A study of the shielding characteristics of metallic casing

Abstract

Electromagnetic interference (EMI) can cause electronic equipment to malfunction via conduction, radiation, or induction. There are statutory requirements with which electronic equipment must comply in order that they do not interfere with each other. To achieve these requirements, EM1 control methods such as grounding, filtering, and shielding are commonly used in the design process of electronic equipment. In theory, shielding by metallic casing is an effective way to provide a good suppression of radiated EM1 for electronic circuits on a printed circuit board (PCB). However, there are many parameters that could degrade the shielding performance of the metallic casing. In this thesis, a study of different parameters affecting the shielding characteristics of metallic casing is presented. Equipment casing comes in different sizes, materials and profiles; the shielding performance of the casing may be affected by these factors. The shielding characteristics of different materials and thicknesses of the casing are studied using numerical simulation. Unwanted electro-magnetic (EM) fields may be induced inside the metallic casing due to the apertures for connecting cable and cooling purposes. The effects of different sizes of the aperture on the EM field distribution and on the induced noise level inside the casing are analyzed by using a numerical method; an experimental study is also used to verify the induced noise level by using a common EM1 source. Electrostatic discharge (ESD), which has a fast rise time and high voltage characteristic, is chosen for the experimental study. ESD discharging on different locations of the casing could cause different noise levels to be induced inside the metallic casing. This effect is investigated by computer simulation, and an experimental study is also carried out to study the noise induced inside the casing. This thesis presents a study of the shielding characteristics of metallic casing for electronic equipment. It is concluded from the analysis that the high conductivity materials for the casing could provide a good EM1 shielding while dielectric materials for the casing could not provide any suppression of EMI. The induced noise level inside the casing is affected directly by the size of the aperture. It is also concluded that the noise level induced inside the casing is sensitive to the location of the EM1 source.

Date of Award	15 Jul 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sai Wing Peter LEUNG (Supervisor)