HOW TO CHOOSE CABLE SHIELDING FOR EMI REDUCTION

Abstract

ABSTRACTThe problem of choosing an adequate shield for a cable generally reduces to one of two cases: In the first case, a radiated field threat is specified and a conducted susceptibility level is known for the equipment under consideration. The problem is to determine what shield construction should be used, in order to insure that no malfuctions due to EMI will occur in the equipment. In the second case, a shielding effectiveness level for the cable shield is specified and the task is to meet this specification by proper shield design.Cable shields are evaluated in terms of their surface transfer impedance Ztc. In this paper, we will define surface transfer impedance; show how it is related to shielding effectiveness; and explain why surface transfer impedance, rather than shielding effectiveness, should be used to specify shield construction. We will then introduce the concept of critical transfer impedance, Zt and show how to derive this quantity from known radiated field and conducted susceptibility levels. Finally, we explain how to use the critical transfer impedance for the system being considered in conjunction with a library of surface transfer impedance curves for various cable shield designs, to arrive at a shield construction which will adequately protect the system from the imposed EMI threat.