Electronics shields

The design of electronic circuits processing low-amplitude signals exposed to electromagnetic stress frequently requires the use of shielding. These include shielded cables, connectors, filters and metal housings, as well as specific functions relating to ground contacts and the involvement of additional protection. This enumeration also implies that the effectiveness of a shield must be harmonized with each component of an installation, to meet electromagnetic compatibility standards, as well as compromises linking the criteria of cost, reliability, electrical safety and dependability.

The scientific analysis of the article is therefore based on the example of generic electronic equipment, configured to collect, transmit and amplify analog signals. These functions and components will thus lend themselves more readily to exposure to various stresses, represented by electromagnetic fields, covering a vast range of frequencies, to which will also be added currents derived in the ground.

We shall see that the use of generic equipment leads directly to the concept of a topological boundary, materialized by the sequence of various types of shielding, opposing the penetration of the constraints mentioned above. Since this physical barrier is imperfect, we'll need to characterize it using parameters that can be measured or calculated for residual voltages reaching a circuit or component considered vulnerable.

In the spirit of the preamble, the first chapter describes the generic equipment and its topological boundary. In the second chapter, we will study the penetration of plane waves into flat shielding. This preliminary step will attempt to relate the attenuation of a shield to its internal structure, depending on whether it is made of a highly conductive homogeneous metal, a metal grid, or a composite substitute. The third chapter deals with shielded enclosures, analyzing their behavior when exposed to electric and magnetic fields covering a wide range of frequencies, as well as to currents derived on their metal structure. The fourth chapter deals with shielded cables, paying particular attention to the evaluation of residual voltages generated by transfer impedance. The fifth chapter, devoted to connectors, focuses on their link with the concept of electromagnetic immunity. The sixth chapter presents a number of processes that complement the action of shielding, such as amplitude limiters, ferrite rings, and filtering or absorbing cables.

As a general rule, each chapter will be accompanied by examples to help you understand the orders of magnitude of the physical data involved, from shielding parameters to electromagnetic stresses.

Readers wishing to delve deeper into the analysis of shielding...