To meet EMI requirements, we only have to put the equipment in an all-metal enclosure. In this situation, the voltage induced in the center conductors as shown is common mode and will be reduced by the common-mode rejection ratio (CMRR) of the receiver.ģ. If the noise currents on the shield are dc or at power frequencies, then an RF capacitive ground connection to the enclosure will shunt RF currents but present a high impedance to the low frequencies and avoid the dreaded audio-frequency ground loop.Īnother approach is to use a differential signal and receiver set. High-power and/or RF currents should be routed on separate and dedicated return conductors that are isolated from chassis. When it comes to high-frequency shielding, the shield must be grounded at both ends.įor the cable shield to be effective when grounded at both ends, one must first avoid using the shield as a return path. It results in electric field shielding with only limited plane wave shielding and very little magnetic field shielding. Audio engineers especially do not like this situation. This difference in ground potentials will create a noise current flow in the cable shield. The most common reason given to do this is to avoid ground loops.Ĭables routed over long distances will cross different ground potentials that may exist. When a shielded cable is connecting two pieces of equipment, engineers were always told that the shield should be connected to chassis or ground at only one end. Cable shields are only connected to ground at one end. This often means that the initial fix will be less effective than expected not that it won’t work.Ģ. Commonly, many sources of emissions exist and when the most predominant one is minimized, then the second one in line will predominate. Often the paths that RF currents take aren’t always obvious, and distributed parasitic components in a circuit may have been ignored. Also, the coupling mechanism in an EMI problem may not have been completely identified, or a number of coupling mechanisms may have been at issue. So, the major reason EMC is seen as “black magic” is the lack of understanding in the fundamental principles of electromagnetics. For example, shielding against magnetic fields using thin conductive materials with a relative permeability of just 1 seems like magic, but has a simple explanation if one knows the frequency of the magnetic field. It’s understandable, then, that this phrase could be applied to some aspects of EMC.Īfter some EMI mitigation work, the result often is contrary to what we expected and sometimes actually counterintuitive. These phrases are used to describe results that we don’t understand or can’t comprehend or even can’t make sense of. We often use the terms “black magic” or “black art” when describing an unexpected or unusual result. ![]() This is probably the most well-known and most common description about EMI/EMC that I’ve heard from engineers and designers in my 40-odd years working in this field. ![]() The intent of this article is to explore some of these myths and provide explanations and clarifications to the reader. Many of these myths were generated simply to make them easy to remember. Myths provide a way to obtain quick answers that are not necessarily accurate, but give us rough estimates. ![]() Most of these myths came about based on attempts to balance reality (accuracy) versus the time and cost spent on successfully solving EMI/EMC issues. Many myths circulate through the field of electromagnetic interference and electromagnetic compatibility (EMI/EMC), some of which are untrue, or only partially true, or based on misunderstanding. ![]() What is a myth? A myth is defined as a popular belief or tradition that has grown up around something an invented story, idea, or concept an imaginary or fictitious thing an unproved or false collective belief. This file type includes high-resolution graphics and schematics when applicable. This article is part of the TechXchange: Delving into EMI, EMC and Noise
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