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Design techniques and tools to help achieve compliance

Electromagnetic compatibility (EMC) is theproperty of an electronic device to coexistpeacefully with its electronic neighbors. This

means that it must not radiate excessively into itsenvironment and, if placed into a noisyelectromagnetic environment, it must not be toosensitive (susceptible). Most electronic productsconsist of printed circuit boards (PCBs) withintegrated circuits (ICs), cables and interconnect, andmetal or plastic infrastructure. The ICs are the sourcesof the electromagnetic interference (EMI), and thecables or metal framing act as radiating antennas. ThePCB can inadvertently contain radiating antennas aswell, but also acts as a conduit, channeling energyfrom the ICs to the antennas (cables).

The job of the PCB designer is then two-fold. First,don’t inadvertently build an antenna into the PCBitself and, second, don’t allow excessive energy to bechanneled onto the external antennas. Anotherapproach would be to pay little attention to the PCBdesign but instead use slower ICs, filter input/output(I/O) lines, enclose the entire product in a metal box,etc. These approaches can work but can delay time tomarket, since they are sometimes done in a reactivefashion, after a problem is discovered. A better plan isto have an EMI mitigation program which is proactiveand prevents EMI from being a problem in the firstplace.

Designing for electromagnetic compatibility is acomplex task best approached by taking a systemsapproach. The important components of an “EMCsystem” are integrated circuits (ICs), printed circuitboards (PCBs), filters, bypass and decouplingcapacitors and antennas. The ICs are usuallyconsidered the source elements which drive the

antennas. Generally, they are electrically small and assuch, do not radiate efficiently. On the other hand,long cables, interconnect, metal chassis, etc. act asantennas and can be electrically large and thus, canradiate efficiently. The PCB connects the source to theantenna and is usually intermediate in electrical sizebut can contain relatively efficient antennas elements.

One example of this is a slot in a return ground plane.Filters can be added by a PCB designer for EMIreduction. For example, filters can be added to slowdown the sources by reducing edge speeds of ICdrivers. They can also be used to filter “noise” fromI/O lines. In both cases they work by reducing highfrequency content of the signal which reduces EMI.Bypass capacitors work by providing return currentpaths closer to the source currents thereby reducingloop area. Less loop area means less radiation. Finally,decoupling capacitors works by reducing the spread ofnoise on the power/ground system. Noisypower/ground systems increase EMI by allowingsignals to propagate to all parts of the PCB.

This article will focus on things that can be done atthe PCB level to improve or preempt electromagneticinterference (EMI) problems. It will be qualitative innature. For a more through discussion, complete withequations, see one of the excellent references available[1] [2].

EMI can always be improved by using slower ICs orshorter cables and interconnect. In fact, EMI can bereduced to zero by putting the whole product in a

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12 CONFORMITY®: NOVEMBER 2003

Stopping ElectromagneticInterference At The PrintedCircuit Board

Dr. Robert G. Kaires

metal box and welding it shut!However, these external factors may becontrolled by others who may not beheld responsible if EMI problems cropup. The discussion here will focus onhow to design a PCB that for a givenset of external conditions, will exhibitthe best EMC performance. This is noguarantee that the product will pass theEMI tests required, only that thechances are improved.

Basic Theory

EMI is specified in terms of the electricfield magnitude in units of dB:V/m.This is defined as 20 • Log(Emax)/10-6),where Emax is the maximum electricfield in volts/meter. The log nature ofthis quantity means that it is relativelyinsensitive to small errors inmeasurement or calculation. This isfortunate since precise and repeatablemeasurements for EMI are hard tocome by. It also means that calculationof EMI from numerical programs orother software need not be exact.

The fundamental source of all EMI isthe time varying currents. It is not anexaggeration to say that, once all thecurrents are known, it is a simple matterto calculate the EM fields. Since thecurrent distribution is a vector field and the EM fields are also vectors, themathematics can be involved. A fact

that may not be obvious at first is thatsmall currents can radiate more thanlarge currents. If a relatively largesignal current on the trace of a PCB hasa return current in a ground plane veryclose beneath the trace in the oppositedirection, the EM field at some distanceaway can be very small. On the otherhand, a relatively small noise current ona cable without a canceling currentnearby can cause a larger EM field andhence, be an EMI problem.

One of the key concepts necessary tothe understanding of EMI from PCBs(and EMI from complete systems) isthat of differential and common modecurrents and subsequent radiation. Thiscan be illustrated by way of simpleexamples.

Figures 1 and 2 show a transmissionline and a dipole antenna, respectively.The currents on the transmission lineare differential in nature and thecurrents on the dipole are commonmode. One aspect of the differentialcurrents is that the source and returncurrents are close to each other andtherefore, the electric (or magnetic)field at a distant point (the so-called“far field”) is relatively small. On theother hand, the currents on the dipoledo not have a cancellation effect1 andtherefore, the fields are larger. This is


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Figure 1: Differential mode current pair in strip transmission line structure. The current isequal and opposite at all points along the line.

1. If the frequency of the excitation is high enough such that the electrical length of the dipole is greater thanone half wavelength, there will be some current cancellation effect.

despite the fact that the currents on thedipole are also smaller, in general, thanthe currents on the transmission line.

Another important aspect of these twoexamples are that the currents (andtherefore the EM fields) for thetransmission line can be predicted usinga 2D static field solver and transmissionline theory. The currents for the dipole,on the other hand, are much moredifficult to obtain and require a fullwave 3D field solver. More on thatlater.

Another very important aspect of EMIis that of antenna efficiency. Antennasgenerally become more efficient withsize. Radiation from loop antennas likethe transmission line of Figure 1 isproportional to loop area and radiationfrom dipole antennas is proportional tolength2. Since the largest structure in asystem is usually an external cable, thesmall common mode currents on thesecables are almost always are a problem.

The goal of the PCB designer is tobuild differential structures like those inFigure 1 and to avoid (inadvertently)building dipole structures like thatshown in Figure 2. This is easier saidthan done. Generally speaking,differential mode currents are designedby intention and common mode

currents are unintentional. On a PCB,one does not intentionally put a voltagesource between two arms of a dipole.The common mode currents areinduced on a dipole structure by nearbyfields generated by differential modecurrents. Intentional, differential modecurrents are converted intounintentional common mode currents.

Figure 3 shows a common way ofconverting a differential mode signalinto a common mode signal [4]. Adipole is created by attaching theground conductor of an external cableto the ground plane of the PCB. Themicrostrip transmission line generates afield which links the dipole and causesa common mode current to flow.

There are other, less obvious ways tocause common mode currents to flow.References [3] and [5] illustrate thatdifferential transmission line structurescan be made to generate common modecurrents by supplying any asymmetriesin the driving voltages or having anygeometric asymmetries in thetransmission lines or terminations.These types of asymmetries abound onreal three dimensional PCBs. Tracesmeander around on circuit boards andcome close to other traces, the edge ofthe board, vias, etc. Two well knownproblem areas are traces that come too

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Figure 2: Common mode currents in dipole antenna structure. This structure can be obtainedfrom that shown in Figure 1 by simply removing the resistor and “unfolding” the transmissionline. The characteristic of a dipole is that the two “arms” have a voltage source between them.

2. This is true so long as the wavelength of the excitation is large compared with the size of the antenna.

close to the edge of a board and traceswhich cross splits in the ground plane.These types of well-known problemscan be addressed with EMC softwarewhich checks the circuit board topologyagainst design rules. More on this later.

Software Tools

Software tools available for EMCanalysis run the gamut fromsophisticated 3D field solvers on theone hand, to expert systems on theother. The 3D field solvers numericallysolve Maxwell’s equations using suchtechniques as finite difference, methodof moments, etc. They can requiresignificant computer resources andengineering expertise. Learning curves

can be steep and the complexity of theproblems they can solve are quitelimited.

Expert systems are quite the opposite.They use approximations, formulas,design rules, and heuristics to take astab at analyzing the whole EMCsystem. They don’t require extensivecomputer resources or engineeringexpertise. Interestingly, the design rules,which can be a part of the expertsystem, are very often developed byexperts using 3D field solvers. Thereare other types of EMC software toolswhich could be classified as betweenthese two extremes. Some of these willbe mentioned later

To demonstrate the use of a numericalEM field solver and to illustrate themagnitude of the common moderadiation versus the differential moderadiation mentioned above, let’s returnour attention to Figures 1 and 2, theloop and dipole antenna. One can use aprogram called MININEC to solve forthe fields due to these structures. This isa “method of moments” program withlimited capability3.

(I used a version called ExpertMININEC Classic which is availablefor free on the internet. For a historicalreview of this program and forinformation on how to obtain it seereference [6].)

Programs such as this are ideal forsolving relatively simple structures toget some basic EMC intuition. They arelimited somewhat in capability and assuch sophisticated design rulegeneration are probably beyond theirscope. For a more comprehensive list offree and commercial EM modelingsoftware, see the University ofMissouri-Rolla (UMR) website atwww.emclab.umr.edu/numer.html.

Figure 4 shows the radiation obtainedfrom the MININEC program for thedifferential line illustrated in Figure 1.

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Figure 3: Dipole-like structure created by adding external cable. The dipole is excited by themicro-strip transmission line.

3. MININEC can handle round wire conductors with length much greater than diameter. The wires are connected together in three dimensions with junctions and lumpedelements. It cannot handle dielectric materials or finite size metal sheets.

Since MININEC only handles roundwires, they were substituted for the flatstrips shown in Figure 1. A round wirewith a diameter of .05 cm (~20mils) isequivalent to a strip with a width of .1cm (~40 mils) [7]. Using thisequivalence, one can use MININEC toapproximate the radiation from the flatstrips. The source and load impedanceused were 10 and 50 Ohmsrespectively.

Note that the figure is labeled “worstcase”. This means that the electric fieldis sampled at multiple points on asphere at 3 meters. The maximum fieldis taken at each frequency and that iswhat is plotted. This is done because itgives a more realistic value for the fieldthat would be measured in an EMC labwhere a product must be rotated toobtain the worst case fields.

It is important to remember thatantennas have “radiation patterns”which can be highly directional.Measuring or simulating a field at onlyone point in space does not give a goodindication of the potential forinterference. Since MININEC does notdirectly output the “worst case” field,the data was post processed to obtainthis format.

Figure 5 shows the radiation for thesame structure open circuited. We note

that there is more radiation at the higherfrequencies. This illustrates the fact thattransmission lines which are terminatedhave better EMC performance.

Finally, Figure 6 shows the radiationfrom the dipole structure shown inFigure 2. The dimensions used for thedipole correspond to the open-circuitedtransmission line being “unfolded”. Wenote that much more radiation isobtained by unfolding the transmissionline to look like a dipole. This is despitethe fact that the current on the structureis smaller by about a factor of two (notshown). This illustrates the point madeabove that common mode currentsgenerally causes more EMI problemsfor reasons stated above.

As mentioned earlier, expert systemsoftware for EMC is also available.(Two commercial tools which are basedon algorithms from UMR [8] areQUIET Expert from Mentor Graphicsand EMC Engineer from Zuken.) Thesetools can address EMI mechanismssuch as those depicted in Figure 7.These tools can estimate the radiationdue to differential currents flowing onprinted circuit board traces, radiationdue to signals coupled onto I/O nets,and radiation from common modecurrents flowing on dipole structures.The dipole arms can be cable to cable,cable to board, and cable to heatsink.

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CONFORMITY®: NOVEMBER 2003 17

Figure 4: Radiation due to a 5 cm long wire, .05 cm in diameter, with a return wire .4 cm away.The source strength is 1 volt, source impedance is 10 Ohms and load impedance is 50 Ohms.

They can also estimate the magnitudeof the noise voltage on the power busnets and the radiated field due to anoise source in a shielding enclosure.These structures are much morecomplicated than those which can bemodeled using full wave field solvers.

However, it is important to keep inmind that these are estimates based onclosed-form expressions. Simplifying

conditions are assumed in order toarrive at these closed-form expressions.For instance, for the prediction ofeffects of a shielding enclosure, it isassumed that the shape of the enclosureis rectangular and that any slots in theenclosure are electrically short, etc. Forthe prediction of radiation fromdifferential mode currents, it is assumedthat all dimensions are small comparedto the wavelength of interest, that traces

are narrow relative to their spacing andlength, etc. It is also assumed that fieldsfrom different trace segments add in ascalar fashion which produces a worstcase scenario. The list goes on.

It is unlikely that these tools willpredict the exact radiation one wouldmeasure in a lab. This fact should notdissuade a person from using thesetools. After all, if one had a large staffof EM engineers, using full wave fieldsolvers on a bank of Cray computers, itis also unlikely that one would predictthe exact radiation one would measurein a lab (from a real PCB design).Why? There are too many unknowns.There can be thousands of traces,complicated ICs with millions oftransistors, complicated frequencydependent material properties, couplingover large areas producing very smallcommon mode currents, parallel platewaveguides directing energy away fromvias, etc. There are so many effects thatEM engineers using field solvers arealways making decisions which effectsare important and which can beignored. Mistakes can easily be made.

The expert system software makes noclaim to compete with full wave fieldsolvers. The idea behind this software isto emulate the thinking of a real expert.Not the kind with PhD in hand whouses the full wave field solver but thekind with copper tape in hand who getsdown and dirty in the EMI lab. Theyuse current “sniffers” to find likely EMIsources, spectrum analyzers to measureharmonic content of clock nets andtrace where those harmonics end up.They look for obvious flaws in thedesign such as void in the return planeunder a trace. As well, they use “rulesof thumb,” many of which are wellknown such as the “20-H” rule [9] [10].

The expert system software can alsoestimate cross-talk, and radiated EMIbased on closed form solutions asmentioned earlier. Even though theseformulae are relatively simple andscalarized, most experts could not solvethese equations in their heads and addup the effects of potentially thousandsof traces.

There is another class of EMI software



Figure 5: Radiation due to a 5 cm long wire, .05 cm in diameter, with a return wire .4 cm away.The source strength is 1 volt, source impedance is 10 Ohms and load impedance is infinite(open circuited).

Figure 6: Radiation due to straight wire 10.4 cm long, driven at the center. The source strengthis 1 volt, source impedance is 10 Ohms.

which accurately predicts differentialmode radiation from circuit boardtraces. They first use a SPICE or othertransmission line simulator toaccurately obtain the currents in thetraces. Once these currents are known,it is relatively easy to accurately predictthe radiation [11]. (Two examples ofthis type of software are Omega Plusfrom Quantic and HyperLynxBoardSim from Mentor Graphics.) Thisapproach is much faster than a fullwave analysis and almost as accurate[11]. However, it can be very slow (forthousands of traces) when comparedwith the closed form approximation.

The disadvantage of this approach isthat common mode current andradiation is not addressed and this canbe significant or even dominant [12]. Inspite of this fact, this type of softwarecan be useful for two reasons.

First, it is possible to obtain significantradiation from differential modecurrents, especially for high speedsynchronous signals on microstrip

traces4. Second, as mentioned earlier,differential mode currents can beconverted into common mode whenencountering asymmetries. Reducing

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Figure 7: Some of the potential EMI mechanisms that an expert system can analyze.

4. Microstrip traces have a cross-section bounded on the bottom by a solid plane and on the top side by dielectric (or air). They are on the outer layer of the board. Striplineline traces are buried between two solid planes on inner layers of the board. Designers sometimes use a stripline configuration for high speed traces for reduced EMI.

differential mode radiation implies thatone has more tightly coupled the differ-ential mode currents. This implies lesscommon mode current conversion andhence less common mode radiation [13].

Conclusion

Printed circuit boards are a costeffective way to route signals betweenICs and other electronic devices.Unfortunately, they also route anddistribute common mode noise whenthe differential currents (intended) areconverted to common mode currents(unintended). The common mode orantenna currents are especially effectiveat producing EMI when they areinduced on long cables or interconnect.This is because the radiation or antennaefficiency increases with the length ofthe conductor. These antenna currentscan also get onto full or partial metalenclosures causing even more EMI.While EMI is certainly a system issue,much progress can be made into itsmitigation by paying close attention to

PCB design and layout since this is thedistribution channel for the noisecurrents.

Software tools are available to help thesituation. Full wave field solvers areuseful at several levels. Modelingsimple structures such as thetransmission line and dipole mentionedin this article are useful for gainingintuition about EMI. More complicatedstructures can be modeled with moresophisticated (and expensive) software.These structures are still veryelementary but can be used to fabricatedesign rules.

Unfortunately the more advanced fieldsolvers are difficult to master andrequire some knowledge inelectromagnetics and numericalanalysis. No tool can model the entireEMC system and accurately predict theEMI that one would measure in a lab.While a full wave field solver may beable to do this in principle, it cannot dothis in practice.

Another approach is to use an EMC“expert system” software approach.This software attempts to model theentire system but the results for radiatedEMI are estimates and all relevantradiation “mechanisms” may not berepresented. This software is still in itsinfancy but shows great promise for thefuture for a number of reasons. It iseasy to use, does not require an expert,is expandable by the user (via designrules), and is constantly evolving viaresearch at the University of Missouri-Rolla and other organizations. �

About The Author

Robert G. Kaires is a software engineerat Mentor Graphics Corporation inWilsonville, Oregon where he works onEMC software applications. He can bereached by e-mail [email protected].

References

1 P.A. Chatterton, M.A. Houlden, EMCElectromagnetic Theory to Practical

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Design, Wiley, West Sussex, 19922. C.R. Paul, Introduction to

Electromagnetic Compatibility,Wiley, N.Y. 1992

3. R.G. Kaires, “The Correlationbetween Common Mode Currentsand Radiated Emissions,” 2000 IEEEInt. Sym. Electromag. Compat., Vol.1, pp. 141-146, Aug. 2000

4. D.M. Hockanson, et al.,“Investigation of Fundamental EMISource Mechanisms DrivingCommon-Mode Radiation fromPrinted Circuit Boards with AttachedCables,” IEEE Trans. Electromag.Compat., Vol. 38, No. 4, Nov. 1996

5. T. Watanabe, et al., “Estimation ofCommon-mode EMI Caused by aSignal Line in the Vicinity of aGround Edge on a PCB,” 2002 IEEEInt. Sym. Electromag. Compat., Vol.1, pp. 19-23, Aug 2002.

6. J.W. Rockway, J.C. Logan, R.G.Olsen, “EMC Applications forExpert MININEC,” IEEE EMCSociety Newsletter, Spring 2003.

7. C. Butler, “The Equivalent Radius ofa Narrow Conducting Strip,” IEEETrans. Antennas and Propagation,Vol 30, No.4, pp. 755-758 Jul 1982

8. N. Kashyap, T. Hubing, J. Drewniak,T. Van Doren, “An Expert System forPredicting Radiated EMI fromPCBs,” IEEE 1997 Int. Sym.Electromag. Compat., pp. 444-449,Aug 1997.

9. H.W. Shim, T.H. Hubing, “20-HRule Modeling and Measurements,”IEEE 2001 Int. Sym. Electromag.Compat., Vol 2, pp. 939-942, Aug2001.

10. Er-Ping Li, Wei-Liang Yuan, et al.,“Analysis on the Effectiveness of the20-H Rule using NumericalSimulation Technique,” IEEE 2002Int. Sym. Electromag. Compat., pp.328-333, Aug 2002.

11. R.G. Kaires, “Radiated Emissionsfrom Printed Circuit Traces includingthe Effect of Vias, as a Function ofSource, Termination and BoardCharacteristics,” IEEE Int. Sym.

Electromag. Compat., Vol 2, pp. 872-877, Aug. 1998.

12. C.R. Paul, “A Comparison of theContributions of Common-mode andDifferential-mode Currents inRadiated Emissions,” IEEE Trans.Electromag. Compat., Vol 31, No. 2,pp. 189-193, May 1989.

13. H. Sasaki, T. Harada, T. Kuriyama,“The Relationship betweenCommon-mode Radiation from theGround Plane and Differential-modeRadiation from Signal Traces on theGround Plane,” IEEE Int. Sym.Electromag. Compat., Vol. 1, pp 195-199, Aug 2002

Acknowledgements

The author would like to acknowledgethe help given to him by Lisa Talebi onthe graphics supplied in this article.



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