NSA measurement of 9KHz-30MHz in darkroom and product low frequency test are briefly discussed_Century Wisdom (Suzhou) Testing Technology Co., Ltd

English

Home page

Technical column

CASES

NSA measurement of 9KHz-30MHz in darkroom and product low frequency test are briefly discussed

Release time:

2020-11-04 00:00

Source:

　　According to the latest CIS/A/1323/CDV document on September 18, 2020, namely CISPR 16-1-4/AMD2 ED4, after several years of research and verification, the darkroom measurement standard for 9KHz-30MHz low-frequency NSA will be promulgated soon . The purpose of the standard is to regulate the performance requirements of the test site in this frequency range.

　　The tests currently involved in the frequency range of 9KHz-30MHz are mainly related products specified in CISPR11. Although the automobile industry also needs to carry out corresponding tests according to GB/T 18387, there is no clear requirement for the quiet zone in the test of automobiles.

　　The quiet zone defined by the standard is as follows:

　　Judging from the current standard discussion and actual test results, there are situations where ±4dB cannot be met at a distance of 10m. The standard itself also gives a clear solution, that is, recalculate the uncertainty, see Appendix L.

　　According to the requirements of Appendix L, the uncertainty calculation results at this time are as follows:

　　For example, when the following measurement results appear (The assumed site validation result is fictive but typical):

　　The calculated uncertainty at this time is as follows:

　　The difference is as follows:

　　The test distance adopted by CISPR11 and GB/T 18387 is 3m, and the maximum theoretical quiet zone is designed to be 1.5m, so whether it is from the standard itself or the product, the test of the car is obviously not suitable for the concept of 1.5m quiet zone.

　　For the design of a semi-anechoic chamber, the factors to be considered mainly include:

　　1. The size of the site meets the specification requirements defined by the standard

　　Judging from the specifications defined by the standard, it is easy to misunderstand that the previous definition of an anechoic room usually refers to the distance from the tip of the absorbing material to the tip, but this document clearly states that the distance is the shielding wall, so the size of the shielding body of the anechoic room is not less than 9.0*6.0 (L*W) can meet the requirements of the standard definition, and the usual height of the three-meter darkroom can meet the requirements, and the ten-meter darkroom does not have similar concerns at all.

　　2. Site material requirements

　　Electromagnetic waves have a certain skin effect. In order to avoid magnetic field leakage, the minimum material thickness requirements are defined in the standard. The standard is as follows:

　　Under normal circumstances, it is impossible for the anechoic chamber to use copper/aluminum as the material. The shielding body of the conventional semi-anechoic chamber is made of galvanized steel plate, and the ground may be made of galvanized steel plate/stainless steel plate and other materials. Therefore, doubts about whether the use of stainless steel plates on the ground meet the standard requirements are actually misunderstandings of the essential structure of the darkroom. Usually the design shielding body of an anechoic chamber can be regarded as a complete and continuous galvanized steel plate closed shell, and the stainless steel plate is only based on a ground inside the shielding shell, even if it penetrates the reflective ground due to the skin effect (the thickness of the ground cannot be greater than 2 times the skin depth (9.0mm), the galvanized steel plate under the elevated floor can also ensure that the magnetic field does not leak, and the current standard is only 1 times.

　　3. Influence of resonant frequency

　　For any rectangular shielding shell, the resonant frequency exists objectively, and the theoretical calculation formula is as follows:

　　here:

　　L=length of half anechoic chamber

　　W=width of half anechoic chamber

　　H=half anechoic chamber height

　　Among them, any two of l, m, and n cannot be 0 at the same time.

　　According to theoretical calculations, only when the length, width, and height are not greater than 7m, there is no resonance frequency below 30MHz, and the lowest resonance frequency is 30.30MHz.

　　In fact, during the previous NSA test above 30MHz, Mr. Wen Yinghong/Zhang Linchang wrote a special article on the influence of the resonance frequency of the darkroom on the measurement results of the normalized attenuation of the site. The download link is as follows:

　　http://www.jinyueya.com/magazine/18954534.htm

　　It is mentioned in the article that when the position of the transmitting antenna and receiving antenna is changed and it is near the resonant frequency point, the NSA near 35MHz may be as high as 7.5dB, which is much higher than the ±4dB required by the standard.

　　The size of a conventional three-meter anechoic chamber is about 9.0*6.0*6.0m, so its resonance frequency is usually higher than 30MHz. With the increase of the size of the anechoic chamber, such as a ten-meter anechoic chamber, there must be resonance within 30MHz frequency, which is theoretically unavoidable.

　　In the standard, there are very clear instructions on the resonance within 30MHz. The original text is as follows:

　　For example, if the largest dimension of a rectangular shielded chamber is less than 7 m, then no resonance will occur below 30 MHz。

　　Large chambers designed for a test distance of 10 m, will show resonance between 10 MHz and 30 MHz. If the return loss of the absorbing material including ferrites is too low, sharp resonances can be observed during site validation.

　　大家通常都认为铁氧体对于低频NSA有一定的影响，甚至业内盛传如果半电波暗室不采用铁氧体工艺进行设计，低频NSA必然存在问题，很显然这是误解了电磁波的本质。

　　在9KHz-30MHz的频率范围内，标准要求的发射天线和接收天线都是环形天线，其测量的本质是磁场而非电场，而目前设计的半电波暗室无论是否采用铁氧体主要看重的还是吸波材料对于电场的衰减，一个典型的铁氧体对于电场的衰减特性如下：

　　为了覆盖更高的频率范围，通常情况下会采用铁氧体+吸波材料的复合尖劈，如果吸波材料的长度<1/4波长，则低频的性能主要取决于铁氧体，而这些性能主要针对电场特性所设计的，众所周知的是磁场是一个封闭的环，即不存在零磁场，而电场则可以衰减为0。在电波暗室中电场能量可以转换为热能而衰减，因此在做辐射抗扰度的时候需要考虑到吸波材料的阻燃特性。因为随着温度的升高，当达到燃烧点的时候暗室就存在着火的可能性，由于此情况导致的暗室自燃现象在国内外都有案例存在，其原理等同于一个巨大的微波炉。

　　It is precisely because the magnetic field is a closed loop, so the shielding of the magnetic field is mainly to use high magnetic permeability materials to form a sealed shell, "trapping" the magnetic field in it to prevent leakage. It can also be clearly seen from the thickness of the previous dielectric material that its thickness is much smaller than the stainless steel plate because the magnetic permeability of copper is much higher than that of the stainless steel plate. Since the real anechoic chamber is constructed in blocks and there is a need to Therefore, the theoretical "complete sealing" cannot be achieved, so the better the low-frequency shielding performance, the better the shielding effect of the dark room on the magnetic field. Similarly, the larger the size of the darkroom, the better its low-frequency characteristics. When the size of the darkroom is infinitely close to that of the open field, the measurement results will inevitably approach the results of the open field.

　　Germany Frankonia has done a lot of research on the standard and also conducted actual tests. The results are as follows:

　　A 10-meter anechoic chamber with a size of 25580*17180*9000mm, a design quiet zone of 6m and no ferrite but only 2400mm absorbing material, the measurement result of the 3m distance is qualified, and the measurement results are as follows:

　　Axis test

　　X direction

　　Y direction

　　Z direction

　　A 10-meter anechoic chamber with a size of 21680*13730*8550mm, a design quiet zone of 3m and no ferrite but only 2400mm absorbing material, the measurement result of the 3m distance is qualified, and the measurement results are as follows:

　　off-axis test

　　X direction

　　Y direction

　　Z direction

　　A 10-meter anechoic chamber with a size of 22800*15980*9000mm, a design quiet zone of 5m and no ferrite but only 2400mm absorbing material, the measurement result of the 3m distance is qualified, and the measurement results are as follows:

　　Axis test

　　X direction

　　Y direction

　　Z direction

　　A 10-meter anechoic chamber with a size of 23030*16540*9000mm, a design quiet zone of 6m and no ferrite but only 2400mm absorbing material, the measurement result of the 3m distance is qualified, and the measurement results are as follows:

　　off-axis test

　　X direction

　　Y direction

　　Z direction

　　However, a designed quiet zone with a size of 14480*10430*6450mm is 3m, and a non-ten-meter chamber using ferrite is used. The measurement result in the Z direction at a distance of 3m is unqualified, and the measurement results are as follows:

　　Z direction curve

　　Exceeding the standard data in the Z direction

　　Therefore, no matter from the point of view of the material itself or the resonance frequency, whether the ferrite process is used or not, there will be no essential impact on the NSA measurement of 9KHz-30MHz, and its fundamental impact only depends on the size of the darkroom / the size of the turntable area Choice of design/test area and nothing else.

　　Of course, when performing metrology, the calibration of the antenna is a very critical factor. In the previous research document "CISPR/A/AHG1_(Jun-Gyu Yang, SW Lee, N.Kim, JH Kim, HS Keum, TH Jang)", it has been very clear that the influence of the antenna on the measurement results is objective. , the calibration of the loop antenna in the later stage is one of the matters that the metrology institution needs to pay attention to.

　　At present, the conventional anechoic chambers are mainly three-meter semi-anechoic chambers and ten-meter semi-anechoic chambers, and the test distances are mainly three meters and ten meters. The test distance based on 9KHz-30MHz is currently 3 meters.

　　For the semi-anechoic chamber of the ten-meter method, the resonant frequency below 30MHz exists objectively, regardless of whether ferrite is used. This means that theoretically all ten-meter anechoic chambers cannot meet the ±4dB requirement defined by the standard. This is also the conclusion from the actual test. Even if ferrite anechoic chambers are used, NSA below 30MHz still cannot Passed case.

　　It is precisely because the ten-meter darkroom objectively has a resonant frequency below 30MHz, and the theoretical quiet zone is only 1.5m at a distance of 3m, so from the perspective of actual testing, the use of the three-meter darkroom is a more reasonable test method. How to test the product is a headache, because the test for the magnetic field is a near-field test, and the theoretical distribution of electromagnetic waves is as follows:

　　That is, the magnetic field attenuates sharply as the distance changes. This can be seen very clearly when using a near-field probe. Both the near-field probe and the loop antenna of the magnetic field measure the magnetic field in essence.

　　Therefore, when the volume of the product is small, for example, the diameter is less than 1.5m, the antenna at a distance of 3m may cover the effective lobe angle of the antenna at one time, and as the volume of the product increases, the product itself is no longer in the quiet zone. Is it necessary to use multiple locations for step-by-step testing?

　　To sum up, the NSA measurement in the range of 9KHz-30MHz is already a trend, and the ten-meter anechoic chamber cannot meet the requirement of ±4dB theoretically, that is, only the three-meter anechoic chamber can fully meet it theoretically. The requirement of ±4dB and the quiet zone can be greater than 1.5m, which is suitable for product testing. The 10-meter anechoic chamber is not the best choice for 9KHz-30MHz testing in terms of measurement results and return on investment.

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