resonances

Many years ago, the author experimented on a metal enclosure of one of his company’s main products. The experiment involved placing an electric field probe inside the empty metal enclosure (no electronics inside) and applying 10 V/m using the IEC 61000-4-3 radiated RF immunity test system.

Most EMI issues are caused by a resonance that is excited somewhere in the system. It may be a resonance of a cable acting as an antenna or a heatsink energized by the power electronics switches bolted to it, becoming a good radiator. In this article, we look at the indicators that signal the presence of structural resonances and provide techniques for fixing the EMI issues. Practical case studies are presented to demonstrate the techniques.

In this article, the author will identify two major types of single-conductor structure resonances and describe some case studies for both. He will then describe how design projects can quickly and easily be ‘de-risked’ from the possibilities of suffering either type of resonance by the use of low-cost field and circuit simulators and/or low-cost bench testing on physical ‘mock-ups’.