Interconnection and Circuit Packaging for Electromagnetic Compatibility

8/9/2019 Interconnection and Circuit Packaging for Electromagnetic Compatibility

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470 IEEE TRANSACTIONS N COMPONENTS, YBRIDS, ND MANUFACTURINGTECHNOLOGY, OL. CHMT-5,NO. 4, DECEMBER 982Interconnection and Circuit Packaging for,Electromagnetic Compatibility

JERRY H. BOGAR, MEMBER, IEEE, AND ERIC VANDERHEYDEN, MEMBER, IEEE

Abstract-Electromagnetic compatibility (EMC) has become a de-sirable and necessary feature of almost all modern electronic pro-ducts. EMC as a design objective requires a careful study and un-derstanding of a technically complex subject and involves such diversetechnical disciplines as shielding , grounding, filtering, and fiberoptics. The unique a ttributes of each of these methods, properlyapplied and implemented, offer effective alternatives in intercon:nection and circuit packaging design. The technical fundamentals ofthese alternatives are presented in this tutorial review.

INTRODUCTION

GVERNMENT and international controls regulating theperformance of electronic equipment required by prolifer-ation of electromagnetic em&ion in our environment havenecessitated a careful study and understanding of the technicalrequirements for effective interconnection and circuit packag-ing design to ensure electromagnetic compatibility (EMC).EMC is having a major effect on the electronics industrywhere it derives impetus from FCC Docket 20780. Similarcontrols have been mandated by the military and internationalregulating bodies. The impact of these regulations has been feltby all sections of the industry and can be compared in scopeto the automotive emission restrictions of a decade ago.Electromagnetic compatibility has thus become an objec-tive in the design of almost all modern electronic products.

The purpose of this paper is to address the technical criteria ofinterconnection and circuit packaging design found to be im-portant in attaining compliance with the various regulations ofwhich FCC Docket 20780 presently is of greatest concern.FCC REQUIREMENTS

FCC Docket 20780 places limits on the permissible con-ducted interference from equipment on power input linesfrom 0.45 MHz to 30 MHz and on radiated interference from30 MHz to 1000 MHz (see Table I.) It a lso defines two generalclasses of equipment to which the limits apply. Class A in-cludes computing devices for use in a commercial, industrial, orbusiness environment. Class B covers computing devices widelymarketed for use in a home or residential environment. A com-puting device is defined as any electronic device that generatesand uses signals or pulses in excess of 10 000 pulses (cycles)per second. The docket also specifies interference test criteriaand requires tests and verification by manufacturers. It speci-fies labelling and information to be supplied to the user and

Manuscript received February 1982; revised July 28, 1982. Thiswork was presentedat the 32nd Electronic ComponentsConference,SanDiego,CA, May 10-12, 1982.The authors are with AMP Incorporated, P. 0. Box 3608, Harris-burg, PA 17105.

TABLE IRadiated Limits Class A Class 0Frequency (MHz)30-8888-218

uvlm at 30 m3050

uvlm at 3 m100150

218-1000 70 200

Frequency (MHz) uv uv.45-1.6 1000 2501.6-30 3000 250

TABLE.11Compliance DatesJanuary 1,198l Personal Computers and ElectronicGamesOctober 1,198l For all other computing devices firstplaced into production afterOctober 1, 1981.October 1,1983 For all computing devices, regard-less of date of first production

sets the date for compliance (see Table II.) FCC Docket 80284is the testing requirement associated with Docket 20780. It ds-fines in detail the test methods, facilities, instrumentation, andconfigurations necessary to verify adherence to the allowab1.emaximum interference limits.

CIRCUITS AND SYSTEMS COVEREDIn general, if a design contains switching or digital circuitsthat employ frequencies of 10 kHz and above, it must at 1ea:stcomply with the FCCs Class A requirements. If, in addition, asignificant market for the product includes use in a home thenit may have to comply with the more stringent Class B rules.At least for the presen t time some equipment has been ex-

cluded, so it is important to check with legal and electroma:g-netic interference (EMI) experts on this. But it is highly likelythat a system that employs a binary clock is driven by areference source in excess of 10 kHz, and/or is sold to thenormal computer market covered. A partial list of componentsand systems covered includel switching power suppliesl mainframe and CPUs with clock frequencies of 10 kHzand abovel memories. CRTs

0148-641 l/82/1200-0470$00.75 0 1982 IEEE


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BOGAR AND VANDERHEYD EN: PACKAGING FOR ELECTROMAGNETIC COMPATIBILITY 471. disks. tape drives0 printers. communications interfacesl modems employing microprocessors.Also covered are systems using all or portions of the aboveincluding dedicated products such as PBXs and business com-

puters. It should be noted that the interconnecting cable andconnectors between equipment can transfer and radiate inter-ference and in many casesare the primary offending sources ofEMI.HOW MUCH RADIATION IS CRITICAL?

Engineers and designers suddenly confronted by the needto meet compliance rarely give much thought to digital circuitsand their interfering potential. So it is no surprise that mostdesigners are not acquainted with the levels of EM1 radiationand conduction to expect from various digital sources. Yet theinterfering energy of such a source can be significant.To understand the relationship between digital pulse char-acteristics and their potential effect on the EMI/RFI environ-ment, it will be necessary to d iscuss some of the basics of timevarying functions and their frequency domain. If a switchedsignal is generated the amount of energy available at any givenfrequency depends upon the signal amplitude (A), the repeti-tion frequency (PRF or l/T), the pulse width (t,,,), and thepulse risetime (t,) and falltime (tf>.These parameters are identified in Fig. 1 . It shows a plot ofthe envelope of the maximum amplitude of the Fourier com-ponents o f a signal versus frequency for the trapezoidal wave-form shown. Note that point X shows the energy level avail-able at frequencies up to frequency fr . It is proportional to

the signal amplitude A and the pulse duty cycle (t,/T, or t, XPRF). This amplitude reaches a maximum at (t,/T) = $, theequivalent of a clock signal with a 50 percent duty cycle. Theradiation envelope begins to drop off (20 dB /decade) at thefrequency fr , determined by I/t,. As t, gets shorter the fre-quency fr of the breakpoint moves up. This represents a bitduration that is directly related to system baud or data rate.At frequency fa , the envelope drops off even more rap idly (40dB/decade). The new breakpoint is determined by the inverseof the pulse rise and fall time (t,. and tf a re assumed to beequal). As pulse transition speed decreases, producing slowerrise and fall times, this point lowers in .frequency and theFourier harmonics at higher frequencies are reduced in ampli-tude. Clearly, if circuits dont require fast rise and fall times, itis best not to use them.Fig. 2 illustrates an example of a practical situation. Itshows the energy available for radiation from a typical 5.0 Vtransistor-transistor logic (TTL) gate driving six loads. (9.6mA) at a frequency of 1 MHz, with a 50 percent duty cycle.The rise and fall times are assumed to be six nanoseconds.Shown in the graph are both the available energy from the cir-cuit in operation and the FCC limits. The graph compares theradiation available to that of a theoretical isotropic radiatoremitting energy equally in all directions at the FCC limits.What may seem surprising in this situation is that even if only

FIRST ROLLOFF20 dB/DECADE

SECOND ROLLOFF40 dB/DECADE

fl f2FREQUENCY (log)

POINT x = MAXIMUM AMPLITUDE = K,A(tw/T)f, = FIRST ROLLOFF FREQUENCY = K2(l/t,)f2 = SECOND ROLLOFF FREQUENCY = KS(l/t,)Fig. 1. Signal components. Graph shows Fourier components of sig-nal versus frequency. Envelope of maximum energy amplitudespectrum generated by trapezoidal signal pulse with equal rise andfall times is plotted.

tPOWER SPECTRUM FOR

1 -MHz TTL CLOCK

88

LIMIT

FREQUENCY (MHz)Fig. 2. Emission levels. Shown are both energy available from 1 MHzTTL gate and emission levels allowed by FCC specifica tion. Graphcompares radiation available to that of theoretical isotropic radiatoremitting energy equally in all directions at FCC limits.1 X 10d3 of the power available at the TTL gate was radiatedinto space the level of EM1 would exceed FCC specifications. .Fig. 3 illustrates an analogous situation for conducted inter-ference. When power levels are much higher than contained inTTL circuits, such as in a switching power supply, the situa-tion obviously becomes much worse.

DESIGNING FOR EMCThe above illustrates the magnitude of the problem facingthe designer of a computing device subject to FCC com-pliance. His problems are especially severe f the digital signalsof his system have fast rise and fall times and a high repetitionfrequency. Since there is a general trend toward faster datarates and the use of corresponding ly faster integrated circuit(IC) families, the EM1 problem w ill in all probability becomeeven more a challenge in the future. Fortunately, modern de-


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472 IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUFACTURIN G TECHNOLOGY, VOL. CHMT-5, N O. 4, DECEMBE R 1982P A VOLTAGE ENERGY SPECTRUM3 107-Y2 106-2 105-2 104-63 103-z0 1020 0 . l 0.45 1 .o 10 30 100 1000

FREQUENCY (MHz)Fig. 3. Conducted interference. Level of conducted emissions al-lowed by FCC specification is compared to RF voltage availablefrom TTL gate. Here, even 10-S of power.available at 1 MHz, con-ducted into power line, would exceed FCC limits.sign alternatives for EMC can alleviate and in many instancescompletely eliminate the problem.The alternatives generally involve shielding, grounding, fil-tering, circuit packaging/design, fiber optics, or a combinationof any or all of the above. The primary concern in any designis cost and it goes without saying that any viable solution hasto also be cost effective. However, manufacturers need not in-cur any additional or excessive cost to meet FCC requirementssince the design for EMC centers on the proper application ofgood engineering practice coupled with an appreciation forand understanding of the underlying principles of the problemand solution.The basics of EMC design include shielding, grounding, fil-tering, and fiber optics and are generally understood. They arecovered here in brief to serve as a quick review and to maketheir relevance to our main topic, Interconnection and Cir-cuit Packaging, more apparent.Shielding

A shield is a conductive (metallic) or magnetic material par-tition used to control the propagation of electric and magneticfields from one region to another. It can be used to containelectromagnetic fields if it surrounds the noise source or it cansimilarly surround a region and protect it from an impingingexternal electromagnetic field.An electromagnetic plane wave striking a metallic surfaceencounters two types of loss. Part of the wave is reflectedwhile the remainder is transmitted and attenuated as it passesthrough the medium. The combined effect of these losses (re-flection and absorption) basically determine the effectivenessof the shield. Absorption losses are due to heating of the ma-terial by the induced currents. The amplitude of an electro--magnetic wave decreasesexponentially as t passes through themedium. The distance required for the wave to be attenuatedto l/e or 37 percent of its original amplitude is defined as theskin depth (6) which can be expressed in terms of frequency(j), relative permeability (py), and conductivity relative to cop-per (u,) as follows:

2.66= (fj,a,y* in-A general rule of thumb is that to be effective in all applica-tions, a shield should be at least three skin depths thick. Note

100802 60

FREQUENCY (MHz)Fig. 4. Skin depth versus frequency. To be effective in all applicationsshield should be at least three skin depths thick. Thickness of five.metals for three skin depths versus frequency are plotted.that 6 is a function or frequency. Fig. 4 shows 36 as a function-of frequency for a variety of metals.Reflection losses are related to the d ifference in characteris-tic impedances of the media through which the wave travels asit encounters a shield as well as when it, leaves a shield. Fo:relectric fields even a thin metallic shield where the primary re-flection occurs at the surface provides good reflection loss. Itshould be pointed out that, generally speaking, the intrinsicshielding effectiveness of the material is of less concern thanthe leakage due to shield discontinuities such as seams andholes.Grounding

The implementation of an effective grounding system isviewed by many as an art rather than a science. Yet propergrounding can be applied with predictable and effective resultsif established and sound engineering principles are followed..Grounding and shielding in combination present a cost effec:-tive means of protecting against EM1 problems.A ground is considered a common voltage reference pointwhere the potential does not change as a function of theamount of current either supplied or removed. A ground canalso be viewed as a plane which ideally serves as a commonreference point anywhere within a system. Noise problems arecreated when a point or plane perceived as a ground is used asa reference, when in actuality it electrically is at a different po-tential. When thishappens in a system and multiple and elec-.trically different grounding points are used to reference vari-ous circuits within the system, the combined e ffect will bedetrimental to the proper and reliable operation of equipment.A grounding system should, therefore, be carefully designedespecially if higher frequencies are involved. Under these cir-cumstances conventional grounding schemes may not be ade-quate and other factors such as cable or conductor length andthe physical size and geometry of the device come into play.For example, a ground lead or cable shield length can easilyexceed a quarter wavelength (h/4) at higher frequencies andhas to be treated more like a shorted quarter wave transmis-sion line than as a ground lead. To avoid such an antenna ef-


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BOGAR AND VANDERHEYDEN: PACKAGING FOR ELECTROMAGNETIC COMPATlBILITY 473feet multiple ground points along the shield or groundingconductor are necessary. The implication for digital systems isthat the spectral content as determined by rise/fall times, pulsewidth, and repetition rates should be taken into account whenoptimizing a grounding system for electromagnetic compati-bility.While multipoint grounding may be desirable at higher fre-quencies, lower frequencies used simultaneously in the samesystem are apt to create ground loop prob lems. In, this case,the use of a hybrid grounding system where low frequenciesare capacitively decoupled may be necessary. The above il-lustrates that proper grounding can be a complex and highlyimportant consideration when optimizing a systemss electro-magnetic compatibility.Filtering

Another design option to a FCC compliance problem is fil-tering. F ilters protect against conducted in terference and inthis case are considered lowpass filters because they protectlow frequency communication signals by excluding high fre-quency noise.The choice of a low pass filter can be narrowed down tothree basic types: feed-through capacitors, lumped-element fil-ters, and distributive dissipative or absorptive filters. Typica lfilter specifications, however, do not include the critical dataneeded to choose the device for a particular application. Suchdata as reflection coefficient, input impedance, and transmis-sion characteristics are usually not covered. Instead, insertionloss per MIL-STD-220, capacitance, insulation resistance, anddielectric breakdown are specified which generally are insuf-ficient to make an optimal choice. For example, the insertionloss of feed-through capacitors is primarily due to mismatchlosses. Their reflection coefficient shows that all energy is re-flected back to the generator with little attenuation and tendsto increase common resistance coupling due to recircula tingcurrents in the ground planes. Lumped element filters consistof L, T, or 71networks made up of inductors, magnetic beads,and coaxial capacitors. The lossy inductive components pro-vide for some dissipative properties but generally this type offilter operates by reflecting energy back to the generator withthe same consequences as mentioned above for feed-throughcapacitors. In addition, the lumped-element-type filter suffersfrom internal resonances which in some critical applicationscannot be tolerated.The distributed dissipative or absorptive filter is con-structed as an integrated monolithic assembly consisting of aferrite-titanate composition (Fig. 5). It can be modeled as atransmission line w ith a propagation constant y = a + jp and acharacteristic input impedance Z,. The real part in this expres-sion carries the symbol cy and is called the attenuation con-stant. It determines the ra te at which waves die out as theytravel along a line and is expressed in nepers per unit length.The imaginary part of y is called the phase constant and hasthe symbol /3 . The phase constant determines the variation inphase position as a signal moves along the line and is expressedin radians per unit length.Since both Q and fi are functions of unit length, insertionloss (IL) can be tailored for each application by simply cut-

BARIUMTITANATEDIELECTRIC

IRE

FREQUENCY (MHz)Cc)

Fig. 5. Distributed element, absorptive filter. Longitudinal cross SC-tion (a) takes form of lossy, coaxial transmission line; equiva-lent circuit is shown in (b). Filtering effect of coaxia l filter is com-pared to ideal capacitor of same value in (c). This filter performsmore effectively than conventional capacitor at high frequencies.ting the filter to the appropriate size. A typical filter , for ex-ample, provides 55 dB of insertion loss at 100 MHz in a 50 asystem. This includes approximately 6 dB in mismatch lossesdue to unequal source, filter, and load impedances. The 3 dBpoint or cut-off frequency f, is similarly size dependent.Fiber Optics

Fiber optics represent yet another design alternative opento manufacturers of equipment trying to comply with the newFCC regulations. Generally immune to EMI/RFI, this designalternative is becoming more attractive as the manufacturers offiber-optic components continue to develop their products,improving performance and lowering prices.Fiber optics is a technology in which light is transmittedalong the inside of a thin flexible glass or plastic fiber. Its com-monest use today is as a transmission link connecting two elec-tronic devices or circuits. The fiber-optic link converts an elec-trical s ignal into light, transmits the light through the fiber,and converts the light back to an electrical signal. It convertselectrons to photons and back. Besides the optical fiber itself,the link contains the circuitry for changing the signal to lightand back. Principally, this means a source of light such as alight-emitting diode or laser, and a detector of light such as aphotodiode. Because the electrical signal-typically a digital-logic voltage-is not directly usable with the source and de-


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474 IEEE TRANSACTIONS N COMPONENTS, YBRIDS, ND MANUFACTURINGTECHNOLOGY, OL. CHMT.5,NO. 4, DECEMBER 98.2TRANSMITTER--,I

-i IG&iz+lzgNSIGNAL iIN I---------------------- J

OPTICALFIBERRECEIVER

L--------,,,,,,,,,,,,,JFig. 6. Basic fiber-optic link.

tector, the link also contains a drive which converts the logicinto a form that will operate the source, and an.output circuitwhich often is simply an amplifier to strengthen the signal.The driver and source together are termed a transmitter; theoutput circuit and detector together are termed a receiver.Presently, a major concern in fiber optics is to make high-quality, reasonably priced connectors to couple a fiber to thesource, to the detector, and to other fibers. Fig. 6 is a block di-agram of a basic fiber-optic link. -A constant concern in communications and data transmis-sion is sending ever-increasing amounts of information withgreater efficiency over a medium requiring less space. But asspeeds increase and more information is handled the dif-ficulties of preserving the integrity of a signal in copper cablealso increase. The problems caused by EMI, crosstalk, and lossof signal power become more troublesome as operating fre-quencies increase. More care and expense are required to pro-tect the signal. In both these areas-signal integrity and infor-mation-carrying capacity-fiber optics offers many advantagesover copper cable. One reason is that optical fibers are dielec-tric materials and can carry the high frequencies of light andstill remain virtually immune to the electromagnetic problemsassociated with copper conductors. So attractive is this newtechnology that many observers predict that fiber optics willbecome the next-generation data transmission medium. Typi-cal fiber optic interconnection products are shown in Fig. 7.

INTERCONNECTION DESIGNThe design of an interconnection system for effective pro-

tection against electromagnetic interference is a multidisci-plined engineering task which may include any or all of theaforementioned subjects of shielding, grounding, filtering, andfiber optics. In addition, an understanding of circuit packagingand careful attention to system bandwidth characteristics is re-quired. Success, however, is still not guaranteed unless the in-terconnecting system is also properly applied.SHIELDED CONNECTORS AND CABLES

The performance of a shielded connector is closely tied tothe type of shielded cable it terminates. One cannot really beevaluated independently of the other. A shielded connectorand cable assembly should ideally incorporate all attributes of

Fig. I. Typical fiber optic components. Metal active device mountand metal connector components shield sensitive detector circuitry.

good shielding and be an extension of the shielded enclosuresit interconnects. Most assemblies, however, are a compromiseof such practical considerations as cost, ease of assembly,cable flexibility, small size, etc. The most important functionof a shielded connector is not to shield the wires inside of it,although this should be done too, but to provide a low im-pedance path between the cable shield and ground. This can beillustrated by the following example.Consider a single wire parallel to and connected to a groundplane through a terminating impedance at each end. If placedin an electromagnetic field it will act as a receiving antenna.Currents will be induced and voltages will appear on the termi-nating impedances. The object is to prevent this wire frombeing influenced by the disturbing field. Using the criteriaestablished previously, i.e., a good conductor, three skindepths or more in thickness, we construct a tubular shieldaround the wire. (See Fig. 8.) If the shield is not connected toa reference potential and is, therefore, free to float, there isno appreciable reduction in the induced energy. The shieldspotential varies under the influence of the disturbing field and.is capacitively coupled to the wire. To be effective a shieldmust be properly grounded. If only one end of the shield isgrounded properly, it will be effective al those frequencies forwhich its length is less than l/6 X. At frequencies higher thanthis, it is necessary to ground the shield at both ends and pos-sibly intermediate points.Ideally the shield should be at grouncl potential so that it is .not affected by the field. For this to be true any wire or strapwhich connects the shield to ground would have to have zeroimpedance so that as he currents induced on the shield flow toground they do not create a voltage across the connection.Any real conductor possesses ome resistance and if it is carry-ing a current, there is an associated magnetic field which im-plies an inductive reactance. The low frequency inductance ofsome representative wires is shown in Fig. 9. Both the resist-ance and inductance are functions of frequency. They are alsofunctions of the geometry of the conductor and the larger thediameter of the wire, the lower the impedance. A #26 drain


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BOGAR AND VANDERHEYD EN: PACKAGING FOR ELECTROMAGNETIC COMPATIBILITY 475

/ CONDUCTING PLANE \Fig. 8. Schematic of shield connected to ground showing intrinsic re-sistance and inductance of the connection.

8060

64321.5.li~, 8-O',MHZ 20 0 1

FREQUENCYFig. 9. Wire impedance versus frequency. Based on wire inductance:L = 0.2 X, lo-6 I (2.303 log 1,~ [41/d] - 1) henries, where I is wirelength in meters and d is wire diameter in meters.

wire I-in long is nof a g00a ground connection and boda notbe used as the sole grounding path between cable shield andground.EFFECTS OF SEAMS AND HOLES

When an electromagnetic wave strikes a shield surface cur-rents are induced. It is bne of the functions of the shield tocarry these currents to ground with a minimum of perturb-ation. If a slot or other obstruction is placed in the path of thecurrent, it is diverted (Fig. 10). The longer current path intro-duces an excess mpedance and hence a voltage drop across theslot. The longer the slot, the greater the voltage. This voltageinduces an E field in the slot and causes t to radiate. If thelength of the slot approaches h/4 it will become a very effi-cient radiator and can actually pump the energy collected overthe entire length of the shield through the slot. To limit theslot effect the rule of thumb for a shielding effectiveness ofabout 60 dB is to allow no slots longer than 0.01 h at the high-est frequency of interest. Seams where contact is only made atintervals or contacts made with spring fingers can be treated asa series of slots (Fig. 11). Designers of shielded connectorsshould be careful when incorporating slots in their design. Aslong as the frequency is such that the perimeter of the con-

CURRENlFLOW

Fig. 10. Diversion of surface current by a slot in a shield.

CURRENTFLOW

Fig. 11. Seam fastened at intervals corresponds to row of slots.CHASSIS

CONNECTOR

CURRENTFLOW

Fig. 12. Proper orientation of a slot to minimize its effect of shielding.nectar is less than h the current path in the connector isknown. The important dimension of a slot is the one normalto the current path. Therefore, if a slot or a seam s required inthe design, its effect can be minimized by placing the long axisparallel to the current path (Fig. 12).A useful concept to describe the performance of a shield isthat of transfer impedance. The surface currents on the out-side of an imperfect shield through some mechanism, such asholes or insufficient thickness, couple energy to the conduc-tors inside the shield. This energy can be represented as a volt-age source inserted in series in the conductors, one source foreach conductor (Fig. 13). The ratio of voltage to surface cur-rent is the transfer impedance ZT:

vizTi=I. s


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476 IEEE TRANSACTIONS ON C OMPONEN TS, HYBRIDS, AND MANUFACT URING TECHNOLOGY, VOL. CHM T-5, NO. 4, DECEMB ER 1982

i = Qi I,Fig. 13. Concept of transfer impedance 2~.Note that the lower Z,i, the better the shield. It is a usefulconcept but difficult to measure in any but circular configura-tions. Fig. 14 presents measured results for two common co-axial cables. RG-%X/U is a single brai,d cable and has ahigher transfer impedance than RG-223/U which is doublebraided. Therefore, RG-58C/U would couple a larger V intothe cable; i.e., it would be a poorer shield.

OTHER DESIGN CONSIDERATIONSShielded connector and cable design takes into account all

aspects of material selection mentioned previously and re-quires attention to such factors as permeability, conductivity,and skin depth as a function of frequency. Attention shouldalso be paid to the following general guidelines:a) low impedance peripheral termination between con-nector and cable shield,b) low impedance between shields of mating connectorhalves,c) low impedance between connector shield and chassisground,d) shielding around the total periphery (360 deg) of the

connector,e) provision for good bonding, e.g., with a strap betweenshield and chassis ground where the connector is notbulkhead mounted and when warranted by systems con-sideration,f) where jack screws are used and extend through the bulk-head into the enclosure, they shou ld be grounded to theshield and/or the bulkhead,g) minimize seams and holes and other shield discontinu-ities,h) for optimum electrical continuity across a joint or seamthe -metal should be protected from corrosion with aconductive finish,i) do not anodize aluminum, use chromate or alodine fin-ishes which are conductive,j) the effect of shield discontinuities such as slots and holesdepends on the degree to which they impede or distortthe uniform flow of shield currents-for example, a nar-row long slot positioned perpendicular to shield currentflow generates considerable leakage in comparison to alarge number of small holes which has little effect onuniformity of shielded currents,k) mating surfaces should be even and smooth to .optimizeshielding performance-they also should be electrochem-ically compatible.To emphasize, the most important consideration in con-

OR01

OR001

-4

0.1

4%\- -

Fig. 14. Transfer impedance versus frequency.nectar design is a low impedance from shield to ground. Drainwires and pigtail terminations of braided shield introduce ahigh inductance path to ground and should be avoided. Reduc-ing this inductance is important and can be accomplished byusing a wide braided or solid copper strap with a maximum as-pect (length/width) ratio of four. In connector design a lowRF impedance can be achieved by providing a multitude ofcontact points around the perimeter of an interface. The ef-fective inductance (RF impedance) is lower for several smallwires, provided that they are spaced apart, than it is for onelarge wire of equivalent cross-sectional area.

APPLICATIONAlthough the foregoing discussion emphasizes a lower im-pedance to ground it should be pointed out that whether orwhere to ground a shielded connector and cable assembly i.sa system consideration which must be included in design. In-terconnecting two different ground references,. for example,when a printer is connected to a computer by means of ashielded cable, could inadvertently produce ground loop cu:r-rents which could couple noise into vital areas of the system.In this case single point ground, i.e., at one end of the cable, is

preferred. At higher frequencies, however, the opposite is trueand multipoint grounding is found to be more effective. Tosatisfy both requirements the connector/cable shield at on.eend can be capacitively coupled to chassis ground. This willstill provide adequate grounding at hi& frequencies while in-hibiting low frequency ground loop currents. Provision forsuch capacitive decoupling is a desirable feature in the designof a shielded interconnection system.SHIELDED CABLES

Some of the more common material for cable shielding aremetal braid, flexible conduit, rigid conduit, metal foil, or highpermeability material. Effective shielding of the cable is highly


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BOGAR ANDVANDERHEYDEN: PACKAGING FOR ELECTROMAGNETIC COMPATIBILITY 477dependent on the type of termination and/or shielded con-nector used with the cable. A shielded system is only as goodas its poorest component. For example, a 60 dB connector ter-minating a 20 dB cable results in an overall system perform-ance of 20 dB. Similarly a system can be limited by the per-formance of the connector. A connector should be designed soas not to degrade the performance of the cable it terminates.

Braided shield is the most common material used. Its effec-tiveness depends on the density of the weave. Higher densitiesmean better performance at higher frequencies. Improved per-formance can be obtained in multilayered braids at the ex-pense of cable flexibility and higher cost.Flexible conduit may have degraded performance at higherfrequencies where wavelengths approach the size of the open-ing between links. The shielding effectiveness of solid rigid orsemi-rigid coax approaches that of a solid sheet of the samematerial and thickness. Although very effective, such coax isexpensive and is used only in critical applications. Cables withsolid aluminum foil shields of adequate thickness provide al-most 100 percent coverage and can be very effective whenproperly designed and terminated. Although not as strong asbraid and harder to terminate, they are becoming more com-mon.

SYSTEM BANDWIDTH CONSIDERATIONSBandwidth is an important factor to be considered in thedesign of an interconnection-system especially where the useof a filtered connector is contemplated in the presence of highspeed digital signals. The bandwidth of a system generally re-fers to the highest frequency f,, or cut-off frequency, belowwhich signals are attenuated less than 3 dB. The smaller thebandwidth the more frequency components of a digital signal

are attenuated and the more distorted such a signal will be-come. Components for interconnecting equipment must trans-mit the desired signals or power without radiating in excess ofthe FCC limits. and without altering the basic shape of datapulses and other communication signals. These two objectivesare not particularly easy to achieve economically while, at thesame time, meeting all the normal mechanical and environ-mental requirements. Fortunately high-density filtered con-nectors are available with cut-off frequencies ranging from 1MHz to 150 MHz. This wide range provides great flexibility inpreserving a systems signal fidelity while maintaining control-lable limits on bandwidth. Typical high density filtered con-nectors are illustrated in Fig. 15.CIRCUIT PACKAGING FOR EMC

Most of the discussion so far has centered around systemconsiderations. However, careful attention at the board andwiring level to circuit packaging and component layout can bea cost-effective way of improving the electromagnetic compati-bility of a system. Circuit packaging for EMC involves thesame fundamentally important techniques discussed previouslyand include filtering, shielding, grounding, decoupling, andbandwidth control. Because of the importance of careful pack-aging it should be included as part of the initial system designconsidera tions. It is at this early stage that attention to board

Fig. 15. Miniature coaxial feedthrough ilters can be assembledntoconnectors to trap interference that might otherwise leave en-closure.TABLE III

LOGIC FAMILIESLogic familyCMOSTTLTTL (LS)TTL SchottkyECL

Maximum useful bandwidth510 12MHz10to 25MHz15to 30MHz

30tolOOMHz50 to 500 MHz

layout, circuit design, component selection, and system band-width pays off.Bandwidth Control

Bandwidth control involves keeping signal spectral contentto a minimum and limiting the systems frequency response toonly those frequencies necessary to operate properly.As seen previously the signal spectral content is related toamplitude, rise and fall time, and pulse duration. The follow-ing guidelines can be used to minimize EM1 by controllingthese pulse parameters.

a) Select basic clock speeds no faster than absolutely neces-sary to operate the system.b) Select logic families according to speed and use the slow-est type that will still do the job. Table III shows logicfamilies and their corresponding operational speeds. Itcan be seen that CMOS should be used when possiblerather than the faster TTL family.c) Circuit design plays an important role in RF1 control forpoorly designed logic functions and improperly timedsignals can result in transient pulses which not only pre-


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478 IEEE TRANSACTlON S ON COM PONENTS, HYBRIDS, AND MANUFACTUR ING TECHNOLOGY, VOL. CHMT-5, NO. 4, DECEMB ER 1982vent proper operation but can also cause severe high fre-quency emissions.d) Logic circuits should be designed to switch only the min-imum energy necessary to accomplish the task.

Limiting the systems frequency response can be accom-plished by filtering or wave shaping. Filtering at the boardlevel serves two functions. When applied to power buses itserves to bypass the dc voltage sources. Applied to signal linesit wave shapes by removing through filtering the unneces-sary higher frequencies which are apt to create RF1 problems.Obviously the more harmonics are eliminated the more dis-torted a digital signal becomes. Fortunately, a signal line sel-dom needs to pass anything higher than the ninth harmonicand waveshaping can generally be applied without affectingcircuit performance. Where high speed logic has to be usedover relatively long runs, filtering, judiciously applied, canmaintain frequencies within a usable spectrum and prevent un-wanted interference. Direct current power distribution circuitsshould be bypassed with large capacitors that are in turn by-passed by smaller capacitors. These serve to cancel the para-sitic inductances and resistances inherent in large value capaci-tors that tend to make them ineffective filters at higher fre-quencies. Fig. 16 illustrates the use of bypass capacitors andtypical capacitance values used to minimize RF interferencefrom power supply leads. Note that 0.01 pf capacitor is recom-mended at each integrated circuit.PC Board Desigrland Component Layout-Out

EMI can also be drastically reduced by proper componentand board layout which limits the length of high current runsand keeps critical circuits separated. Careful planning at thisstage can reduce the amount of spurious radiated noise whichotherwise would have to be suppressed elsewhere by means ofshielding or filtering. Proper design can also prevent excessiveenergy from being coupled into surrounding circuits by keep-ing conductors close to a ground plane and far removed fromone another. Coupling, inductive and capacitive, can further bereduced by keeping conductors short and frequencies low. An-other form of coupling which can be avoided by proper boarddesign is common mode impedance coupling which occurswhen more than one signal source share a common returnpath. This common path has a finite impedance which espe-cially at higher frequencies can be significant. Voltages devel-oped across this impedance due to one signal induce a corres-ponding voltage in others and vice versa, To prevent this, cir-cuits should be arranged so that ground returns are direct andas short as possible. Good grounding can be incorporated byedging the board with a l/8-in to l/4-in wide ground conduc-tor. In critical applications double sided boards with groundson both sides and frequent plated through holes assure goodground distribution.Shielding at board level has many advantages; its lesscostly, more effective, and simpler to implement here than atsubassembly or higher levels. Bulk and weight can often begreatly reduced by the use of less complex localized shieldingof components such as clocks, oscillators, drivers, and switch-ing power supplies. Attempts at shielding for EMC should,

MEDIUM FREQUENCY HIGHCOMPONENTS OF FREQUENCYCOMPONENTSLOWEST FREQUENCIESOF BOARD CURRENT C,RCU,T

BRANCH NODE

OF CURRENTADDITIONALBOARD POWERINPUT

Fig. 16. Bypass capacitors minimize RF emissions from power supplyleads. High frequencies are bypassed closer to integrated circuitsource, medium frequencies at circuit branches, and low frequenciesat board input. This approach minim izes high frequency currentspikes in long interconnection paths back to power su pply.

therefore, start at the lowest possible level. Small light weightshields covering specific functions or components and madeout of thin sheet metal can eliminate the need for heavy metalenclosures. Shielding should be.contemplated when all othermeasures mentioned above have been applied. Even then shieldonly the sources suspected of being the worse offtinders.

CONCLUSIONInterconnection and packaging for electromagnetic compat-ibility is a complex, yet important aspect of system design thatcan have a great impact on the performance of the final prod-ucts.Although design for EMC involves a multitude of engineer-ing disciplines it is primarily the application of good practicalengineering coupled with a senseof appreciation for the prob..lem that brings designs to successful conclusion. This papelserves as a quick survey of some of the technical subjects thatshould be considered and included in the planning stage ifEMC is a design objective. The subject has far greater depthand detail than could possibly be covered in this short presen,.tation. For those interested, it is suggested that the accom,.panying list of references be used to delve further into thistopic.

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REFERENCESH. W. Ott, Noise Reduction Techniques in Electronic Systems.NewYork: Wiley, 1976.B. E. Keiser, Principles of Electromagnetic Compatibility. Ded-ham. MA: Artech House, Inc., 1979.C. A. Harper, Handbook of Wiring, Cabling and Interconnectingfor Electronics. New York: McGraw-Hill, 1972.R. E. Matick. Transmission Lines for Digital and CommunicationNetworks. New York: McGraw-Hill, 1969.E. M. Reyner II, Controlled Interconnection System s for HighSpeed Digital Equipment. Harrisburg, Pa: AMP Incorporated.J. H. Bogar and E. M. Reyner, Miniature low-pass EMI filters,Proc. IEEE . vol. 67, no. 1, Jan. 1979.M. Schwartz, Information Transmission Modulation, and Noise.New York: McGraw-Hill, 1959.D. R. J. White, Electromagnetic Shielding Materials and Perjorm-ante. Gainesv ille. VA: Don White Consultants, Inc., 1980.E. L. Bronaugh, Circuit. grounding, and shielding designs forsuppressing electromagnetic emissions, presented at 1980 Mid-con, Dallas, TX.I. Straus. Design digital equipment to meet FCC standards.EDN, June 5, 1980.Introduction to Fiber Optics and AMP Fiber-Optic Products, AM FHandbook HB 5444, AMP Incorporated, Harrisburg, PA.

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Interconnection and Circuit Packaging for Electromagnetic Compatibility

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