| impedance should be 150Ω , and the tolerance is ±60Ω in the extended 80MHz-300MHz frequency range . The receiver cooperates with the CDN and is tested in a shielded room or in a normal environment, that is, the CDN method.

      Due to the different waves emitted by the test site, the test site has a significant impact on the results in the EMC test. This may cause the same test instrument to obtain different test results in different test sites. At present, the radiation disturbance test methods of lighting equipment are divided into direct test method (anechoic chamber method) and alternative test method ( CDN method). Test sites include shielded room or normal environment ( CDN method), open field or anechoic chamber (anechoic chamber method).

      According to the requirements of CISPR 15 and GB 17743 , the anechoic chamber method should be tested in an open field or anechoic chamber that meets the requirements of the CISPR 22 standard, and should be tested according to the test methods specified in Chapter 10 of CISPR 22 .

      The CDN method can be carried out in a shielded room. According to the independent method of radiation disturbance measurement in CISPR 15 or GB 17743 , when the lighting equipment meets the relevant requirements of Appendix B in CISPR 15 or GB 17743 , it can be considered that the radiation disturbance of the equipment at the frequency of 30MHz-300MHz meets the requirements of the standard Require. 

3. The relationship between the anechoic chamber method and the CDN method

      Because the anechoic chamber method described in CISPR 22 is costly and time-consuming, and in the early stage of product development, designers urgently need a low-cost test method to quickly obtain feedback on product changes. Therefore, the CDN method came into being . The radiation characteristics for lighting equipment are mainly determined by the cables connected in this system. This means that, for products with a single cable, a relationship can be found between measuring the electric field strength according to CISPR 22 and measuring the common mode current through the CDN in a conducted test configuration .

      For anechoic chamber method and CDN method test results to be usefully compared, an equipment under test ( EUT ) that generates sufficient field strength levels over the entire frequency range is required. According to the proposal of Mr. SBWorm of Philips Research Institute at the 13th Zurich International Electromagnetic Compatibility Conference in February 1999 , a crystal oscillator with a basic frequency of 10MHz and a built-in battery-powered analog test device were used. According to the electric field intensity measured according to CISPR 22 when the terminal impedance is 0Ω , 50Ω , 150Ω , 300Ω , 500Ω and open circuit, we can obtain the frequency spectrum of the 10MHz oscillator measured in a semi-anechoic chamber using different common-mode terminal impedance , as shown in Figure 1 : 

Figure 1  Spectrum of a 10MHz oscillator measured in a semi-anechoic chamber using different common-mode termination impedances

      At the same time, the sample under test (EUT) with a built-in power supply 10MHz oscillator is measured by the CDN method , the output line is connected to the S1 type CDN , and 150Ω is generated through the shielded coupling of a single coaxial cable, and the cable lengths are respectively 10cm , 25cm , and 1m ; the common mode terminal voltage spectrogram of the equipment under test ( EUT ) and the cable height of 5cm and 10cm , as shown in Figure 2 below : 

Figure 2  Spectrum diagram of common mode terminal voltage on CDN with different geometric configurations

      It is not difficult to find from Figure 2 that the difference in the results below the frequency of 100MHz is less than 5dB, and  the geometric configuration becomes important at high frequencies. This can explain that the changes in Figure 2 are systematic.

      We normalize the electric field intensity curve in Figure 1 to the curve of L=10cm common-mode voltage response in Figure 2 , and we get the following Figure 3 :  

Figure 3  The relationship between field strength and common mode current with different terminal resistances

      The transfer curve of the conducted-to-radiated test results is obviously dependent on the common-mode termination impedance used in the radiating configuration, and average transfer curves can be obtained for 150Ω and 300Ω .

      In the actual process, of course, no resistance terminal is used for the power line of the equipment under test. Combining with the CDN common-mode impedance characteristics, the relationship between the radiation and conduction test results can be obtained, as shown in Figure 4 below : 

Figure 4  The relationship between field strength and common mode current 

4. The difference between the anechoic chamber method and the CDN method

      Although it is considered in CISPR 15 that when the lighting equipment meets the relevant requirements of Appendix B in CISPR 15 , it can be considered that the radiated disturbance of the equipment in the frequency range of 30MHz – 300MHz meets the requirements in the standard. Some EMC testing institutions are still relatively cautious in using the CDN method, and the electromagnetic compatibility radiation disturbance test within the frequency of 30MHz-300MHz is mainly carried out by the anechoic chamber method.

      At present, there are different test sites, measurement equipment and environments between the anechoic chamber method and the CDN method, and the differences caused by the objective uncertainties must exist objectively. There are also objective differences between the anechoic chamber method and the CDN method due to the difference in test principles . At the same time, compared with the anechoic chamber method, the CDN method adopts the space acceptance method. In the high frequency part, the quality of the cable connection between the EUT and the measurement equipment will directly affect the test results. 

5. Conclusion 

      In CISPR 15 and GB 17743 , the electromagnetic compatibility radiation disturbance test of lighting equipment should be tested and judged according to the corresponding test method. Although there are certain differences between the CDN method and the anechoic chamber method, the CDN method, as an alternative method to the anechoic chamber, still has its advantages that cannot be ignored in the early stage of product design. This method can be considered under the premise of clarifying the product characteristics.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

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