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Development trend of low frequency NSA metering in darkroom

Release time:

2016-01-25 00:00

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       The 2015 CISPR Annual Conference was held in Stresa, Italy from September 20th to October 1st . The meeting lasted for 2 weeks , and the content related to the CISPR/Athe A branch plenary meeting and 3 special project group meetings ( ad-hoc) , 2 working group meetings , as a P member, the delegation of China A Branch is headed by Cui Qiang, China Institute of Electronic Technology Standardization, from China Institute of Metrology, the Fifth Electronics Institute of the Ministry of Industry and Information Technology, and Shaanxi Haitai Electronics Co., Ltd. Five people from the company and other units participated in various meetings of the CISPR/A branch. One of the topics involves the performance research of NSA in an anechoic chamber below 30MHz, and the results of the topic are as follows:

Site confirmation project group meeting below       30MHz

On the morning of September 22 ,        2015 , Kriz from Austria presided over the meeting as the convener of this project, mainly discussing and dealing with the opinions of the National Committee on the CISPR/A/1101/DC document , namely the CISPR/A/1107/INF document .

       The main technical contents involved are as follows:

       a)  Obstacle-free area and resonance-free area

       Britain and Italy pointed out that the maximum size of the rectangular shielding room recommended in DC is 7m , and it is considered that resonance will not occur below 30MHz when the size is less than 7m , but the measurement distance is allowed to be 10m . This statement is contradictory. After discussion, this recommended size will be removed. Japan pointed out that the effectiveness of sites below 30MHz mainly depends on the size of the test site and the performance of ferrite absorbing materials. Due to the poor performance of carbon absorbing materials below 30MHz , the minimum distance from the center of the test space to the wall of the anechoic chamber should not be calculated from the tip of the absorbing material, but from the shielding wall of the anechoic chamber. The convener also accepted this proposal.

       b) 9kHz ~ 30MHz NSA calculation

       The representative of Japan pointed out that for the frequency below 100kHz , there is a large error in the calculation of NEC , and at the same time, it is proposed that increasing the diameter of the loop antenna can improve the low-frequency performance. Qu Pengfei from China Academy of Information and Communications Technology pointed out that there is an error in the NEC code in DC . After discussion, it was decided that the convener Kriz , Dr. Fujii from Japan , and Qu Pengfei should further study the problem of the difference below 100kHz . Dr. Fujii from Japan introduced the research results of Japan's 9kHz ~ 30MHz site confirmation test, explained the problem of the zero point at 25MHz , and proposed a new "Hz layout " scheme, that is, the feeding point of the transmitting loop antenna and the receiving loop antenna Arranged in a diagonal distribution. After actual measurement, it is better than the original Near (the feeding points of the transmitting loop antenna and the receiving loop antenna are arranged close to each other) and Far (the feeding points of the transmitting loop antenna and the receiving loop antenna are arranged far away from each other). Subsequent simulation calculations of NSA theoretical values ​​will be based on this scheme.

      As an alternative experimental site for the open field, the anechoic chamber currently involves radiated emission tests below 30MHz including:

1. RE101 (25Hz-50KHz) and RE102 (10KHz-30MHz) tests for military products, according to the standard GJB151B2013

2. The three-ring test of lamps and lanterns ( 9KHz-30MHz), according to the standard GB17743

3. The low-frequency magnetic field emission of civilian products is 9KHz-30MHz, according to the standard GB4824

4. Low-frequency emissions of electric vehicles, including electric and magnetic fields 9KHz-30MHz, according to the standard GB 18387

     Among them, RE101 and RE102 of military products use loop antenna (133mm diameter) and rod antenna respectively. The requirements for environmental level and absorbing material in anechoic room are as follows:

(See page 8 of GJB151B-2013) Table 1: Absorption loss at normal incidence

The requirements for the electromagnetic environment level are all 6dB lower than the limit.

       It can be seen from the above that the latest GJB151B-2013 has no requirements for darkroom performance below 80MHz, but compared with GJB151A-1997&GJB152A-1997 16 years ago, the distance between the absorbing material and the EUT has been improved. It’s just a clear requirement (see the specific chapter). This has nothing to do with performance, but only a size issue. From the perspective of the development trend of the US military standard, no similar trend has been seen so far.

      Relatively speaking, the low-frequency radiation emission tests of several other products are classified under CISPR requirements. From the trend point of view, the measurement of anechoic chamber performance below 30MHz is an inevitable trend. As for the change of product standards, it must be based on the measurement requirements of anechoic chamber performance. Further research can only be carried out on this basis, which is bound to be a long-term process.

The current mainstream test of the three-loop of the luminaire is placed in a shielded room. Due to the huge size of the three-loop antenna, higher requirements are placed on the size of the shielded room. CNAS review experts are currently testing the three-loop antenna that is not placed in the shielded room. The conclusion of the low-frequency magnetic field test is that rectification is needed, but the ordinary three-loop antenna with a height of 3m obviously cannot meet the requirements. Therefore, the recommended height of the shielded room with a three-loop antenna test is at least 3.5m (the height of the three-loop antenna itself with its bracket is about 2.6 meters) , the floor and decoration are estimated to be 0.4m, and the distance from the normal recommended distance is 0.5m).

       The low-frequency electric field test in electric vehicles is completely similar to RE102. According to the research on this type of test, it has nothing to do with the performance of the darkroom itself, because the impedance caused by the wavelength problem is unstable, so the latest military standard is also They are mainly based on the layout to ensure the repeatability of the test. The mainstream antenna manufacturer Schwarzbeck whip antenna VAMP9243 in Germany has launched the latest option Opt . MIL 461F Bonding Kit .

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Test layout

      Taking into account similar influencing factors, FORD has also added similar layout requirements in the latest FMC1278 test of RE310 (above 30MHz, FMC1278 page 18):

     In the long run, it is necessary to address the layout requirements of these test cables, because the sources that affect the test results include:

1. The performance of the darkroom ( NSA/VSRW, etc.)

2. Test the performance of the antenna

3. Test the performance of the cable

4. The performance of the receiver

5. Test layout

      From the comprehensive consideration of the above aspects, the performance of the anechoic chamber/test antenna/test cable/receiver has very clear and detailed requirements in the corresponding requirements of CISPR16, and there is no special specification requirement for the test layout at present. Most laboratories have a very clear and understanding of the layout of the EUT, but do not have a special understanding of the layout and requirements of the test link, and even now some laboratories do not have enough test tables with low dielectric coefficients for high-frequency testing. attention.

       A typical radiated emission uncertainty calculation formula :

Em=[E r *A f *C l ]*R x *A d *A h *A p *A i *D v *S i *M m

       Where E r is the measurement receiver reading, A f is the antenna calibration factor, C l * is the cable attenuation calibration, R x is the receiver performance index, A d is the antenna directivity, A h is the antenna factor, A p is the antenna phase Center change, A i is the antenna factor frequency interpolation, D v is the change of the measurement distance, S i is the site imperfection factor, M m is the mismatch between the receiver and the antenna.

       A typical uncertainty calculation result:

       Among them, the uncertainty calculation formula of darkroom performance S i = darkroom performance/√6, when the darkroom performance is ±4.0dB, the uncertainty is 1.633, when the darkroom performance is ±3.0dB, the uncertainty is 1.225, and the darkroom performance is ± When the uncertainty is 2.0dB, the uncertainty is 0.816. It can be seen that when the anechoic performance is improved from ±4.0dB to ±2.0dB, the contribution to the uncertainty is only 0.816dB. In fact, the current mainstream anechoic performance (NSA) can basically ensure Within 3.5dB, the difference between the contribution of a ±3.5dB and a ±3.0dB darkroom to the uncertainty is 0.203dB.

       Therefore, from the calculation of the uncertainty itself, it is not necessary for the anechoic chamber itself to define too high performance. From the perspective of actual uncertainty, the influence of antenna calibration factor, receiver performance, antenna and receiver mismatch factor More importantly, relatively speaking, the performance of the receiver has little probability of changing over time under normal conditions, but the aging of the antenna (and consequent receiver and antenna mismatch) is inevitable over time. Therefore, it is recommended that the laboratory measure the antenna regularly if conditions permit. If there is a relatively large decline in antenna performance, it is recommended to replace the antenna.

       According to the above analysis, the only item that may affect the low-frequency radiation emission test and darkroom related items is the low-frequency magnetic field radiation emission test. Usually, the low-frequency magnetic field emission uses a loop antenna, and the conventional size is a loop antenna within 60cm in diameter, which is also GB4824 And the antenna type defined by GB 18387.

       Referring to this CISPR discussion, there is a big controversy whether the product standard will introduce this standard, because according to the premise of the performance research "the feeding points of the transmitting loop antenna and the receiving loop antenna are arranged in a diagonal distribution", while At present, the turntables of the anechoic chamber (rectangular anechoic chamber) are distributed on the central axis. Even if the NSA of the anechoic chamber below 30MHz can meet the requirements of performance research, the turntable cannot be used in the actual test.

       For the theoretical calculation of NSA, it is not only the absorption characteristics of the absorbing material itself that is considered. Like the NSA above 30MHz, the energy loss of the electromagnetic wave incident on the object can be divided into emission loss, absorption loss and multiple reflection loss. In this way, the total attenuation includes the sum of these three parts, that is, the formula is as follows:

SE T =SE R +SE A +SE M

       Among them, SE R is the reflection loss, SE A is the absorption loss, and SE M is the multiple reflection loss. The respective calculation formulas are as follows:

       From the perspective of this research topic, for a specific darkroom, the absorption loss is obviously a relatively fixed value. In fact, for low frequencies (especially below 10MHz), whether it is ferrite or long wedge The performance is extremely poor (usually the absorption performance of 10MHz is <6dB, the characteristics of the material below 10MHz are still under consideration, and the high-frequency characteristics above 30MHz are usually at least >20dB), obviously it is not a feasible method to increase the absorption loss. Because if the low-frequency characteristics are increased, the high-frequency characteristics will inevitably decrease. In fact, there is no material that can cover the low-frequency to high-frequency perfect characteristics, especially for the built darkroom.

       Therefore, increasing reflection loss including multiple reflection loss is the direction of research, because these two kinds of losses can only be realized by changing the arrangement, and the incidence angle is reduced when the diagonal distribution is used. In fact, even for the same material, different The performance brought by the incident angle is also completely different, refer to the following figure:

       Therefore, this is also the reason why foreign manufacturers recommend not to use the central axis arrangement when measuring NSA, and it is the same for the research of low-frequency NSA.

       From the above analysis, it can be seen that the performance research of low-frequency NSA is a long-term process. At present, only the theoretical method of simulation has been determined. The research on different materials, different sizes of darkrooms and the performance decline curve of these materials over time is all important. It is a long process, and it is also an extremely complicated process, and the actual measurement after the simulation and the formulation of the standard are a long-term process. The core is whether it is necessary to introduce this performance test into the product standard, which is another key issue worthy of discussion.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

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