emi design techniques

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APPLICATIONNOTE

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February 1996

EMI Design Techniques forMicrocontrollers inAutomotive Applications

CHRIS BANYAITechnical Marketing EngineerIntel Automotive Operations

DARYL GERKEEMC Consulting EngineerKimmel Gerke Associates Ltd

Order Number 272673-001


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Information in this document is provided in connection with Intel products Intel assumes no liability whatsoev-er including infringement of any patent or copyright for sale and use of Intel products except as provided inIntels Terms and Conditions of Sale for such products

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Contact your local Intel sales office or your distributor to obtain the latest specifications before placing yourproduct order

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Intel CorporationP O Box 7641Mt Prospect IL 60056-7641

or call 1-800-879-4683

COPYRIGHT INTEL CORPORATION 1996


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CONTENTS PAGEAUTOMOTIVE EMI PROBLEMS 1

EMI DESIGN STRATEGIES 8

EMI CIRCUIT BOARD GUIDELINES 10

INTEL SPONSORED TEST PROJECT 18

CONTENTS PAGESUMMARY 21

REFERENCES 21

APPENDIX A A-1


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Electronics content of automobiles and other vehicleshas grown rapidly in recent years Embedded micro-controllers are used in a wide range of vehicle applica-tions for control convenience and comfort Examplesrange from sophisticated engine and braking controls toautomated radios and individual passenger temperaturecontrols

As the electronics content of vehicles have increased sohave the electromagnetic interference (EMI) problemsThese range from annoyances (jamming an on-boardAM of FM radio) to upset or damage (blowing out anengine control module due to power transients ) Theproblems are expected to get worse as system clockspeeds and logic edge rates increase due to increasedEMI emissions and decreased EMI immunity

This application note describes the automotive EMI en-vironments and then discusses how to identify and pre-vent many common EMI problems at the design stageAlthough a range of solutions will be addressed em-phasis is on printed circuit board design methods

This application note also describes some recent Intelsponsored research efforts that investigate EMI to on-board FM radio receivers Several different design ap-proaches were tested using both two layer and multi-layer circuit boards The test program was based on anABS (anti-lock braking system) control module thatuses the Intel 80C196KR microcontroller The resultsand recommended low noise design concepts howev-er apply to any microcontroller design used in vehicu-lar applications

AUTOMOTIVE EMI PROBLEMS

Vehicle electronics are affected by several factors in-cluding harsh environments high reliability and ex-treme cost sensitivity Fortunately these problems can

be overcome through good EMI design techniques

The automotive environment contains several threatsincluding power transients radio frequency interfer-ence (both to and from nearby or onboard radio trans-mitters and receivers) electrostatic discharge and pow-er line electric and magnetic fields Since vehicles cango almost anywhere the worst case situations must beassumed

Vehicular electronics must be designed for extremelyhigh reliability Even a single failure over millions of vehicles may not be tolerated Furthermore any systemthat affects vehicle safety must be fail-safe Systemsmust also be easy to install test and repair And of course all of this must be done at the lowest possiblecost

Here some comments on common EMI threats that arefaced by the designers of vehicle electronics They aredivided into two broad classes susceptibility (also re-ferred to as immunity) and emissions In the first casethe automotive electronics are the victim of EMI andin the second case the automotive electronics are the source of EMI

Automotive EMI Susceptibility

Since the automotive environment is so severe manyautomotive electronics designers are already well versedin dealing with the following EMI problems Neverthe-less well review them here since understanding theproblems is the first step toward solving them

Power Transients Vehicle electrical systems are a richsource of power transients Seven of the most severehave been characterized and have become a suite of standard EMI test pulses as described in SAE J1113Electromagnetic Susceptibility Procedures for VehicleComponents These transients include pulses that sim-

ulate both normal and abnormal conditions includinginductive load switching ignition interruption or turn-off voltage sag during engine starting and the alterna-tor load dump transient The last is particularlyharsh and can destroy unprotected electronic devicesFor more information on these transients see SAEJ1113

All of these transients can damage or upset electronicsystems Digital circuits are particularly susceptible totransients which can result in false triggering orflipped bits in memory As electronic devices becomefaster and smaller they become even more vulnerableto these transient spikes

Radio Frequency Immunity Since vehicles are often aplatform for land mobile radio transmitters the on-

board electronics systems may be exposed to very highradio frequency (RF) electromagnetic field levels Thevehicles can also be exposed to high levels from exter-nal threats like high powered radio stations or airportradar systems

The electric field levels from these threats can easilyreach 50100 volts meter In order to protect againstthese threats test levels of up to 200 volts meter arespecified for automotive applications Since typical fail-ure levels for unprotected electronic systems are in the110 volt meter range substantial RF protection mustbe provided for electronic systems operating in the au-tomotive environment

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Although both digital and analog circuits are vulnera-ble to the RF threat low level analog circuits such assensors are the most vulnerable A common failuremode is RF rectification which occurs when an ana-log circuit is driven non-linear by a large signal inducedby large RF fields Voltage regulators can also be af-fected if the RF energy gets into the feedback loop of the regulator Due to their higher operating marginsdigital circuits are not as vulnerable to this threat buteven they can be affected at high RF levels when usingtwo layer boards

Its easy to predict electric field levels if you know thetransmitter power and distance using the following for-mula

E e 5 50AP d

where E is the electric field level in Volts meterP is the transmitter output power in wattsA is the gain of the antenna (the product of AP iseffective radiated power)d is the distance from the antenna in meters

This formula assumes an isotropic or uniform pointsource and assumes no intervening shielding betweenthe transmitter and the vulnerable electronics Never-theless its quite accurate particularly at the citizensband land mobile and television frequencies of 25 MHz and higher Most radio frequency interferencesusceptibility problems occur at these higher frequen-cies where the cables and even circuit traces can act asefficient antennas

Table 1 Electric Field Levels vs Distance andPower (Free Space Isotropic Source)

1 10 100 100 000Watt Watts Watts Watts

1 meter 5 5 V m 17 3 V m 55 V m 1730 V m

10 meters 0 55 V m 1 73 V m 5 5 V m 173 V m

100 meters 55 mV m 0 17 V m 0 55 V m 17 3 V m

1 kilometer 5 5 mV m 17 mV m 55 mV m 1 73 V m

Table 1 gives some examples Note distance is criticaland even a low power hand held (walkie-talkie) radio iscapable of high field levels when it is close to the victimelectronics A 1 watt hand held radio at 1 meter resultsin about 5 volts meter while a 100 watt transmitter(typical of many fixed mobile transmitters) at 1 meterresults in over 50 volts meter Both levels are muchhigher than from a 100 000 watt radio broadcast stationlocated 1000 meters away In vehicles the local on-

board transmitter is usually the biggest RF threat Evenlow powered cellular phones can cause interferenceproblems if their antenna is close to victim electronics

Electrostatic Discharge (ESD) Since almost every ve-hicle has humans on board and since humans are aready source of ESD this is another major EMI threatto vehicular electronics

Most ESD requirements are based on the human bodymodel which characterizes typical voltages currentsand risetimes associated with a human ESD event Al-though ESD discharges are usually specified in pre-discharge voltage levels its actually the current pulsethat causes most of the problems Like water runningdown a river bed after the dam breaks the ESD currentcan upset or destroy any vulnerable electronics in itspath Furthermore the electromagnetic field associatedwith the ESD event can also radiate into nearby elec-tronics systems causing even more upsets This isknown as the indirect effect of ESD

Figure 1 shows the ESD current pulse resulting from ahuman ESD event Note that this ESD current has avery rapid risetime in the 12 nanosecond range withpeak currents of 10 amps or more A 1 nanosecondedge rate has an equivalent bandwidth of over300 MHz so ESD is very much a high frequency issueand requires high frequency design solutions

Because of this high frequency content a direct hit isnot necessary The intense electromagnetic fields caneasily upset a nearby system from an indirect hit of ESD This effect has been observed up to 20 feet (6meters) away and is a reason that indirect testing isnow included in recent international ESD test specifica-tions

As mentioned above the test specifications are usuallygiven in the pre-discharge voltage levels Most auto-motive electronics are designed to withstand at least15 KV a severe level that actually exceeds most humanESD levels

Power Line Fields Since vehicles can go almost any-where and since power lines (and transformers) can bealmost anywhere this threat must also be addressed invehicular designs

Normally this is not a problem for microcontrollerbased systems since at 50 or 60 Hz electromagneticfield coupling is not very efficient Nevertheless verylow analog level circuits can be affected by straymagnetic and electric fields Thus power line field re-quirements are usually imposed on vehicular systems tomake sure no upsets occur due to this threat

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2726731

Figure 1 Typical Waveshape of ESD from Human Body

Automotive EMI Emissions

Almost all automobiles today have sensitive AM FMradio receivers (or perhaps even a land mobile VHFradio) so emissions from digital circuits are one of the biggest EMI problems facing todays designer of vehicular electronics Most of the time the problem is annoy-ing but in the case of emergency vehicles (police fireambulance) jamming a radio receiver could be lifethreatening As a result most vehicle manufacturersnow require suppressing the offending emissions to ex-tremely low levels

Why Commercial EMI Limits Dont Work This prob-lem is similar to the radiated emissions from personal

computers which results in the well known FCC (Unit-ed States) or CISPR (European) EMI limits for com-puters These commercial limits are aimed at protectingnearby television receivers (3 10 meters away) frominterference The military has similar limits with lowerlevels designed to protect their radio communicationsand navigation systems from EMI

Radiated emissions in the vehicular environment ismuch more severe than in the commercial or militaryenvironments First automotive radio receivers aremuch more sensitive than television receivers and sec-ond the offending circuits are much closer to the radioreceivers (commercial or military) typically within1 meter of the antenna

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2726732Comparison of Radiated Emission Specifications Commercial (FCC) Military and Automotive

Figure 2

Several vehicle manufacturers have responded withtheir own radiated emission limits that are well belowthe commercial or military limits Figure 2 comparesthe commercial FCC limits with both military (MIL-STD-461) with the General Motors vehicle modulelimits (GM9100) for radiated emissions (All three setsof limits have been normalized to a 1 meter measure-ment distance ) In the FM broadcast range(88108 MHz) the vehicle limits are about 6 mV m(15 dB mV m) which is about 300 times (50 dB) morestringent than corresponding commercial emission lim-its and about 6 to 20 times (1525 dB) more stringentthan corresponding military limits

Most vehicle problems occur in the FM broadcast band(88-108 MHz) rather than the AM broadcast band(550 kHz1600 kHz) At FM frequencies the cablesact as efficient antennas due to the shorter wavelengths(3 meters for 100 MHz vs 300 meters for 1 MHz) soradiated emissions are much more efficient at thesehigher frequencies On board VHF land mobile receiv-ers operating in the 140 170 MHz (amateur policefire ambulances) can also be similarly affected Onboard UHF land mobile receivers in the 450 MHzrange are not usually affected by todays microcontrol-lers but they will be as the clock speeds and edge ratescontinue to increase

Microcontrollers as Unintended Transmitters The pri-mary sources of emissions from microcontroller basedsystems are the clocks and other highly repetitive sig-nals The harmonics of these signals result in discretenarrowband signals that can often be received well intothe VHF and UHF radio ranges Although digital sys-tems are often classified as unintentional radiatorsthese harmonics are easily radiated by cables wiringand printed circuit board traces

Fourier analysis is a useful tool to understand theseharmonic emissions The Fourier series shows that anynon-sinusoidal periodic waveform is composed of a fun-damental frequency plus harmonic frequencies Fortu-nately as the frequency increases the amplitudes of theharmonics decrease Unfortunately square waves usedin digital systems decrease at the slowest rate (20 dBdecade) and therefore are rich sources of high frequen-cy harmonics In a sense a digital system is like a seriesof small radio transmitters that broadcast simulta-neously on a wide range of frequencies

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Fourier Predictions

2726733Digital circuits are rich in harmonics and act like multiple transmitters

Figure 3

Figure 3 shows the relationship between the time andfrequency domains for a repetitive digital signal such asa clock Although in the time domain it is customary toexpress data in a linear fashion in the frequency do-main it is customary to express data in a logarithmicfashion This Bode-plot approach provides additionalinsights as slopes and break points become readily ap-parent

A widely used EMI frequency is the transition fromb 20 dB decade to b 40 dB decade which occurs at

1 (q t r) This is often referred to as the logic band-width because this is the bandwidth necessary to passthe signal without degrading the edge rate For exam-ple the logic bandwidth of a 10 nanosecond edgerate is 32 MHz while the logic bandwidth of a 1 nsecedge rate is 320 MHz Since many systems today haveedge rates in the 1 3 nsec range these systems arehot with VHF and UHF energy Note that thisbandwidth is only effected by the edge rate andnot the clock rate Decreasing the edge rates will de-crease this bandwidth and thus decrease the high fre-quency content of the digital signal

Figure 4 shows the effect of increasing a clock rate butkeeping the edge rate constant In this case the specificamplitude at a given frequency increases with the clockrate Note that BOTH edge rates and clock rates contrib-ute to the radiated emissions that interfere with radio receivers Furthermore the harmonics not the funda-mental clock frequency cause the problems

Cables and PCB Traces as Unintended Antennas Foremissions to be a problem we need both a transmit-ter and an antenna Weve already seen that repeti-

tive digital signals act like multiple hidden transmittersNow well look at how cables and board traces act asmultiple hidden antennas

Figure 5 shows the effect of coupling the harmonicsdescribed above to a hidden antenna such as a cableAlthough the harmonic frequency content decreaseswith frequency the antenna efficiency increases withfrequency The net result is a pretty good system fortransmitting VHF and UHF energy even if it is unin-tended

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How Fast Risetimes Cause Radiated Interference

2726734High speed sources plus long cables equals emissions

Figure 4

Clock Rate Affects Emissions Too

27267353x clock rate e 10 dB higher emissionsThe faster the clock frequency the higher the emissions

Figure 5

Any conductor will act as an efficient antenna when itsphysical dimensions exceed a fraction of a wavelengthSince cables by their very nature are among the long-est conductors in a system they are usually the mostsignificant antennas At higher frequencies howevereven the traces on the circuit boards become longenough to radiate The secret to success for radiatedemissions is to prevent high frequency energy from everreaching the hidden antennas

As a rule of thumb any wire over 1 20 wavelength isconsidered an antenna for EMI purposes Most com-munications antennas are 1 4 wavelength or longerbut even short antennas are significant radiators Ta-ble 2 shows some typical frequencies and lengthswhich are related by the formula l e 300 f where l isthe wavelength in meters and f is the frequency inMegahertz Also shows are equivalent risetimesbased on the logic bandwidth frequencies of digitalsignals using the formula t r e 1000 f q where tr is

the risetime in nanoseconds and f is the frequency inMHz

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Table 2 Frequency Wavelength Rise Time

FrequencyApprox

Wavelength1 20

t r Wavelength

300 kHz 1 msec 1000 meters 150 meters

1 MHz 300 nsec 300 meters 15 meters3 MHz 100 nsec 100 meters 5 meters

10 MHz 30 nsec 30 meters 1 5 meters

30 MHz 10 nsec 10 meters 50 cm

100 MHz 3 nsec 3 meters 15 cm

300 MHz 1 nsec 1 meter 5 cm

Based on the 1 20 wavelength criteria a cable that is1 5 meters long is a good antenna for any frequencyabove about 10 MHz Even a 15 centimeter meter cableis an effective radiator at 100 MHz which is right inthe middle of the FM broadcast band At 300 MHz thecritical length drops to about 5 centimeters so thateven the board traces themselves become significant ra-diators at frequencies above 300 MHz

Both the board traces and the cables connected to thecircuit board can radiate as shown in Figure 6 Thecable contribution is normally much more severe dueto the large physical size of the cable One can use an-tenna theory to predict the electric field levels radiated

by these unintended antennas For board radiation as-sume a small loop antenna and for cable radiation as-sume a larger dipole antenna The formulas are as fol-lows

E e 265 x 10 b 16 IAf2 r (Differential mode currents

on the board)

E e 4 x 10 b 7 IfL r (Common mode currents on thecable)

Where E is the electric field intensity in Volts meter Iis the current is amps A is the loop area on thecircuit board L is the cable length and r is the distancefrom the antenna

For example 1 milliamp of differential mode current at100 MHz in a 1 square centimeter loop on the circuitboard results in an electric field intensity of about26 mV m at a distance of 1 meter For the same electricfield intensity only 200 picoamps of common modecurrent at 100 MHz would be needed on a 1 meter longcable for the same field intensity

For this example both of these levels are still wellabove typical automotive limits of 6 mV m First itsobvious that if even a fraction of a percent of the PCBcurrents end up on the cables as common mode cur-rents weve got a serious emissions problem Secondits obvious that all high frequency currents (commonmode and differential mode) must be controlled at thecircuit board There should be no doubt that the hid-den antennas can cause serious EMI problems

Physical Dimensions (Antenna Effects)

2726736Loop Radiation (Differential Mode) On the board

Signal LoopsPower Loops

Cable Radiation (Common Mode) Off the board

Figure 6

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EMI DESIGN STRATEGIES

With all these potential EMI problems its apparentEMI protection must be designed in from the begin-ning Unfortunately there is no magic solution forEMI Rather it must be addressed throughout the de-sign

Different Approaches for DifferentThreats

Each of the EMI threats discussed earlier have pre-ferred strategies In many cases one strategy may ad-dress several threats at the same time In all cases mul-tiple levels of protection are needed

Power Transients The design strategies for this threatare to provide primary transient protection on all mod-ule input power lines plus secondary protection such asfiltering at the circuit level The load dump transientis usually the biggest concern Designing noise toler-ant software is also very effective in controlling sus-ceptibility to power line transients

Radio Frequency Immunity The design strategies forthis threat are to keep the unwanted energy from reach-ing vulnerable circuits This requires high frequency fil-tering on cables (both power and I O) which act asantennas plus careful circuit layout and circuit decou-pling Cable and module shielding are also effective butare not popular in vehicular designs due to the costs

Electrostatic Discharge The design strategies for ESDare to limit damage by transient suppression or highfrequency filtering on I O and power lines and to limitupsets by local filtering and decoupling careful circuitlayouts and perhaps even shielding Many of the designtechniques for RF emissions and immunity work equal-ly well for the indirect ESD effects due to the tran-sient electromagnetic fields

Power Line Fields The design strategies are usuallyinstrumentation oriented and include local shieldingand filtering of the most critical circuits The designtechniques necessary for RF emissions and immunityalso minimize this threat This is normally not a seriousthreat

Radiated Emissions The design strategies for thisproblem are to suppress the emissions at the source bycareful circuit layout filtering and grounding or to contain the emissions by shielding For automotive de-

signs the emphasis is usually on suppression and care-ful circuit layout since shielding is costly and difficultfor most high volume automotive products Neverthe-less shielding is seeing increasing use in vehicular ap-plications

EMI at the IC Level

As an integrated circuits vendor we are often asked tomake our circuits more quiet or more rugged inhopes of solving the EMI problems After all everyonealways says to suppress it at the source or harden atthe victim So why not just incorporate all the EMIfixes at the IC level and be done with it

If only life were so simple While many equipment de-signers would like to push all the EMI responsibilityback to the chip vendors this approach is not verypractical due to constraints of chip speed cost per-formance and wide temperature range Lower EMIusually means slower speeds which in turn means low-er performance Yet the trend in automotive ICs is

toward higher performance devices with increasingclock speeds and edge rates

Nevertheless as chip manufacturers we are workinghard to help control EMI at the IC level For exampleat Intel weve designed in the capability to turn off cer-tain high speed control lines when not needed Weveredesigned clock drivers and weve incorporated highfrequency power and grounding schemes right on thesilicon We are working with a Society of AutomotiveEngineers Task Force to develop methods to measureand control EMI at the chip level Our research intothese areas continues But while these efforts help wecant do it all and the real battle against EMI must stillbe waged at the circuit and module stage

EMI at the PCB LevelThis is where the most effective automotive EMI re-sults can be achieved today Its also where the designerhas the most control Many solutions at this level areinexpensive or even free particularly when dealing withboard layout and routing Later in this applicationnote well provide some specific PCB design guidesbut here is some general EMI advice

First plan for EMI right from the start Identify themost critical circuits and decide what to do aboutthem Consider multi-layer boards and allow for plentyof filtering and decoupling in the initial designs Youcan always remove components later if you dont needthem

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Second stay involved with the design Most good EMIdesigns are the result of the design engineer and thePCB layout specialist working closely as a team Asclock speeds and edge rates increase this becomes evenmore important

Third test early and often Unfortunately EMI is notan exact science Better to identity potential problemsearly in the design when the alternates are inexpensiveand plentiful Late in the design cycle EMI solutionsmay be expensive and painful

EMI is a necessary part of any vehicular electronic de-sign and building in EMI suppression and hardness atthe PCB level is very sensible engineering Not only willyou end up with an EMI-proof design but youlllikely have a more reliable design as well

EMI at the Cable and InterconnectLevel

It is not usually cost effective to attack automotive EMIat this level Cost and weight constraints generally pre-clude using shielded cables or expensive filtered con-nectors Even external clamp on ferrites commonlyused with personal computers are not practical Youcan however provide cost effective filtering and tran-sient protection at the I O interface on the circuitboard

Fiber optics may change this in the future Fiber opticscan reduce and even eliminate many cable related EMIproblems but this approach is still too expensive formost automotive applications Even when fiber be-comes practical however the power wiring will stillneed EMI protection

EMI at the Module Level

Like cable shielding module shielding is often consid-ered too expensive for automotive applications Never-theless this approach should not be overlooked It maybe less expensive to shield and filter a module than toadd a lot of components on the circuit board This isparticularly true if the module must be enclosed forenvironmental protection

There are two simple secrets to success with EMI mod-ule shielding First seal all seams and second filter allpenetrations For most EMI problems the material isnot critical and even thin conductive coatings providevery high levels of protection For example nickel painton plastic typically provides 60 dB (a factor of 1000 ) or

more of protection and vacuum plating or electrolessdeposition on plastic typically provides 80 dB (factor of 10 000) of protection A steel or aluminum box canprovide well over 120 dB (factor of 1 000 0000) of highfrequency shielding These levels are easily attainable if the leaks (seams and penetrations) are sealed

Good shielding need not be expensive Consider the tel-evision-tuner with its interlocking metal box and fil-tered input and output lines This approach has beensuccessfully used for almost fifty years to provide costeffective EMI control in a highly cost sensitive indus-try There is an increasing trend toward similar shield-ed modules in vehicles

EMI at the Software Level

Although normally not effective against emissions (al-though there have been cases where it did make a dif-ference) software can be very effective for EMI suscep-tibility Like fault tolerance you should build noisetolerance into your software

There are two simple objectives catch the errors be-fore they upset the system and then gracefully recoverIt doesnt take much to provide this protection andoften times just a few lines of code can work wonders

To detect memory errors add checksums to blocksof data in memory to tell when a memory state haschanged To detect program errors add tokens tomodules of code Save the token on entering a moduleand then check it on leaving the module If they are notthe same an error has occurred since the module wasentered illegally To detect I O errors do type andrange checking on the data

A watchdog can prevent becoming lost in an endlesssoftware loop This is often a separate device although

many microcontrollers incorporate an internal watch-dog If the watchdog is not reinitiated in a predeter-mined time it resets the entire system bringing it backto a known state

Many of these techniques are already used in vehicularapplications with very good results Often times theyare incorporated for safety reasons but they are alsoeffective tools in the battle against EMI For more de-tails on software techniques see the Bibliography

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EMI CIRCUIT BOARD GUIDELINES

Having examined problems and strategies lets look atsome specific solutions Well focus on circuit boardissues as this is where equipment designers have themost control Well share some details on how to attackpotential EMI problems right at the root of almost allEMI problems at the circuits and their interconnect-ing traces

Guideline 1 Identify Critical Circuits

The first step is to identify the most critical circuits onthe circuit board Experience shows that most EMIproblems can be traced to a few key circuits The goodnews is by identifying these circuits early many EMIproblems can be prevented with very little effort

Emissions The most critical circuits for EMI emissions are the highly repetitive circuits such as clocksaddress enables and high speed data busses Even sig-nals with low repetition rates such as address bit 0 cancause problems with sensitive automotive radio receiv-ers Consider adding a ferrite bead or small resistor(1033 ohms) in series with any clock or other highspeed output right at the driving pin This will helpdamp any ringing and also helps provide an impedancematch

Both the signal traces and the power traces are poten-tial sources In the latter case remember a chip that isswitching is consuming current in pulses which canradiate just as well as signal current pulses This meansthat any switched device is a potential source of emis-sions - not just the microcontroller Figure 7 shows typ-ical emission paths from critical circuits

Since clock lines are critical position the chips to mini-mize any clock runs As previously mentioned keep theclock traces and crystals away form any connectorsRoute the high speed lines first and keep those linesshort and direct Consider hand routing the criticallines but if you use an autorouter be sure to check tosee where the lines have been routed

Susceptibility The most critical circuits for EMI susceptibility are the reset interrupt and control lines Theentire system can be brought to a halt if one of theselines are corrupted by EMI Even though these circuitsmay have slow (or even nonexistent) repetition ratesthey are still vulnerable to transients and spikes whichcan result in false triggering Use high frequency filter-ing such as small capacitors (0 001 mf typical) and fer-rite beads (or 100 ohm resistors) to protect these linesThese components should be installed right at the inputpins to the microcontroller

Radiation and Coupling from Critical Circuits

2726737

Figure 7

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Be especially careful with the reset circuitry particular-ly when using external devices for watchdogs or detect-ing power loss Any false triggering of these externalcircuits can cause a false reset so these external circuitsmust be protected against EMI as well Once againsmall capacitors and ferrite beads or resistors are veryeffective as filters against spikes and transients

Guideline 2 Plan your board layout

After identifying the most critical circuits the nextconcern is where to place the circuits on the boardAlthough this sounds trivial EMI success hinges onhow well this stage is implemented Different circuitsinteract in subtle and unexpected ways so you mustplan your layout

Group the circuits by speed of operation While thisseems obvious this simple principle is often overlookedwith devastating EMI results Figure 8 shows an exam-ple of a well partitioned board Note that in this exam-ple the high speed digital circuits are separated fromthe lower speed digital and analog circuits Further-more the high speed circuits are physically separatedfrom the I O connectors to minimize parasitic high fre-quency coupling

Use special care with the input-output circuits sincethey connect to the outside world Even though mostautomotive I O circuits operate at relatively low fre-quencies the cables still act as antennas for high fre-quency energy A key concern here is location A com-mon problem is placing a clock or crystal next to anI O port which results in parasitic coupling to the I Owiring A similar problem is routing reset or interruptlines next to I O lines which allow them to pick uptransients from the outside world Keep these compo-nents and traces at least 25 mm (1 inch) away from anyinput output or power connectors and wiring

CLOCK MEDIUM LOW LOWCLOCK BUFFERS SPEED LOGIC FREQUENCYHI-SPEED LOGIC DIGITAL I O

MEMORY AD AD LOWFREQUENCYANALOG I O

RIBBON CABLE DC POWERCONNECTORS INPUT

Figure 8 Board Partitioning

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Guideline 3 Select an appropriateboard type

The type of board has major impact on EMI issue bothemissions and susceptibility For years automotive de-signers have used two layer boards due to cost con-straints That trend is changing and many new higherspeed designs now use multi-layer boards There is nodoubt that multi-layer boards minimize and often elimi-nate EMI problems Yet many simpler designs can bestill implemented with two layer boards if proper de-sign techniques are used

Multi-Layer Boards Preferred Multi-layer boards of-fer several EMI benefits First the power and signalloop areas are minimized both reducing emissionsand increasing immunity at the board Second thepower and ground impedance levels are lowered (oftenby several orders of magnitude) which reduces powerand ground perturbations Third the presence of powerand ground planes greatly minimize crosstalk betweentraces As a result multi-layer boards typically offerten-fold to hundred-fold EMI improvements over twolayer boards

The multi-layer miracle occurs because of the image-plane effect Place a current carrying wire close to ametal surface and most of the high frequency currentreturns directly under the wire (At high frequenciesthis path has the minimal loop area and thus minimalinductance ) A transmission line is formed by the wiresmirror image located over the metal surface Withequal and opposite currents these transmission lines donot radiate well nor do they pick up external energyThis is shown in Figure 9

Note that both the power and ground planes act assignal ground planes at high frequencies Since theyare decoupled through bypass capacitors and their owncapacitance between the planes they are actually at thesame potential at high frequencies For signal purposesboth the power and ground planes are treated in anidentical fashion

The paths under the traces (in both the power andground planes) must be continuous and unbroken If aplane is cut or if someone decides to borrow some of the plane area for trace routing the return currents areforced to divert around the cut or break creating aloop Watch out for this problem in connector cutoutareas as currents may be forced to divert around thecutout This is particularly troublesome as this hotspot can result in increased coupling to or from theattached wiring

Some Two Layer Techniques Due to cost concernstwo layer boards are still widely used in automotivedesigns In spite of their EMI disadvantages some twolayer boards can be designed to approach multi-layerEMI performance For example by routing criticallines with dedicated returns and by routing all powertraces as power return pairs loop areas can be mini-mized just as with a multi-layer board Filling in un-used areas with groundfill helps lower the groundimpedance

2726738

Figure 9 Image Plane Effects

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One technique is to use one layer as a dedicated groundplane You dont need multiple planes to benefit fromthe image-plane effect In fact on two layer boardsallocating one side as a dedicated ground plane produc-es benefits almost as good as a multi-layer board Thisworks best on boards with low routing densities

As an alternate to a dedicated plane ground grids canbe used on two layer boards One method accomplishesthis by running horizontal ground traces on one side of the board and vertical ground traces on the other sideThey are connected together at the crossover pointswith vias In this way both surfaces may be used forrouting and yet a ground grid is provided for the highfrequencies Although not as good as a plane the gridapproach is still much more effective than randomtrace routing

Even with care however its difficult to achieve EMIsuccess with two-layer technology on designs withclocks above about 15 MHz As a result we recom-mend multi-layer boards for new high speed microcon-troller designs

Design Guideline 4 Isolate WithCare

Isolated or split planes have become very popular asa method of maintaining high frequency isolation on a

circuit board This technique has been used for years onboards with mixed analog and digital circuits and westrongly recommend it for those applications In otherapplications Intel recommends using extreme care withthis method It is a useful technique but if you do apoor job of isolating or segmenting the planes you mayend with more problems that if you stayed with solidplanes

Two errors must be avoided First make sure the corre-sponding planes are aligned as shown in Figure 10Failure to do so will allow high frequency energy tocouple at the overlapped areas Second make sure thesignal traces are continuously routed over the appropri-ate return plane If the traces go over and back youcreate loops that couple energy between the isolatedareas

Use care at any signal or power interfaces between theisolated areas If high frequencies must pass betweenthe two areas the power and ground points should be joined at a narrow bridge and the signal tracesshould be routed over this bridge If high frequenciesdo not need to pass between these two areas then theconnecting traces can be isolated with ferrites induc-tors or common mode chokes These are shown in Fig-ure 11

2726739Plane isolation is defeated by overlapping unrelated power and ground planesLayer capacitance couples all surfaces at high frequency

Figure 10 Align the Planes

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Micro-Island This term was coined by the EMI con-sulting firm of Kimmel Gerke Associates to describe anisolation design technique that has proven effective incontrolling EMI in embedded control systems Proper-ly implemented it provides many of the benefits of amulti-layer board even when using two layer boardsThe technique typically yields ten fold reductions inradiated emissions over standard two layer boards Thetechnique also works with multi-layer designs but theyare generally more quiet in the first place

For many automotive electronic systems the embeddedmicrocontroller is the only high speed source of EMIon the board If one can confine that high frequencyenergy to a small area (Micro-Island) on the boardEMI emissions can be minimized This is accomplishedby providing a local ground plane under the high speed circuitry (typically the microcontroller and perhapsRAM or ROM memory devices) and then filtering every trace (signal power and ground) entering or leav-ing the isolated island A small shield could be added tocompletely encapsulate the island for even higher levelsof suppression although this is rarely needed

Heres how to create your own Micro-Island See Fig-ure 12 for details

First define the boundaries of the island to encom-pass all high speed circuitry (microcontroller crys-tal RAM ROM etc ) Fill this area with a groundplane

Second isolate every signal entering or leaving theisland with a T-filter (ferrite-capacitor or resistor-capacitor) The capacitors are connected to theground plane through a short lead

Third isolate every power and ground trace with aseries ferrite bead Decouple the power and groundwith a 0 01 mF capacitor at the capacitor energypoint

Fourth any signal not starting or ending on Micro-Island must be routed around the island Later inthis application note well share some test results of this technique

Note that this example actually shows two islandsone for analog and one for digital If you are not usinglow level analog signals one island is sufficient

27267310

Figure 11 Routing Between Islands

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27267311

1 Dedicated Digital Power Ground Planes2 Dedicated Analog Power Ground Planes3 Digital and Analog Power Decoupling4 I O Decoupled with Ferrite Resistor and Cap5 Grounded Crystal or Resonator

1 Ground Plane Under Micro-Controller2 Separate Analog Ground Plane3 Power Decoupled with Ferrite and Cap4 I O Decoupled with Ferrite Resistor and Cap5 Crystal or Resonator over Digital Ground Plane

Figure 12 Micro-Island

Guideline 5 Decouple the power

Many high frequency radiated emissions are caused bypoor power decoupling While everyone intuitively un-derstands that high speed signals cause EMI some for-get about the power perturbations Whenever a digitalcircuit switches it also consumes current at the switch-ing rate These pulses of power current will radiate justas effectively as pulses of signal current In fact theyoften cause more radiation since the power current lev-els are usually much higher than those on an individualsignal line

High speed CMOS devices are particularly vexingsince they exhibit high peak currents due to the mo-mentary short across the power rails when theCMOS devices switch In fact CMOS peak currents areoften higher than other technologies so emissions mayactually go up when a CMOS device (such as an80C31) is used to replace an HMOS (8031) device eventhough the average power is much lower with CMOSIts the peak current not average power causing EMIemissions The solution is to improve the power decou-pling

Circuit decoupling We recommend local power de-coupling of every integrated circuit on the board Fordevices with multiple power and ground pins each pairof pins should be decoupled High frequency capacitorsin the 0 010 1 mf range should be installed as close aspossible to the device For multi-layer boards run ashort trace from the power pin to the capacitor andthen drop the other lead into the ground plane For twolayer boards fat traces (with a length to width ratioof 5 1 or less) should be used on both the power and

ground sides of the capacitor to minimize inductanceIn both cases keep the leads as short as possible

Additional protection can be provided by inserting aferrite in series with the V CC line to the microcontrol-ler This must be installed on the V CC side of the capac-itor not on the IC side This small LC filter furtherisolates the V CC traces from current demands of theswitched device We strongly recommend this tech-nique for two layer and Micro-Island designs its op-tional for multi-layer designs

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Power entry decoupling We recommend high frequen-cy decoupling at the power entry points In addition tothe standard 110 mf bulk capacitors add a0 010 1mf high frequency capacitor in parallel at thepower entry point Due to internal resonances the bulkcapacitors are useless at frequencies above about 1MHz The high frequency capacitors are there to inter-cept any high frequency energy that tries to sneak outthe power interface For more protection series ferritescan also be added

Be sure to keep the leads short on the decoupling ca-pacitors The self-inductance of wires and traces isabout 8 nh cm (20 nh inch) so even a few millimetersof wire length can defeat the decoupling at high fre-quencies due to the inductance Figure 13 gives severalexamples of how lead inductance defeats the decouplingcapacitor Note that once you are above the resonantfrequency using a larger capacitor provides no addi-tional benefits as the inductive reactance prevails

One more power decoupling hint Add high frequencycapacitors (0 001 mf typical) to the input and outputs of all on-board voltage regulators This will protect thesedevices against high levels of RF energy (which canupset the feedback) and will also help suppress VHFparasitic oscillations from these devices Keep the ca-pacitors close to the devices with very short leads

Guideline 6 Bulletproof theinterfaces

Weve already seen that cables and wiring can act asunintended antennas and that even low levels on rela-tively short lengths can still cause EMI problems Thuswe strongly recommend that you plan for filtering atyour power and signal connections to the module Youmay find that you dont need all of the filtering but youcan always remove components depending on EMI testresults

Power Interfaces We already discussed high frequen-cy protection at the power inputs A few high frequencycapacitors here is very cheap insurance and they areoften needed for automotive designs to meet the ex-tremely low emission requirements For immunity ad-ditional low frequency filtering may also be neededplus transient protection or energy storage for the auto-motive power transient requirements Typically meet-ing the load dump transient satisfies the other tran-sient requirements as well

Signal Interfaces These need some protection tooDont assume that just because an interface is slowthat high frequency energy wont try to enter or leavethe system at that point

27267312Combination capacitance plus internal and external lead lengthLarger capacitor moves resonant frequency down without providing any high frequency improvement

Figure 13 Capacitor Resonance

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As a minimum provide the space and pads for highfrequency shunt capacitors and or transient protectionon every line Better yet provide for a series resistor aswell These can be zero-ohm resistors connected astraces between two pads Cut and replace with actualresistors if necessary

Dont overlook the ground leads in the signal interfaceas these can provide sneak paths for common modecurrents into and out of the system We recommendadding a small ferrite bead in the ground lead to com-plete the filtering of the interface

Here are some additional recommendations on how toground shunt capacitors if filters To contain emissionsconnect the capacitor to the signal ground The objec-tive is to keep these currents on the board To enhanceimmunity connect the capacitor to a chassis ground (if

available) not signal ground The objective here is tokeep the offending currents off the circuit board

If there is no chassis ground then a compromise isnecessary for immunity Connect the capacitor to thesignal ground PLUS add a series element (ferrite orresistor) to limit the EMI current shunted into the sig-nal ground This is mandatory for ESD if not usedthe ESD current will likely bounce the ground withupset or damage as the result

Guidelines Summary

Figure 14 summarizes some EMI-quiet circuit boardsolutions By using these simple techniques many EMIproblems can be minimized or eliminated

Some EMI-Quiet PCB Solutions

27267313

GOOD1 Parallel Power Ground Traces2 Parallel Signal Return Traces3 Power Decoupling at Chip4 Separation from I O5 Separate I O Power6 High Frequency Filter on I O7 High Frequency Capacitor on Power

BETTER1 Multi-Layer Board2 Power Decoupling at Chip3 Separation from I O4 Isolated I O Power Ground Plane5 High Frequency Filter on I O6 High Frequency Capacitor on Power

Figure 14 Summary

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INTEL SPONSORED TEST PROJECT

In 1994 the Intel Automotive Operations in ChandlerArizona commissioned a research project to investigatethe effects of different printed circuit board techniqueson radiated electromagnetic interference emissions Theproject involved laboratory testing of several designvariations of a typical automotive electronics moduleusing the Intel 80C196KR microcontroller The em-phasis was on practical low cost solutions that could beused by Intel automotive customers

Test Methods and Procedures

The primary objective was to test several different cir-cuit board configurations for RF emissions in the 301000 MHz range The test procedures were based onthe module level radiated emissions tests of GM9100These procedures are used by General Motors to quali-fy electronic modules supplied by GM vendors and areaimed at minimizing interference to vehicular radio re-ceivers when the modules are installed in the vehicleThe test levels are very stringent with levels of 15 dBmV m (6 mV m) at 1 meter over most of the frequencyrange of interest These levels are 50 to 100 timestougher to meet than those for personal computers

Six board configurations were tested All were based onan actual ABS module design The representativeboards were populated with an 8XC196JT microcon-troller regulator load dump diode and hex buffersThe fail-safe ASIC and other circuitry was simulatedby placing appropriate capacitor and resistor loads onthe test board The component placement on the testboards was approximately the same as a fully populatedABS module This was done since it imposed realisticPCB routing constraints on the test boards and provid-ed realistic coupling paths on the test boards Actualproduction connectors and cables were also installedduring the tests

The six test configurations were as follows1 Standard Two Layer Layout2 Two Layers with Micro-Island Isolation Poor

Implementation3 Two Layers with Micro-Island Isolation Better

Implementation

4 Standard Four Layer Layout5 Standard Four Layer Layout with Micro-Island Iso-

lation6 Customer Supplied ABS Module Partially Popu-

lated

Note that a key layout technique studies was the Mi-cro-Island technique discussed earlier While this isnot a new concept this method is a practical techniqueto isolate high frequency sources to one portion of thecircuit board The intended effect is to eliminate highfrequency escape paths to the rest of the board and theinterconnecting cables This high frequency isolationwas achieved by placing the control (80C196JT) andfail-safe microcontrollers on an island and then filter-ing all traces bridging the island with ferrites and ca-pacitors

Micro Island Anomalies Note also that two versionsof the Two-Layer Micro-Island configuration weretested Due to layout routing difficulties the PCB de-signer compromised the micro-island technique bycutting up the ground plane to route needed tracesThe PCB designer believed erroneously that taking afew traces from the ground plane area would have min-imal impact As is often the case in real designs therewas no easy way to route all the traces without bor-rowing from the ground plane

Rather than give up on the Two Layer Micro Islandapproach the cuts were bridged with high frequencycapacitors The objective was to create a high frequencygrid to minimize the effects of the cuts for the tracesWhile simple wire jumpers could have been used ca-pacitors were chosen since this technique is often usedto stitch planes of different voltages together at highfrequencies As it turns out these two configurationsyielded some very significant test results

Test Results Conclusions

Figures 1519 show the emissions from the various testmodule configurations The tests showed primarysources of emissions were clock harmonics as expectedAt frequencies below 150 MHz the interconnecting ca-ble was the primary antenna At frequencies above150 MHz both the cable and the board contributed tothe radiation This is also consistent with expectationsgiven the wavelength and dimensions of the cables andboard traces

Here are some conclusions based on the test resultsfrom the different board configurations

The standard two layer board had the poorest per-formance and would likely fail GM9100

The four layer micro-island board was the best andwould likely pass GM9100

The four layer standard board would also likelypass GM9100 However it had higher emissionsabove 150 MHz than the four layer micro islandboard

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The two layer micro island board without the ca-pacitors would likely fail GM9100 but would likelypass with the addition of the auxiliary capacitorsand jumpers

The experiments showed some other significant resultsas follows

The four layer standard board was 10-25 dB morequiet than the two layer standard board

The properly executed two layer micro island boardapproached the results of the four layer micro-is-land board

The poorly executed two layer micro island boardwas almost no better than the standard two layerboard Cutting the plane essentially destroyed themicro-island protection

A local shield positioned over the microcontrolleryielded 6 dB reductions This was believed to be dueto reduced capacitive pickup of energy by the exter-nal cable

Additional near field tests were done on these testboards using strip lines loop probes and theEMSCAN board measurement system While morefrequency components were noted in the near fieldtests it was difficult to correlate these results with thefar field data

In conclusion the multi-layer board performed muchbetter than the same layout on a two layer board Themicro-island approach is a viable solution but it mustbe properly implemented Finally achieving the lowemission levels necessary for automotive applications ispossible but difficult with little room for design errors

27267314

Figure 15 Two Layer Standard

27267315

Figure 16 Four Layer Standard

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27267316

Figure 17 Two Layer Micro-Island(Cuts in Ground Plane)

27267317

Figure 18 Two Layer Micro-Island(Capacitors to Repair Cuts)

27267318

Figure 19 Four Layer Micro-Island

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SUMMARY

Automotive EMI problems are harsh The susceptibili-ty levels are often high and the radiated emission levelsare extremely low The emission levels are particularlygrueling as they require suppression to levels up to1000 times below commercial designs and up to 100times below those for personal computers Unfortunate-ly there are no simple solutions to these problems

We at Intel are committed to helping you solve theseproblems Well continue our research and develop-ment at the chip level doing what we can to control theEMI problems at that level It should be apparenthowever we cant do it all EMI control must also beaddressed at the circuit board and module levels Wellcontinue our efforts at these levels too

For additional help with these problems we invite youto contact your Intel or Distributor Applications Engi-neers Many have received introductory EMI trainingand may be able to help you with basic questions Formore involved problems they can refer you to EMIdesign experts

Acknowledgments The main sources of the informa-tion for this application note are listed in the Referencesection Daryl Gerke PE of Kimmel Gerke Associ-ates Ltd was responsible for supplying much of thisinformation and conducting the Intel sponsored testprogram His firm specializes in EMI design trouble-shooting and training courses Figures 114 are fromthe firms EMI training courses and are used herecourtesy of Kimmel Gerke Associates Ltd Mr Gerkecan be reached in St Paul Minnesota at612-330-3728

REFERENCES1 Gerke D D and Kimmel W D EDNs Designers

Guide to Electromagnetic Compatibility CahnersPublishing Newton MA 1994

2 Williamson T W Designing Microcontroller Sys-

tems For Electrically Noisy Environments Intel Ap-plication Note AP-125 1982

3 Gerke D D and Kimmel W D Focus on Auto-motive EMI Kimmel Gerke Bullets (EMI News-letter) Vol 2 No 4 Kimmel Gerke AssociatesLtd 1991

4 Kimmel W D and Gerke D D Microproces-sors and VHF Radios Mutual Antagonists EMI EXPO 1989 Symposium Record pp C6 9-C6 22August 1989

5 Gerke D D Designing Microcomputer Systemsto Tolerate Noise Society of Automotive EngineersTransactions 870787 April 1987

6 Yarkoni B and Wharton J Designing ReliableSoftware for Automotive Applications Society of Automotive Engineers Transactions 790237 Febru-ary 1979

7 SAE J1113 Electromagnetic Susceptibility Mea- surement Procedures for Vehicle Components Soci-ety of Automotive Engineers August 1987

8 SAE J1752 (Draft) Electromagnetic Compatibility Measurement Procedures for Integrated CircuitsSociety of Automotive Engineers August 1994

9 GM9100P Automotive Component EMC Specifica-tion General Motors

10 Gullett C The Hidden Schematic CommonGround (Intel Automotive Newsletter) Volume 1Number 5 Intel Corporation February 1993

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APPENDIX AAutomotive EMI Test Techniques

For The Design Engineer

EMI testing can be complex and expensive Further-more it takes many years of experience to develop ahigh level of EMI test expertise As a result most de-signers send their products to an EMI test laboratory todemonstrate compliance to the appropriate EMI testrequirements

This testing is often done near the end of a design proj-ect so if problems occur they can be painful and ex-pensive to fix Fortunately there are a number of teststhat can be done during the design phase that can iden-tify potential problems when they are still easy to fix

The tests well discuss here are engineering tests andnot compliance tests As such high degrees of quantita-tive accuracy are not necessary The objective of these engineering tests is to improve the probability for suc-cess of the eventual compliance tests The goals are touncover problems early and to demonstrate design im-provements Here are some comments on EMI teststhat you should consider during the design phase

Power Transients Several test equipment manufactur-ers offer test systems that generate the automotive tran-sients described in SAE J1113 (Electromagnetic Suscep-tibility Procedures for Vehicle Components) Be sure thetest system includes the load dump which is a verysevere transient Most design engineers can run this testin their engineering lab with the appropriate equip-ment

Power Line Electric and Magnetic Field ImmunitySince this is rarely a problem it is probably not worthengineering tests You can do these tests in an engineer-ing lab if you really insist on it using Helmholtz coilsfor the magnetic fields and parallel plates for the elec-tric fields

Electrostatic Discharge Several test equipment manu-facturers offer ESD test systems A good guideline forthis test is IEC 801 2 (Electromagnetic Compatibility

for Industrial Process Control Measurement and Control Equipment Electrostatic Discharge Requirements)This widely used test method is based on the humanmodel for ESD and is easy to do in the engineeringlab Be sure and do both the direct and the indirectESD tests as described in the 1991 version of IEC 801 2

Radio Frequency Immunity Full comprehensive RFimmunity tests can be difficult since they often requireantennas amplifiers and shielded rooms Small mod-ules however can be tested in a TEM cell which is aspecial test fixture that is a piece of expanded transmis-sion line The cell is completely shielded so you dontneed an antenna or shielded room You still need asignal generator and a power amplifier to develop theappropriate test levels A modified version of the TEMcell known as a GTEM cell has proved popular forthese applications

For frequencies below 100 MHz special probes may beused to inject RF energy directly on the cables Thiscan be useful since at frequencies below 100 MHz thecables are the most likely antennas for picking up theRF energy

Crude RF immunity tests can be done by keying smallVHF UHF hand held radios near the equipment undertest At 1 meter a 1 watt hand held radio generates anelectric field of about 5 volts meter At 1 foot thatincreases to about 15 volts meter At less than one footthe levels are not as meaningful since the unit undertest is the near field Keep in mind these radios onlytransmit on select frequencies Nevertheless if a failureoccurs you know you have problems

Radio Frequency Emissions This can also be difficultin an engineering lab since full tests often require ashielded room antennas and sensitive spectrum ana-lyzers or EMI receivers The automotive test thresholdsare several orders of magnitude below commercial lim-its so a shielded room is almost mandatory In somecases a TEM cell or strip line antenna can be usedbut the correlation with final levels can be difficultThese methods are useful however for making relativemeasurements such as assessing design changes

Two useful troubleshooting tools for emissions are cur-rent probes and sniffer probes which are connectedto a spectrum analyzer The former are single turn cur-rent transformers that are clamped over a cable to mea-

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sure the high frequency current on the cable The latterare magnetic loop antennas that show the presence of high frequency magnetic fields (and thus currents) in acircuit or cable While useful you can not correlatethese measurements with actual emission measure-ments since they only measure the current and do noaccount for the antenna effects

A relatively new system for testing circuit boards foremissions is the EMSCAN system The board undertest is scanned for both frequency and location Theresults can be plotted to show RF hot spots similarto thermal hot spots Like the current probes and sniff-er probes however the measurements do not correlatewith final emission levels since antenna effects are notincluded Nevertheless many manufacturers havefound this system useful in designing RF quiet boards

Finally crude emission tests can be done by placing theantenna of an FM or VHF radio receiver near the unitunder test A typical test distance would be 1 meteraway using a representative antenna such as a verticalwhip If you can hear emissions on the radios youare probably above the limits

Summary Engineering level tests will never replace fi-nal EMI compliance tests but they can still prove veryuseful Here are two final notes of advice For immuni-ty testing youll likely need to add light emitting diodesor other devices to indicate failures as external equip-ment may mask the test results For emissions testingpay attention to software and be sure to exercise allperipherals to assure that maximum noise levels aregenerated

INTEL CORPORATION 2200 Mission College Blvd Santa Clara CA 95052 Tel (408) 765-8080INTEL CORPORATION (U K ) Ltd Swindon United Kingdom Tel (0793) 696 000

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