fundamentals of emc design

7/28/2019 Fundamentals of EMC Design

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F u n d am e n t a l s of EMC De s i g n :Ou r P r o d u c t s Are T ry i ng To Help Us

KEITH ARMSTRONGCherry Clough Consul tantsS ta f f o r d , United Kingdom

1. IIMTRODUCTIONW e often design electronic products only to f i n d that when wetest them for electromagneticcompatibility ( E M C ) , their emissions an d/or immuni ty are not as good as we needthem to be.

Usually, at thi s ti me, we feel as i f we arefighting against the laws of physics to contain the conducted and radiated emissions,or to reduce susceptibility.

But in fact the laws of physics - Maxwell's Equations - are causing our designto have the best emissions and immuni tythat the physical stru ctu re allows . We migh tsay that our product is doing the very besti t can to reduce its emissions and improveits immunity!

(I am using the word "product" to meanevery typ e of elec tron ic assembly, f r o mmodules, subsystems, equipment and systems, to installations.)Hie key issue - is that all curre nts (in cluding strays) always f l ow in closed loops,and always take the path of least impedance,whether this path is along conductors orth rough the air (or other dielectrics) between them.

Current flows in the path of least impedance to min im iz e the energy in its associated electric and magnetic fields, rather l i k ethe way a drop of water i n air assumes a

globular shape to mi ni mi ze the energy inits surface tension.

Because currents natura lly take the pathsthat resul t in the lowest EM f i e l d energies,they automatically give us the best emissions and immuni ty of which our designis capable. Rather than fighting the lawso f physics, what we are fighting is our ownlack of unde rsta ndin g of how the laws ofphysics work. Once we under stand this wecan work w i t h these laws f r o m the starto f our design, to easily and quickly createcost-effective products that meet their EMCspecifications.

Unfortunately, the way that Maxwell'sEquations are taught doesn't show how easyit is to derive (without any mathematics!)the easiest, simplest, most profi table way todesign products using good EM C engineering techniques [1].

Signal Integrity (SI) and Power Integrity(PI) are subsets of EM C engineeri ng, so employing good EM C design techniques f r o mthe star t of a new project ensures excellentS I and PI (see [2 ] ) .

This has the effect of conside rably reducing the numb er o f design iterations, genera l l y reduc ing overall cost of manufac ture,and reducing time-to-market.

Time-to-market has, since 2000, becomethe most important issue for a financiallysuccessful e lect ronic product . This is shownby the industry responses to Question 6i n [3], see Figure 1, and I have seen otherreports f r om simi lar prestigious organiza tions that show the same for most electronicapplications.

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Question 6;What are the op issues youface injenerating continued revenue andprofit?

Gettng the product toiTrarket on timeGettng the product costwithin Ititdgst

Incorporatng changng featureretiuirenients in product desgnIjefore product reeaseReducng the number of desgnIrevsons Itefore protiuct is reeased flie snpprers not asked! i

Rapdy changng consmerinterests, wants, or needsCompetton from new atidrorunex>ected sources

Taken from:" T h e C o n s u n i e r E l e c I r o n i c s B o o m How S e m i c o n d u c t o r an d C o n s u m e rE l e c t r o n i c s C o m p a n f e s Ca n improve C o s t .Timedo-fdarttet. anrt P r o d u c t Quaity "hyKPMG L LP ' s Information, Communl cedon

and Entanainment (ICE) Ptactica. 2008.www,kpmo.dB/doc*/20030701_The_Conaumar_E lacnoni ca_Boom,pdf

CE = C o n s u m e r E l e c l r o n i c sIC = Integrated C i r c u i t s

CE anti ICrespondentssayprodact tinie-to-market is thetiiost iinportatit business issue,followed by prodiictcost.

Figure 1. Time-to-market and cost.

I t isoften found inpractice that empl oying good EMCdesign techn iques frona the star t of a project improve s functional perforrnance, sometimes even giving signal qualityand functi onal specifications better tha n anyth ing that hadever been achieved before.

Unfortunately, some project/engineering managers insisto n the lowest B i l l Of Materia ls (B OM) cost, bel ieving thatthis w i l l somehow lead tothe most profitable product.

Where it prevents us f r o m working with the laws ofphysics, we often find ourselves fighting this ill-advised andplainly incorrect approach (see [ 4 ] ) . The result isa numbero f additional delays and cost-increases (e.g. adding filteringand shielding topass EMC tests) that increase the overallcost-of-manufactur e, delay mark et in tr odu ct io n, reducep r o f i t a b i l i t y and increase financial risks.

F o r example, the ideal printed circuit board (PCB) layerstack for good EMC design of agiven product might haveeight layers, but the mi n i mum SI and functional specifications can bemet w i t h just six. Tiae cost-saving achieved byusing the six-layer board is considerably outweighed by theextra delay and cost of addi ng filtering and shielding at theend of the pro ject tomeet its EMC specifications.

Tiae overall cost ofmanufacture ends upbeing nauchhigher than would have been achieved w i t h an eight-layerP C B , and the(more important ) time-to-market isdelayedby several weeks - which in some situations can naake thedifference between aproduct's success and its failure.

Hals article b r i e f l y introduces the laws ofphysics asthey apply to ourproducts' SI, PIand EMC design issues,developing an "EM Design T o o l k i t " . It then b r i e f l y describesapplying that t o o l k i t toaPCB assenably example.

I wrote asimilar type of article onapplying these sanaelaws ofphysics toease the EMC design of systems andinstallations orany size, [ 5 ] , which might beof interest tosome readers.

2. EXTERNA L AND I NTERNAL EMCApar t f r o m DC issues such asthe fan-ou t of DC signals orthe voltage drop caused by resistance in DC power conductors, all SI and PI issues are just subsets of EMC, asFigure2 tries toshow (also see [ 2 ] ) . They might be called "internal

Figure 2. Good EMC design also takes care ofSI and PI.

E M C " - the product interfering w i t h itself. For more deta ilo n this, seeChapter 8of [6 ] or2.10 of [ 7 ] .

3. EVERYTHING HAS PERMEAB I L I TY ( p) ANDPERMITTIVITY (e)A l l media and materials inthis universe haveconductivity, permea bili ty (p) and permi t t iv i ty (e).

I n vac uum (and air): p^ =4 T T I 0 ^ Henries/metere = (1/ 36nl0' ') Farads/meter

Other media andmaterials arecharacterized bytheirrelative permeability (p^^j and permittivity (e^) - dimension-less numbers, just multi plie rs for the vacuum permea bili tyand permitt ivi ty - sotheir overall permea bili ty is: p p j andtheir overall permitt ivi ty is: e^E^

Permeability isassociated w i t h induc tive EM energy,which we draw asmagnetic field contour lines.

Permitt ivi ty isassociated w i t h capaci tive E M energy,which we draw aselectric field contour lines.

Conductivity (and its reciprocal, resistivity) is associatedw i t h energy loss, i.e. the conversion of E M energy (magnetico r electric) into thermal energy.

The shape andsize ofconductive structures carryingcurrent, and the p p ^ ande^e^ of themedia or materialsthey are embedded in , cause inductance (L ) and capacitance( C ) , respectively.

This means that whenever there isafluctua ting voltage( V ) there isalways anassociated curr ent (I).

A n d vice-versa: whenever there isa fluctuating current( I) there isalways anassociated voltage (V) .

Some digital designers assume that because the input resistance of a CMO S gate is several M O , PCB traces carryingdigital signal voltages carry no (or a very t i n y ) current. TTiis isincorrect because it ignores the inevitable (and unavoidable)stray capacitance of the traces and the gate input.

F o r example, w i t h agate input capacitance of3pF anda 3V o l t digital signal rise-time of 300ps (quite slow thesedays) the peak curr ent re qui red jus t to charge up thi s singleinput gate alone is about 3 0mA . This intense c urr ent "spike"must flow ina loop that includes the DCpower supplydistribution network, socan cause ail manne r of S I , Pi and

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Magnetic fieldT h e H f i e i d i s o r t h o g o n a l to

T h i s o n l y s h o w s o n e l i n e v e c t o r (z) in a 3D s p a c e , at o n e i n s t a n t in t i m e

Figure 3. Visualizing a linearly-polarized EM wave in free space.

E MC problems.I n insulators and dielectrics (e.g. air, PVC, fiberglass) \xyp^

and e^e,; cause analogous effects to inductance and capacitance - so whenever there is a fluctuating elect ric field (E)there is always an associated magnetic field (H).

A n d vice-versa: whenever there is a fluctuating magnetic f i e l d (H) there is always an associated electric f i e l d (E).

Chapt er 2 of [6] and 2.3 of [7] have more details onthe above.

4. B E C A U S E OFM A X W E L L ' S E Q U A T I O N S . . .Every flu ctu at ing voltage or curr ent is really E M power(Watts, i.e. rate of f l o w of elect rical energy), propa gatingas a wave in the medium w i t h velocity v = l/^dx^H^Eff)m/s (= 3.108 m/s in air or vac uum) a nd crea tin g el ect romagne tic (E M) fields as it does so.

This applies to every k i n d of elect rical event, whethe rwe call it elec tric al power; electroni c or radi o signals;infra-red; l i g h t ; l ightning, etc., and including ail mains60Hz power; analogue, digital and switch-mode powerand signals; data commun ica tion s; radio-frequencies (RF)and microwaves, etc., including ail electrical, electronic,or radio "noises".

Figure 3 is an att emp t at vis ua li zi ng a single vector ofan E M wave at a sing le frequency, as it propagates in freespace. Its shows that the E and H fields are per pen dic ula rto each other, and that they both fluctuate in directionsperpendicular to the direc tion in which the E M power ispropagating.

The usual analogy is w i t h waves on the ocean, whichpropagate wave energy across the surface of the oceaneven though the molecules of seawater in a wave onlymove up and down.

A co mm on way of vi su al iz in g the E and H fields associated w i t h voltages and cur ren ts i n cond ucto rs, isshown in Figure 4, for a sen d/r et urn pair of condu ctor sshown in cross section. E-fieid lines always terminate onconductors, perpendicular to their surface, and H - f i e i dlines never terminate on anything.

These lines should be considered l i k e contour lines on

Figure 4. Cross section of fields associated with a pair o f send/return conductors.

a geog raph ical map - they are not real, but thei r densi ty(num ber of lines per inch ) indica tes the st ren gth o f thef i e l d ( l i k e the slope of a h i l l ) . So we can see that the E andH f i e l d stre ngth s are high est in betw een the send andre turn conductors.

The electrical power associated w i t h the current in thewires propagates alo ng the le ngt h of the wires. BecauseFigure 4 shows the wires in cross section, the electricalpower (i.e. propa gati ng E M energy) is f l o w i n g perpendicular to the surface of the page or screen w i t h which you arereading these words, and the E and H fields it sketches aref luctuating in the plane of the paper or screen.

Maxwell's famous four equations include Amperes Law,which says that currents always f l o w in closed loops, andFaraday's Law of electromagnetic induc tion , which saysthat currents always f l o w in such a way as to minimizetheir loop areas.

Maxwell himself invented the concept of displacementcurrent, sho wing how a fluctuat ing current could f l o wth rough capacitance even though there was no conductive path for it.

5. B E C A U S E OF T H E LAW OF C O N S E R V A T I O N OFE N E R G Y . . .Ignoring the virtual particles in the "quant um vacuum",[ 8 ] , there is always zero E M power at any poi nt in space.The E M power enteri ng a poi nt must be exactly balancedby the E M power lea ving it.

This is Kirchoff's current law, which is often descri bedas: "the sum of the cur ren ts at any po in t equals zero", andis equivalent to Ampere's Law.

Another way of putting this is to say that all currentsf l o w in closed loops. If some current could escape f r o ma loop and go wa nde ri ng off on its own , never to r et ur n,the n at the poi nt where i t l e f t the main loop there wouldbe an imbalance i n the current . Curr ent would accumulateat that p oin t, and the Law of Co nse rva ti on of Energy tel lsus this can't happen (in our univer se, anyway).

So we see th at Co nse rva ti on of Energy (i n thi s con tex t

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sometimes caiie d the Law of Conse rvat ion of Charge)means we couid rewrite Kirchoff's cu rr en t iaw as: "thesu m of the EiM power at any po in t equals zero".

This means that at any circuit node that sends a current(whether power, signals, noise, etc.) also simultaneouslyemits an antiphase current that we call the return current.

These send and re tu rn cur rent s propagate th ro ug hthe impedances of the variou s media (air, condu ctor s,etc.), eventually meeting up to create what we th ink ofas send/re tur n c urr ent loops. At any insta nt in time, thecurre nts i n the send and retu rn curre nt paths balanceeach other out.

Notice that because ail power, signal and (stray) noisecurrents, of any k i n d , f l o w in closed loops, this means thatthe con nec ti on to the safety eart h/ gro un d electrodes generally has no relevance at ail for good SI, PI or EM C design.

( I n poor EM C designs, stray curr ent loops can trave lthrough the safety eart h/g rou nd, u sing it as a conveni entconductive structure, and causing high levels of emissionsand poor immunity.)

6 . B U T I T ' S R E A L L Y A L L B E C A U S E O F QUANTUME L E C T R O D Y N A M I C S ( O E D )H o w di d the retu rn currents "k now" what paths to f o l l o wt o exactly match up w i t h thei r respective send current s?Prof Feynman's s l i m book, [9], says that propagating E Menergy ( l i g h t is also E M energy) takes the path of leasttime - which is also the path of least energy - which isalso the path that gives the best SI, PI and EMC possiblef o r a given geometry and media/mat erials (althou gh thislast conclusion is not found in [9]).

T o f i n d out how propa gati ng EM energy "knows " todo this, we have to integrate over the whole of space andtime, including negative time. This was Prof Feynman'sgreat insight, which made the w o r l d of qu ant um electr odyn amic s amenable to calc ula tio n, and is responsiblef o r much of moder n electronic t echnologies.

B u t when Prof Feynman's students asked h i m whatunderpinned this natural behavior, he said no one knewand there was simply no point in even asking the quest i o n . It was ju st the way natu re w ork ed. However, someprogress is now being made in answeri ng this qu esti on,w i t h the favored soluti ons being the "many wor ld s" or"parallel universes" theory, which is known to be truebecause otherwise quantum computers wouldn't work.

A character isti c of QED is that i t defies co mmo n senseand destroys the time relationship between cause andeffect, w i t h some outcomes that can seem very weird.Apparently, w i t h sensitive enough instruments listenerscouid hear what the ou tco me of a bail game would be bylistening to radio broadcasts f r om the future! Unfor tunately it only reaches a few femtoseconds into the future- not enough time to place a winning bet.

Also, QED permits the power budget for a point todeviate f r o m zero for a few femtoseconds, bu t after thatthe Law of Conse rvat ion of Energy insists that the powerbooks have to balance to zero once again, as described

i n 5 above.Maxwell's Equ ati ons and re lat ed laws of physics de

scribe a common-sense, cause-and-effect w o r l d in whichund erst andi ng basic concepts makes it quit e easy andquick to design iow-ov erai i-c ost good SI, PI and E MC -b u t the QED concepts t hat un der pi n thi s are very weirdand wonderful.

Despite its weirdnes s, QE D is the most wei i- pro ventheory ever known, and has been prov en to be accu ratet o abou t 11 orders o f mag ni tu de mor e than has (so far)been possible for gravity.

Happily, for ail SI, PI and EMC work, engineers needgo no deeper than Maxwell's Equations and Conservationo f Energy (or Charge).

7 . WHAT D O E S A L L T H E A B O V E MEAN F O R SI,P I AND E M C ?7.1 EM power d i v i d e s between al ternate p a t h sa c c o r d i n g to the ir a d m i t t a n c e sI n the "far f i e l d " of an E M source, E and H fields exp er ience the "wave impedance" of the media or mate ria ls the irE M power is propagating through:

i n air or vacuum: \l{pjEf=120nD. (near enough 377n)i n other media (e.g. PVC, o i l , fiberglass, etc.):120nV(p, /e , )n

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These simple wave impedance formulae are only truein the "far field", typical for radio transmission and reception, whereas in the "near field" the impedance situation ismore complex, and the dominant effects on the impedanceof a path through the air or other dielectric are inductiveand capacitive coupling - often caiied "stray" or "parasitic"inductance and capacitance. See Cha pte r 2.4 of [6] or 2.3.3of [7], for more on this, including how to calculate whetherwe are in the near or far field.

For E M waves propagating along conductive structures(what we call power, signals or stray curr ents flowing incables and PCB traces), the medium surroundi ng themhas an important effect on impedance, but so does theshape of the structures carrying the current and the shapeand proximi ty of nearby conductors - most especially thereturn conductor(s), but any other conductors in the nearfield will also have an effect.

So EM waves propagating a long conductors can experience i mpedanc es that are lower, or higher, than theimpedance of the medium surrounding them.

This means that for a fluctuating current travel lingalong a conductor there are always alternative paths inthe air and other diele ctrics, so its send/ ret urn curr entloop is never a simple one.

I n fact, ail curr ent s always split and flow in multiplealternative paths, in proportions according to the admittances of each of the paths (a path's admittance is thereciprocal of its impedance).

This is conceptually no different from the way that aD C current flowing through a bunch of parallel resistorswill divide up acco rding to their various conductances(reciprocal of their resistances ) - with the highest current flowing in the resistance with the lowest value (i.e.the highest conductance).

The big difference for fluctuating currents is what issometimes caii ed "the invisible schematic" - the impedances of the stray capacitances and inductances, wh ich arealternative paths for E M energy to flow in, whi ch success ful practical E M C engineers l earn to visuali ze wheneverthey look at conductive structures.

E a c h part of a current loop has several alternativepaths. The paths can be along conductors or throughcomponents and devices, or through the stray paths inthe ins ulation, PCB substrate, air, etc.

I t simply doesn't matter to a propagating E M wave. Theconductors, components and devices that we designed, andthe stray capacitive/inductive coupling and "accidentalantenna" emissions (see 7.2) that we didn't design and notwanted (but can't be prevented entirely) ail just look likedifferent admittances (reciprocals of their impedances ).

F o r example, a signifi cant port ion of the E M wavepower might leave a conductor and continue on its path bytravelling through the air - for example as a (capacitive,E-field) displ acement curre nt - if it sees that air path ashaving impedance comparable with that of the conductor.

When a conductor resonates (i.e. is not a wei i-matched-impedance transmissi on line, see 7.6) in a way that creates

a high impedanc e, a "stray capacitance" path through theair can easily create a lower loop impedance, causing mostof the current to travel as displacement currents.

And where an air path resonates in a way that createsa low impedance, it couid easily create a path with muchless loop impedance than that of the intended conductors,so once again most of the current can trave l as E-fieiddisplacement currents in the air.

We couid say that our main task of S I , PI and/or E M Cdesigners is to reduce the proportion of the E M waves(wanted currents) that "leak out" of our conductors - "escaping" into nearby conductors via stray capacitance andinductance (what we call cros stalk), and also "escaping"into the air as far-field E M waves (what we call E M emissions and measured with antennas in test labs).

I t is important to understand that every current loop,however formed, with however many branch ing curr entpaths going wherever, always has to return exactly 100%of the E M energy back to its source, to comply wi th theiaw of conservation of energy.

Actually, the reality of power and signal propagationis not that a current starts off from a voltage source andeventually returns back to it - having flowed ar ound aloop or loops - but that the send and return curr ents areactually generated simultaneously by the source, and balance each other out at every instant thereafter.

Anyway, this perspective that curren t flows in multiple paths according to their admittances, shows that- to achieve good SI, PI and/or E M C - all we need to dois control the impedances in the various paths that areavailable to our wanted signals or power currents, so thatthey travel predominantly in the loops we want them to.

For example, i f it was possible to design so that no signalor power current was "lost" to alternative paths, then wemust have no crosstalk , no emissions, and - as a direc tresult - our product's SI and PI must be perfect and itsE M emissions zero (see [2]). Al so, by the Princip le of Reciprocity (see 7.2 below), its R F immunity would be perfect.

O f course, perfection is never achieved but we can getclose enough to reduce emissions to sufficiently smallamounts, and improve immuni ty by as much as is needed,without adding significantly to the overall cost of manufacture, simply by working with the laws of physics.

For more detai l on this topic, see Chapt er 2 of [6], 2.3of [7] or 10.1.4 in [10].

7.2 All c o n d u c t o r s ar e " a c c i d e n t a l a n t e n n a s "A transmitting antenna is merely a conductor that inten-tionally leaks its voltages and curre nts as EM power intothe air. A rece iving antenna is simply a conductor thatintentionally picks up voltages and curre nts from the E Mfields around it.

O f course, the more usual situation is that we don'twant our conductors to transmit (leak) some of their E Mpower, or pick up noise from the environment. E M C engineers usually call the fact that they always do leak andpick-up: "accidental antenna behavior" or "unintentional

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A ben t w i re , 20mm abo ve a shee t me ta l c h a s s i su s e d as its current return path

W = 1.4 mF r o m : - M o d e l i n g an d S l m u l a t l o n o f P o w e r l r a i n s t o r E l e c t r i c a n d H y b r i d V e h i c l e s " . M a r c o K l i n g e r . W o r k s h o pF R - A M - < - 1 . I E E E 2009Int I E M C S y m p o s i u m , A u s l l n . T X . A u g 1 7 - 2 1 . 2 0 0 9 .C P - R O M I S B N : 9 7 8 - 1 - t 2 4 4 - l 2 8 5 -0

P lan v iew o f t he ben t w i rea b o v e the sh ee t me tal c h a s s i s

10 Hz 1 00 Hz

1 kHz 10 kHz 1 00 kHzA s f r e q u e n c y i n c r e a s e s , r e t u r n c u r r e n t f l o w s e v e r c l o s e r to path ot ben t w i re |

Figure 7. Examp le of a bent wire with a sheet metal chassis for its Figure 8. Computer simulations of the return c urrent p ath for a wirereturn current. above a plane.

and EMC.I have seen this sort of simuiation done many times, w i t h

wire-over-sheet structures i i k e Figure 7 or w i t h PCB tracesover pianes (e.g. slides 46-50 in [13]), and I have also seen itdone as practical demonstrat ions using ciose-fieid probes. Tireresults are the same, up to however many GHz one cares to go.

7.5 P o w e r and s i g n a l s in c o n d u c t o r s ha ve twom o d e s of w a v e p r o p a g a t i o nD i f f e r e n t i a l Mode, D M (also caiied transverse or metal licmode) is what we c a l l our "wanted" power and signals.

Com mon Mode, CM (also known as "longitudinal mode"or "antenna mode") is caused by the stray, leaked, "unwante d"E M energy when a D M loop's near-field E or H fields meetanother conductor, as shown in Figures 5 and 6. It also occurswhen f a r - f i e ld E M waves couple power f r o m the wanted signali n its intended c i r c u i t , to another c i r c u i t - accidental radiotransmission and reception.

Figure 9 shows the relative paths of the D M and C M currents in a s i m p l i f i e d system.

Paraphrasing 7.1 above - the electricity does not ail stayin the w i r e !

Some of it travels as stray C M cur rents, w h i c h - i i ke ai lcurrents - must flow in closed loops.

Because C M loops are generally very much larger than theD M loops that caused them, their E and H field patterns aremuch more w i d e l y spread. T i i e result that C M is generallythe major cause of "accidental antenna" effects causing EMproblems for emissions and immunity over the frequencyrange f ro m I M H z to I G H z .

Figure 10 shows tha t C M currents also couple w i t h " v i c t i m "circuits through H - f i e i d coupling, simila r to how D M currentscouple (in Figure 6).

Reducing the size of the C M loop reduces its H - f i e i d coup l i n g into the v i c t i m , in the same way that reducing the size ofthe D M loop does in Figure 6. An d reducing the size of the C Mcurrent loop also reduces the amount of E- f i e id coupling intothe v i c t i m , in the same way as for the D M E- f i e id in Figure 5.

So, just as it is im por ta nt for good 51 , PI and EMC to m i n i mize the area enclosed by a i l wanted ( D M ) curre nt loops, it isalso im por ta nt for ail unwa nte d, accidental, CM curre ntloops. For more de ta il on thi s topi c, see Chapt er 5.5 of [ 6 ] ,2.7.5 of [7] or 10.1.5 o f [10].

Figure 9. An example of DM (wanted) signals causing CM noises,for a 'floating' load.

CM s e n d path( i .e . both of t h e D M c o n d u c t o r s )

CM re tu rn pa th(e.g. l o c a l me ta l vyo rk )

rx#V 'c t im c i r c u i t1

\

CM H f i e l dsCM H f i e l d f l ux l i nes t ha t p a s s t h r o u g h t h e

v i c t i m c i r c u i t ' s c u r r e n t l o o p= m u t u a l I n d u c t a n c e w i t h t h e C M c i r c u i t

Figure 10. Example of CM H-fie Id coupl ing.

112 I N T E R F E R E N C E T E C H N O L O G Y E M C D I R E C T O R Y S, D E S I G N G U I D E 2012


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A R M S T R O N G

T hiL I S not an eartti" or " g r o u n d " for EM taut :it rs part ot thp s l i p ' s c o n d i i c l i u p stniotiirp that

h e l p s to return s t r ay CM c u t t e n t s

: pWCelM-lleMeetiteeueeelie j

Figure 11. This copper busbar is not an"earth" or "ground" tor SI, PI or EMI.

Figure 12. These are not "earths" or"grounds" for SI, PI or EMI either.

Figure 13.. .. these are also not "earths" or"grounds" for SI, PI or EMI.

Figure 14. ... and neither are these "earths"or "grounds" for SI, PI or EMI.

7.6 R e s o n a t i n g c o n d u c t o r smake perfect a c c i d e n t a la n t e n n a sThere are various causes of resonances in contductive str uctur es, atcertain frequencies...

a) Wh en the L and C reactanceshappen to be equal

b) Due to geometry interactingw i t h wavelength

The second item concerns trans-naission-iine matching. Wh en mismatched conducto r characte ris ticimpedances cause propagating wavesto be refle cted, under cer tai n condi tions and at cer ta in frequencies theycan cause standing waves to arise,which are a type of resonance.

A t resonant frequencies, loopimpedances fluctuate w i l d l y , in therange between the conductor's seriesresistance (possibly jus t a few m f l ) , upto the stray shunt resistance (possi blya few i V i n ) ,

Accidental antenna effects (straycouplings, wheth er near-f ield or far-f i e l d ) are significantly amplified byresonances, often between 10 and100 times (20 to 40dB), possibly more,affecting both emissions and immun i t y equal ly due to the P rinci ple ofReciprocity.

7.7 T h e r e is no s u c h thing as" e a r t h " or " g r o u n d " for SI, PIan d EMCCurrents always f l o w in closed loops.So the idea that the earth/groundelectrodes prov ide a perfe ct zero-impedance sink that we can use toabsorb, or otherwise make unwantedelectrical power, signals or noises goaway, can't possibly be true - it is atotal myth, pure and simple, havingno basis in reality in this universe.[13] has more on this, especially itsslides 32, 33 and 79.

Even if a zero-impeda nce ear th/ground could exist (which it can't,because eve ry th ing has i mpedance)- if we sent some unwanted currentinto it, the curr ent would come backvia some other route to complete itsloop. So, then: no current sinks (inthis universe).

Earth/ground is only a v a l i d concept (can only have any effect) for

hu ma n safety, where i t an issue ofpreventing electric shock by l i m i t i n gthe ma xi mu m pote ntia l differencesthat someone could come into contact w i t h , whether they are caused bymains electricity leakage currents orfaults, or l ightning strokes.

Even when ear th /g ro und electrodes are doing their thing for safetyreasons, the relevant currents s t i l lf l o w in closed loops.

Figures 11 through 14 show someexamples of what are com mo nl ycaiied ear ths or gro unds , but arereally just elements of a product's ,equipment's, bui lding's or site's conductive structures that help returnC M curre nts back to the ir sources.Whether these structures are connected to safety ear th /g ro und electrodes, or not, is of no consequencefor S i , PI or EMC.

O f course, 1 am not the f i r s t personto comment on the meaningiessnesso f the t er m eart h or gro und for SI, PIand EMC . Dr Bruce Archa mbeau lt isan I B M Dist ingui shed Engineer anda mainstay of the IEEE EMC Society,and many years ago he produced thegraphic copied in Figure 15, as a wayo f making the same point, but in amore amusing way, see [13].

Because it is natural to assume thatsomet hing caiied "earth" or "ground"is an i n f i n i t e sink for noise cu rrent s~ even though such a thing simplycannot exis t - the use of such wordsor thei r graphical symbols encouragesincorrect design for SI, PI and EM C,and I have seen millions of dollarshave been wasted over the years forthis exact reason.

So I always strongly recommendthat the words "earth" or "ground"and their graphical symbols are neverused in electronic design (except whena safety earth or ground is actuallyinte nded - and then for elect ricalsafety purposes o n l y ) . Instead, call theconductive structures by other namesthat mean what they say, e.g. RF Reference (see 8 below), C M Retu rn Path,or whatever.

Using words such as "chassis","frame", "enclosure", "shie ld" or "Faraday Cage" can also lead to the.sameconceptual design errors as "earth"

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F U N D A M E N T A L S OF E M C D E S I G N : O U R P R O D U C T S A R E T R Y I N G TO H E L P U S

phi.-.V . T ' - 4 - .

Sy r^.^r

(("

' ^ ' ' P ^ ^ A . . u r r

p . - A 4 f "K - . 1 4 P . . P

,4--

iE?yp77T\

4 4 W M D t L g C A J H p 4X / p,,D K . r i K i

V p . j . > ,

F r o m S l i d e 42 of: " T h e " G r o u n d " M y t h " . B r u c e A r c h a m b e a u l r . P h . D . . IB M D i s t i n g u i s h e d E n g i n e e r .F I E E E . 18 N o v e m b e r 2 0 08 . h n p ; / / e w h . i ee e . o r g y r 6 /p h o e n t x / p h o e n i x e n K / P C B - D e s i g n . p < l f

Figure 15. "Ground" is meaningless for SI and EMC.

or "gro und" - so it is im po rt an t to be very careful to onlyuse them to mean what they actually are (i.e. mechanicalstr uctur es made of metal) rather th an assume they are(mythical) i n f i n i t e sinks for noise currents.

For more de ta il on thi s, see 5.7 of [6] or 2.7.7 of [ 7 ] , also11.1.2 and 11.1.3 of [10].8. A P P L Y I N G T H E S E " EM D E S I G N T O O L S " TO AR E A L - L I F E P C B A S S E M B L Y8.1 Introduction to the exampleSect ions 2 to 7 above have gi ven us a set of E M designtools - r eal ly ju st ment al concepts for how the E M energythat we call our power and signals actually prefers to f l o wto maxi miz e SI, PI and EMC .

Notice that i n sections 2 th ro ugh 7 I intentionally usedvery l i t t l e math; it is not necessary for an unders tandi ngo f these im po rt an t concepts, i n fact, using equations canobscure what is really going on, which every successfulEMC designer learns to "see" w i t h his/her "mind's eye"just by looking at the conduct ive st ru ctu re of a product .

W i t h the compl exi ty of moder n products it is best forthe designer to underst and the concep ts and have "theeye" for the m, leaving the calcul ati ons to the approp riat etypes o f E M f i e l d solvers.

Anyway, now for a real-life example - controiiing theE M emissions and immuni ty of the typical electronicproduct sketched in Figure 16.

To minimize the overall cost of manufacture, this PCBassembly should have good EM C chara cter isti cs, so tha ta lot of money and time does not have to be spent (andadd weight and size) by shielding and f i l t e r i n g it to get itto pass its EMC tests.

Because our E M design tools are ail concerned w i t hcon t r o i i ing E M f i e l d patterns to minimize unwanted"noise" coupling, the exact same tools also improve immun i t y (e.g. maximizing immunity to nearby walkie-talkies,cellphones, CPRS, 3C, W i - F i and Bluetooth transmitters,and also transients, ESD and lightning).

The assumpti ons made in the i n i t i a l des ign of the ex-

11 4 IN TE R FE R E N C E TE C H N OL OG Y

ample were not in accordance w i t h the "Laws of PhysicsBased E M Design Tools" ou tl in ed in 2 th ro ugh 7 above.Instead, they represent what are unfortunately s t i l l commonplace bad practices in many electr onic product designdepartments.

One bad practi ce used in our example is the use ofso-called "single-point ear thi ng/ gro undi ng" (sometimescaiied "star ear thi ng/g rou ndin g"), using OV plane splitsbetween (and on) the PCBs. This is assumed to keepdevices' circulating return currents confined to certaincircuit areas, prevent ing cross talk of noise between the m(e.g. digital noise in analogue ) - but it only works w e l lbelow a few tens of kHz.

Splitting OV pianes ignor es the fact tha t fluctuatingcurrents always divide up acco rdi ng to the admit tanc eso f the various alternative paths, including "stray" pathsthrough the air or insul at io n (see 7.1 above). For thi s reason, since 1980, the author has always found that whenmicropro cesso rs and swi tch -mode convert ers are used,single-point ea rt hi ng/ gr ou ndi ng has always been a baddesign practice for Si, PI and EMC . Other s w i l l no doubtbe able to give examples f r o m before 1980.

Another bad design practice used in the example isthe assumpt ion that ach ievi ng the lowest B O M cost issufficient to produce the most profita ble pro duct . So thenumbe r o f boar d layers and amo unt o f power decoupl ingwas reduced to the minimum that achieved the functionalspecifications. Also, provision has not been made for f i t t ing E MI filters to ail of the cable connections, becausethis would have increased the board's area.

Section 1 ment ioned that relying on achiev ing the l o w est B O M cost to create profi table product s has been knownto be an incorrect practice since 2000. Plain commonsense easily reveals the fallacy inherent in this overly-simplistic approach - we only have to consider a producttha t had a BO M cost t hat was h a l f (or less) o f that of ai lits competitors - but suffered a 100% warranty returnrate. Clearly, this would not be a successful product , sothere is very much more to a product's p r o f i t a b i l i t y thanits B O M cost.

I see many designs i i k e the example in Figure 16 every

Figure 16.Overview of the example PCB assembly.

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A R M S T R O N G

Near- f ie ld i n t e n s i t y

Figure 17. Near-field plot of the example (simulated, or measuredwith near-field probes).

year. They all suffer poor f unc tio nal perfo rmance at f i r s t ,especially poor signai-to-noise (S/N) ratios and unr eliablesoftware that take many design iterations to solve, causingproject delays, increasing costs and reducing p r o f i t a b i l i t y .

Once the f unct iona l problems are solved, they then f a i ltheir EMC tests, requir ing many more design iterationsto solve, causing more delays and more project costs, plusrequiring the addit ion of f i l t e r s and shi eldi ng that increaseB O M cost, weight and size and reduce p r o f i t a b i l i t y evenmore. They also suffer higher-than-hoped-for warrantyreturn rates, w h i c h erode p r o f i t a b i l i t y even more.

A plot of the near-f ield emissions 20 mm above thePCB assembly, at the stage where it meets its functionalspecifications but has not yet been test ed for EM C, isshown in Figure 17.

What do such near-fields mean? This close to the PCBand its com pone nts they are the wa nt ed D M signals, plusD M and C M crosstal k and noise. H i g h levels mean reduc edS/N ratios in analogue ci rcu its , and reduced noise mar ginsi n d i g i t a l cir cui ts - l eading to unrel iable softwar e.

I n EMC testing, high levels of near-fields over largeareas indicate high levels of cond uct ed and radi ated e missions, and corre spon ding ly poor condu cted and radi atedimmunity.

I n real l i fe , high levels of near-fields over large areasmeans a lower prop or ti on of satisfied customers (increasi n g the cost of future sales, because it is easier to sellproducts that customers l i k e ) , and higher levels of warranty costs. A i l causing lower p r o f i t a b i l i t y .

We understand, f r o m the laws of physics discussed insections 2 through 7, that:

ail currents (including D M and C M "noise"currents) f l o w in closed loops

current loop shape and area govern f i e l d patterns currents naturally "prefer" to f l ow in the loops

that have the lowest impedance - hence thesmallest f i e l d patterns and best internal andexternal EMC.

So we can see how to make a number of improvements

to the c i r c u i t design and PCB layout, to reduce the areaso f the D M and C M cur ren t loops and make thei r nearfields more compact.

8.2 Imp ro vement #1: C r e a t e an RF R e f e r e n c eWe replace the multiple PCBs, w i t h a single PCB that has acommon conductor (almost always a OV plane) over its entirearea, w h i c h I shall c a l l the RF Reference. You may chooseyour o wn name for it, as long as it is not "earth" or "ground".

Th e RF Reference in a PCB is at least one s o l i d , continuous, copper plane layer, w h i c h lies unde rne ath - andextends w e l l beyond - ail devices, comp onen ts, traces an dpower plane areas.

There should be no traces "snuck i n t o " this plane layer,and any gaps in it mus t be unavoid able and as sma ll aspossible.

Cellp hone designers fo und that their produc ts' closep r o x i m i t y between 2 Wa tt UH F or microw ave RF trans mitters, naicrophone amplif iers and d i g i t a l processorsmeant that even the clearances ar oun d via holes addedtoo muc h imp eda nce to the ir RF Reference planes, sodeveloped microvia PCB manuf actu ring technology (alsocaiied " H i g h Density Interconnect" or H D I , or " B u i l d Up")that provides 100% s o l i d copp er RF Reference planes.

A n RF Reference achieves v ery low im ped anc e (Z),the value of w h i c h depends on the devices and the EMCreq uir eme nts s pec ifi cat ion to be met - but it mus t alwaysbe i n over the frequency range that must be cont roi iedto avoid causing/suffering E M I .

'T/ie frequency range that must be controlled to avoidcausing/suffering EMI" is ai l of the D M frequencies created i n the devices on the PCB, and ai l of the frequenc iesexisting in the operational envi ron ment and/or in theimmunity test standards (if they r equir e immunity overa larger frequency range).

Designing a profitable produ ct is ail about sa tisfyingcustomers w h i l s t selling a legal product at an overallp r o f i t , and there can be many more EM I requi reme ntsinvolved in satisf ying customers tha n merely passing them i n i m u m requireme nts of the m i n i m u m set of E MC teststandards required for legal sales.

The p oi nt of cre ati ng an RF Reference is that it automatically provides a low-imp edance (hig h-admi ttanc e)ret urn pat h for ail possible power/s ignai/noi se curre nts,and C M noise curre nts o n the PCB. Because it is in veryclose p r o x i m i t y to the PCB's components, devices andtraces, ail these current loop areas are small - just whatwe need for good SI, PI and EMC.

I t is important to realize that we don't have to "make"the return currents f l o w in the RF Reference and so havethe least E and H f i e l d emissions - we only have to providean RF Reference plane an d they w i l l nat ura lly "prefer" tof l ow in it rather than elsewhere! (See Figures 7 and 8).The RF Reference pl ane wor ks best w i t h lower-profilecomponents, so we also replace any t a i l components anddevices w i t h ones tha t lie close to the PCB and its RFReference plane iayer(s).

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F U N D A M E N T A L S O F E M C D E S I G N : O U R P R O D U C T S A R E T R Y I N G T O H E L P U S

See C ha pt er 7 . 4 of [ 7 ] , 3 an d 4 of [ 12 ] and 11.2.2 of [ 10]f o r m o r e d e t a i l o n c r e a t i n g e f f e c t iv e l o w - i m p e d a n c e R FReference Planes i n PCBs.8 . 3 I m p r o v e m e n t # 2 : D e c o u p l i n g t h e D C s u p p l i e sW e d e s i g n t h e d e c o u p l i n g b e t w e e n D C p o w e r r a i l s a n d t h eR F Reference t o a c h i e v e l o w Z , t h e v a l u e o f w h i c h (as for8 .1 ) d e p e n d s o n t h e d e v i c e s a n d t h e E M C r e q u i r e m e n t ss p e c i f i c a t i o n t o b e m e t - b u t m u s t a l w a y s b e i n o v e rt h e f r e q u e n c y r a n g e t h a t m u s t b e c o n t r o l l e d t o a v o i d causi n g / s u f f e r i n g E M I .

T h i s p e r m i t s th e f l u c t u a t i n g D M c u r r e n t s i n t h e p o w e rr a i l s t o f l o w i n m u c h s m a l l e r l o o p s v e r y close t o t h e d e v i c e s t h a t cause t h e m - w h i c h t h e y n a t u r a l l y " p r e f e r " t od o , r a t h e r t h a n f l o w i n g m o r e w i d e l y i n t h e R F Reference- m a k i n g s m a l l areas o f D M n e a r - f ie l d s t h a t create lessC M n o i s e e m i s s i o n s t h a n l a r g e r areas w o u l d .

P C m o t h e r b o a r d s n o w n e e d t o a c h i ev e p o w e r s u p p l yi m p e d a n c e s o f m u c h less t h a n 0 .2 5 m H t o f r e q u e n c i e sm u c h m o r e t h a n I G H z . T h i s is i m p o s s i b l e t o a c h i e v ew i t h l o w - c o s t d e c o u p l i n g c a p a c i t o r s , because a b o v e a b o u t3 0 0 M H z t h e y a r e b e y o n d t h e i r s e l f - re s o n a n t f r e q u e n c ya n d s o a c t i n d u c t i v e l y - t h e i r i m p e d a n c e rises w i t h f r e q u e n c y - m a k i n g l o w - e n o u g h i m p e d a n c e s i m p o s s i b l e .

H o w e v e r , because w e n o w h a v e a R F Reference p l a n ei n t h e P C B , w e c a n p a i r i t w i t h a d j a c e n t p o w e r p l a n e s t op r o v i d e d i s t r i b u t e d d e c o u p l i n g capacitances w i t h i n t h ePCB's f i b e r g l a s s d i e l e c t r i c , w h i c h c a n m a i n t a i n v e r y l o wi m p e d a n c e s u p t o a n y n u m b e r o f G H z .

See C ha pt er 7 . 5 of [ 7 ] , 5 of [ 12 ] an d 12.1.3 of [ 10] f ord e t a i l s o n h o w t o d o e f f e c t i v e d e c o u p l i n g o n PCBs.8 . 4 Im p r o v e m e n t # 3 : C a b l e fi l t er i n gW e a d d d i r e c t b o n d s o r f i l t e r s t o t h e R F Reference o n a l ltraces c o n n e c t e d t o o f f - P C B c o n d u c t o r s , w h a t e v e r t h e i re l e c t r i c a l / e l e c t r o n i c / o t h e r p u r p o s e ( i n c l u d i n g m e t a l m e c h a n i c a l p a r t s ; a n d m e t a l h y d r a u l i c / p n e u m a t i c p i p e s,e t c . ) .

F i l t e r s o n i n p u t s c a n o f t e n b e j u s t a c a p a c i t o r c o n n e c t e dt o t h e R F Reference, b u t f i l t e r s o n o u t p u t s w i l l g e n e r a l l yn e e d a series r e s i s t o r o r s o f t - f e r r i t e c h o k e s o t h a t a d d i n gt h e c a p a c i t o r t o t h e R F Reference does n o t s i g n i f i c a n t l yincrease t h e p e a k o u t p u t c u r r e n t .

O f c o u r s e , w e m i g h t n e e d t o m a k e m o r e c o m p l e x f i l t e r sb y c o m b i n i n g c a p a c it o r s w i t h r e s is t o r s a n d / o r s o f t - f e r r i t echokes a n d / o r C M chokes - b u t t h e r e a r e f a r t o o m a n yd e t a i l s i n v o l v e d t o e v e n s t a r t t o a d d r e s s t h i s t o p i c i n t h i sa r t i c l e . F o r m o r e d e t a i l s o n f i l t e r i n g , s ee C h a p t e r 5 o f [ 7 ] ,2 of [12] or 13.2 of [10] .

These d i r e c t b o n d s o r f i l t e r s a r e p l a c e d w h e r e t h e tracesc o n n e c t t o t h e o f f - b o a r d c o n d u c t o r s , t o p r o v i d e l o w - Zp a t h s f o r C M c u r r e n t s t h a t w o u l d o t h e r w i s e " l e a k " f r o mt h e P C B i n t o t h e c o n d u c t o r s . A s f o r 8 . 1 , t h e v a l u e s o f Zt h a t a r e r e q u i r e d d e p e n d s o n t h e d e v i c es a n d t h e E M Cr e q u i r e m e n t s p e c i f i c a t i o n , b u t m u s t a l w a y s b e i n o v e rt h e f r e q u e n c y r a n g e t h a t m u s t b e c o n t r o l l e d t o a v o i d causi n g / s u f f e r i n g E M I .

8 . 5 I m p r o v e m e n t # 4 : Us i n g m a t c h e dt r a n s m i s s i o n l i n e sW h e r e d e v i c e d a t a sheets s p e c i f y t h e u s e o f m a t c h e dt r a n s m i s s i o n - l i n e s - u s u a l l y f o r h i g h - s p e e d c l o c k s o rs e r i a l d a t a l i n e s - d e s i g n e r s a l m o s t a l w a y s r e m e m b e r t oc o n t r o l t h e i r t r a c e g e o m e t r y a n d m a t c h i n g i m p e d a n c e s .

B u t t h e y g e n e r a l l y d o n o t c o n s i d e r t r e a t i n g a l l o f t h eo t h e r traces a s m a t c h e d t r a n s m i s s i o n l i n e s , u n t i l t h e y a r ei n v e s t i g a t i n g d i g i t a l s i g n a l o v e r / u n d e r s h o o t s , r i n g i n g o ro t h e r u n w a n t e d noises t h a t cause i n c o r r e c t o r u n s t a b l es o f t w a r e o p e r a t i o n la t e i n a p r o j e c t - t h e stage w h e r ed e l a y s a n d d e s i g n changes a r e m o s t c o s t l y .

These o v e r / u n d e r s h o o t s o r r i n g i n g a r e i n d i c a t i o n s o fs t r o n g e m i s s i o n s ( a n d p o o r i m m u n i t y a t t h e e m i s s i o nf r e q u e n c i e s ) , a s s h o w n i n [2 ] . S u p p r e s s i n g t h e m t o g e tg o o d E M C , e i t h e r b y f i l t e r i n g a t t h e i r d r i v e r s o r b y u s i n gm a t c h e d t r a n s m i s s i o n l i n e s t o r e d u c e " a c c i d e n t a l a n t e n n a "effects a n d p r e v e n t resonances, r e s u l t s i n v e r y l o w o v e r /u n d e r s h o o t s a n d n o r i n g i n g . I t also r e d u c e s c r o s s t a l k a n dm a k e s ( b u g - f r e e ! ) s o f t w a r e w o r k v e r y r e l i a b l y in d e e d .

E M C t e x t b o o k s o f t e n m a k e r e c o m m e n d a t i o n s a b o u tw h e n t o t r e a t a P C B t r a c e o r cable as a m a t c h e d t r a n s m i s s i o n l i n e , b u t d i g i t a l d e v i c e r i s e - a n d f a l l - t i m e s a r e n o wg e n e r a l l y s o s h o r t ( t y p i c a l l y < 0.5ns f o r 74-series g l u el o g i c a n d < 0.2ns f o r m i c r o p r o c e s s o r s a n d m e m o r i e s ) t h a ta l m o s t a l l p r a c t i c a l t r ac e a n d cable l e n g t h s n o w n e e d e i t h e r t o b e f i l t e r e d t o s i g n i f i c a n t l y r e d u c e t h e i r f r e q u e n c yc o n t e n t , o r else b e t r e a t e d a s m a t c h e d - i m p e d a n c e t r a n s m i s s i o n l i n e s .

See C hapters 4 . 7 and 7 . 6 of [ 7 ] and 6 of [ 12 ] f or more ond e s i g n i n g w i t h m a t c h e d t r a n s m i s s i o n l i n e s .8 . 6 T h e i m p r o v e d e x a m p l eT h e appearance o f t h e e x a m p l e P C B i m p r o v e d b y 8 .2t h r o u g h 8 .5 a b o v e , i s s h o w n i n F i g u r e 1 8 . N o t i c e t h a t i ts t i l l h a s o n e p l a n e s p l i t , u n d e r t h e m a i n s s a f e t y i s o l a t i o nt r a n s f o r m e r - w h i c h c a n n o t b e a v o i d e d .

D e s p i t e i n c r e a s i n g t h e n u m b e r o f b o a r d la y e r s t o p r o v i d e R F Reference a n d P o w e r S u p p l y p la n e s , a n d a d d i t i o n a lp l a n es fo r c o n t r o l l i n g t r a n s m i s s i o n - l i n e i m p e d a n c e s , a n dd e s p i te i n c r e a s i n g t h e n u m b e r o f d e c o u p l i n g c a p a c i t o r sa n d f i l t e r s , i t i s q u i t e n o r m a l t o f i n d t h a t t h e o v e r a l l costo f m a n u f a c t u r e ( n o t th e B O M ) is l o w e r . T h i s i s becauset h e i n t e r - b o a r d c o n n e c t o r s a n d t h e i r cables h a v e been r e m o v e d - s i g n i f i c a n t causes o f a s s e m b l y e r r o r s a n d r e w o r k ;u n r e l i a b i l i t y a n d w a r r a n t y r e t u r n s .

F i g u r e 1 9 s h o w s t h e n e a r - f i e l d p l o t 2 0 m m ab o v e t h ei m p r o v e d P C B a s s e m b l y , w h i c h n o w h a s o n l y s m a l l r e dareas a r o u n d t h e c o m p o n e n t s . These a r e a l m o s t e n t i r e l yt h e D M f ie l d s associated w i t h t h e w a n t e d p o w e r a n d s i g n a l s , w h i c h w e c a n n o t e l i m i n a t e w i t h o u t e l i m i n a t i n g t hep o w e r o r s i g n a l s t h e m s e l v e s .

R e m e m b e r , a l l f l u c t u a t i n g c u r r e n t s ( w h e t h e r p o w e r ,s i g n a l s o r n o i s e ) a r e r e a l l y E M e n e r g y p r o p a g a t i n g asw a v e s , s o t h e best w e c a n d o is p r o v i d e s t r u c t u r e s t h a ta l l o w these c u r r e n t s t o n a t u r a l l y f l o w i n l o o p s o f l o w i m pedance ( h i g h a d m i t t a n c e ) s o t h a t t h e y n a t u r a l l y create

1 1 6 I N T E R F E R E N C E T E C H N O L O G Y E M C D I R E C T O R Y & D E S I G N G U I D E O D I S


12/12

A R M S T R O N G

Figure 18. Th e improvedexample PC B assembly.

v e r y s m a l l a n d l o c a l f i e l d p a t t e r n s , w i t h great benefits f o rS I , P I a n d E M C . W h e n w e have a c h i e v e d t h i s , as s h o w ni n F i g u r e 1 9, w e s ee v e r y l i t t l e f i e l d - s p r e a d i n g i s seen d u et o C M noise c u r r e n t s .8. 7 Improving by using cable shieldingW h e r e t h e u se o f f i l t e r i n g a n d u n s h i e l d e d cable techniques( C h a p t e r 4 . 4 o f [ 7 ] , 2 o f [ 1 2] a n d 13.1.8 o f [ 1 0 ] ) c o u l d n o tsuppress t h e D M o r C M f i e l d s a r o u n d a cable b y e n o u g h ,s h i e l d i n g m i g h t b e necessary f o r some ( o r a l l ) cables a n d /o r p a r t s o f (o r th e w h o l e ) P C B assembly.9. CONCLUSIONSA l l e l e c t r i c a l a n d electronic a c t i v i t i e s a r e r e a l l y E M energies t r a v e l l i n g as p r o p a g a t i n g w a v e s , a n d c o n n e c t i n g t osafety e a r t h / g r o u n d h a s n o effect o n t h e m s o is u n i m p o r t a n t a n d unnecessary f o r S I , P I a n d E M C .

W e c a n easily d e s i g n c i r c u i t s a n d PCBs t o create s m a l l ,l o w - Z c u r r e n t l o o p s f o r b o t h t h e w a n t e d D M a n d t h e s t r a yC M c u r r e n t s , t h e E M waves n a t u r a l l y p r e f er t o f l o w i nthese r o u t e s . S o, b y w o r k i n g with t h e l a w s o f p h y s i c s , w ea u t o m a t i c a l l y achieve v e r y compact f i e l d p a t t e r n s , w h i c ha r e best f o r i n t e r n a l a n d e x t e r n a l E M C a n d f i n a n c i a lsuccess.

Because these techniques c o n t r o l f i e l d p a t t e r n s t o m i n i m i z e u n w a n t e d "noise" c o u p l i n g , because o f t h e p r i n c i p l eo r r e c i p r o c i t y t h e exact same techniques also m i n i m i z es u s c e p t i b i l i t y , f o r example m i n i m i z i n g u n w a n t e d "noise"c o u p l i n g s .

T h e p r i n c i p l e s o f g o o d d e s i g n techniques f o r S I , P I a n dE M C a r e v e r y clear, easy t o u n d e r s t a n d , a n d easy f o r e v e r y o n e t o i m p l e m e n t at l o w cost i n p r a c t i c e . P r o d u c t s r e a l l ya r e d o i n g t h e i r best t o h e l p u s pass E M C tests a n d meetE M C r e q u i r e m e n t s p e c i f ic a t i o n s - a l l w e need t o d o i s g ivet h e m a l i t t l e h e l p , f r o m t h e s t a r t o f t h e i r d e s i g n process.10 . REFERENCES

[ 1 ] . A r m s t r o n g , K . " K e y k n o w l e d g e f o r t h e e f f ic i e n t d es i g n o f electronicp r o d u c t s a n d t h e i r E i V lC - t h a t w e w e r e never t a u g h t a t u n i v e r s i t y " ,K e i t h A r m s t r o n g , ANSYS Seminar " N e . x t G e n e r a t i o n S i g n a l I n t e g r i t y

Figure 19. Near-field plot of the improved PC B assembly (simulated,or measured with near-field probes).

a n d E M I S i m u l a t i o n " , 2 3 r d M a r c h 2 0 11 , O x f o r d , U K . w w w . a n s y s . c o m /staticassets/AKiSYS%20UK/statica,ssets/I

fundamentals of emc design

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