AN-643| Application Note

EMI/RFI Board Design

National Semiconductor Application Note 643 Joe Cocovich April 2001

INTRODUCTION The control and minimization of Electro-Magnetic Interference (EMI) is a technology that is, out of necessity, growing rapidly. EMI will be defined shortly but, for now, you might be more familiar with the terms Radio Noise, Electrical Noise, or Radio Frequency Interference (RFI). The technology's explorations include a wide frequency spectrum, from dc to 40 GHz. It also deals with susceptibility to EMI as well as the emissions of EMI by equipment or components. Emission corresponds to that potential EMI which comes out of a piece of equipment or component. Susceptibility, on the other hand, is that which couples from the outside to the inside. In HPC designs to date, we have looked at noise situations ranging from 2 MHz to 102 MHz. EMI, in some cases, can affect radio reception, TV reception, accuracy of navigation equipment, etc. In severe cases, EMI might even affect medical equipment, radar equipment, and automotive systems. This Application Note will define ElectroMagnetic Interference and
describe how it relates to the performance of a system. We will look at examples of Inter-system noise and Intra-system noise and present techniques that can be used to ensure ElectroMagnetic Compatibility throughout a system and between systems. We will investigate and study the sources of noise between systems through wire-harness and backplane cables and connectors. Active circuit components can be contributors of noise and be susceptible to it. The fast switching times of CMOS devices fabricated in today's technology can cause incredible noise in a system. This noise typically is made up of crosstalk, power supply spiking, transient noise, and ground bounce. The minimization and suppression of EMI can be obtained by utilizing proper control techniques. Intra-system noise, noise within a single module, sometimes can be controlled with methods such as filtering, shielding, careful selection of components, and following good wiring and grounding procedures. Controlling noise between systems, Inter-system no
ise, uses subtler techniques such as frequency management and time management, etc. Appropriate time and resources should be spent during the design of a system or systems to insure that no problems will be encountered due to effects of EMI. Design guidelines will be presented that can be used to increase ElectroMagnetic Compatibility between systems by reducing the effects of noise between them. Above all, don't forget that the development tools used are also systems and are important to consider in your planning. A brief look will be taken at the environment and tools required for different levels of noise testing. Relative risk-costs between preparing for EMC or excluding EMI concerns from the project will be listed. DESCRIPTION OF NOISE ElectroMagnetic Interference EMI is a form of electrical-noise pollution. Think of the time when an electric drill or some other power tool jammed a nearby radio with buzzing or crackling noises. Sometimes it
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got so bad that it prevented you from listening to the radio while the tool was in use. Or the ignition of an automobile idling outside your house caused interference to your TV picture making lines across the screen or even losing sync altogether making the picture flip. These examples are quite annoying but not catastrophic. More serious, how about a sudden loss in telephone communication caused by electrical interference or noise while you are negotiating an important business deal? Now EMI can be economically damaging. The results of EMI incidences can be even farther reaching than these examples. Aircraft navigation errors resulting from EMI or interruption of air traffic controller service and maybe even computer memory loss due to noise could cause two aircraft to collide resulting in the loss of lives and property. These were just a few examples to help you identify the results of EMI in a familiar context. To help understand an ElectroMagnetic Interference situation, the problem can be divided into
three categories. They are the source, the victim, and the coupling path. Secondary categories involve the coupling path itself. It the source and victim are separated by space with no hard wire connection, then the coupling path is a radiated path and we are dealing with radiated noise. If the source and victim are connected together through wires, cables, or connectors, then the coupling path is a conducted path and we are dealing with conducted noise. Incidentally, both types of noise can exist at the same time. ElectroMagnetic Interference Situation

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ElectroMagnetic Compatibility If you think about the examples given, one can understand that EMI or electrical noise is of national concern. The Government and certain industry bodies have issued specifications with which all electrical, electromechanical, and electronic equipment must comply. These specifications and limitations are an attempt to ensure that proper EMC techniques are followed by manufactures during the design and fabrication of their products. When these techniques are properly applied, the product can then operate and perform with other equipment in a common environment such that no degradation of performance exists due to internally or externally conducted or radiated electromagnetic emissions. This is defined as ElectroMagnetic Compatibility or EMC.

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