| As technology advances,electronic devices,electrical appliances and electronic systems have become more and more compact.The resulting electromagnetic environment has become more and more complex.Shielded cable is widely used due to its good shielding effect.At present,the shielding performance of cables is mainly measured by the transfer impedance and screening attenuation.The in-depth research on the shielding performance of cable and its testing technology has been conducted at home and abroad,and the relevant international organizations have given the testing standards for the related testing technologies.These test methods basically require that the impedance of the test system be matched with the characteristic impedance of the cable.For the test cable that does not match the test system,an impedance matching adapter needs to be tested between the test device and the test cable.But the design of the impedance matching adapter is cumbersome and requires high precision.The cable with different characteristic impedances require different impedance matching adapters,so the testing process takes time and effort.If the shielding performance of the unmatched cable can be directly calculated through testing,not only the rapid test of the cable but also the error that may be caused by the impedance matching adapter can be eliminated.In order to obtain the shielding performance of the unmatched cable directly without impedance matching adapter,this paper starts with the influence of cable matching status on the cable shielding performance test,derives the correction formula for the shielding performance of unmatched cable,and verifies the effectiveness of the formula through experiments.The main research content is as follows:1.The basic structure and main parameters of the braided cable are described,and the theoretical calculation of the transfer impedance of the braided cable is summarized.The characteristics of the related test technique of cable shielding performance were analyzed,and the triaxial method and GTEM cell method were introduced in detail.2.On the basis of theory of the transmission line,the unmatched end-face model of the tested cable was established,and the unmatched end face of the unmatched cable connected to the instrument and connected to the terminal resistor was modeled and analyzed.the influence of the model of the cascaded network on its shielding performance test was analyzed.The impact of the two different match states of single-ended cable mismatch and double-ended mismatch on the test is analyzed,and the related specific numerical analysis of the SYV-75-5-41 coaxial cable is given.3.A multi-conductor transmission line model of a triaxial coaxial test system was established,and the cable transfer impedance calculation formula was obtained based on the solution of the frequency domain multi-conductor transmission line equation.Therefore,the correction factor of the unmatched cable is introduced,and the relationship between the correction factor and the scattering parameter under different matching conditions is analyzed.The mismatch attenuation caused by the mismatched cable is obtained.4.According to the mismatch attenuation,the correction formulas of the transfer impedance and shield attenuation of the test cable were derived,and the transfer impedance and shield attenuation of the unmatched cable were tested by the triaxial method and the GTEM cell method,respectively.Comparing the results from the impedance matching adapter test results and the correction formula,the effectiveness of the correction formula was verified.5.This article provides a simple(mismathched)-short-short triaxial method for testing mismatched cables—flexible triaxial method,in which conductive copper foil or cable braid instead of the metal tube of the rigid test set-up.And compare and analyze the test results to determine the effective test frequency range of flexible triaxial method. |
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