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Data Acquisition Part 9

Tham khảo tài liệu 'data acquisition part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Data Acquisition Systems for Magnetic Shield Characterization 231 Fig. 1. Magnetic field loop source close to indefinite conducting plate. consideration it can be often the case that the geometry of the shield and the dimensions of the electronic device under test do not make it possible to place the chosen loop probe in the desired position and as a consequence to correctly perform the SE measurement. In some cases this problem is solved by the use of smaller-size magnetic sensors. However the averaging problem still stands and the estimation of the SE could be more properly defined by averaging the field over a spatial region better representative of the victim . As written in the previous text an effective method for shielding problem consists in the application of the Schelkunoff s theory. Here we limit to recall some basic aspects of this approach with reference to the case of a plane wave incident on an indefinite conducting lamination and to the previous case of a loop coupled to an indefinite plate. Due to the impedance discontinuity at the air-metal and metal-air boundaries and to the diffusion equation governing the field inside the shield part of the energy is reflected at the two interfaces and part absorbed by the shield turning into heat energy. It is common notation to express the overall SE as the sum of three separate contributions SE R A M 3 where all the terms are expressed in dB and R represents the reflection losses at the left and right interfaces A is the term of the absorption losses while M is an additional terms due to multiple reflection effects. In the case of Fig. 2 SE can be analytically evaluated and its expression is SE 20log10 no n 2 4non 2Ologio e J 20logi0 n _ n2 I . 1 _ n0 n t Se-i2flt n n 4 where it is possible to recognize the reflection R absorption A and mutual reflection M terms Paul 1992 . n0 and n represent the intrinsic air and shield impedance respectively. The Schelkunoff s theory can be extended to the case of shield .