Shielded vs. Unshielded Square Magnetic Field Loops for EMI-ESD Design and Troubleshooting

8/10/2019 Shielded vs. Unshielded Square Magnetic Field Loops for EMI-ESD Design and Troubleshooting

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- conductor nearby the current carrying conductor/ such as a side of thesquare loop above/ will pick up an open circuit voltage of6

e& 7 di dt !)#

where is the mutual inductance between the current carrying wire andthe nearby wire per unit length.

Since 8 and are constants/ then e4 and e& are only di=erent by aconstant. must be smaller than 8 for two parallel wires !due tothe magnetic >u0 that >ows between the wires instead of enclosingboth#/ so e4 is a lower bound estimate for the magnitude of e& and hasthe same wave shape.

?sing that principle/ a simple square loop/ such as the one above/ can beused to estimate the voltage drop across conductors. @hen the probe isheld up to a conductor carrying high frequency current/ the probeAs opencircuit output voltage is a lower bound for the voltage between thecorners of the probe along the current carrying conductor as measuredby the magnetic "eld captured in the area of the loop.

The probe should be connected to an oscilloscope or spectrum analy1er

using a coa0ial cable terminated in its characteristic impedance. Thisresistive load on the loop in combination with the self inductance of theloop forms a low pass "lter on the probe output. 3or a loop with sides of4 cm/ this corner frequency will be between & and )


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Figure ": #uare loop rom a coa$ cable unshielded %le t& andshielded %right&.

3igure ) shows the construction steps for quickly and simply making ashielded magnetic loop in the lab.

Figure ': Building a shielded loop.

Step 46 Take a piece of sti= copper wire or rod !4, gauge is best for"tting in B5C connector#. The wire is covered with heat shrink tubing andthen copper tape is wound around the heat shrink tubing so that it iscovered with three layers of tape. 2n that way/ when the wire is bent toform a square loop/ the outermost layer may crack slightly. Since the


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crack is only on the outside of the bend and it is small/ it does not posemuch of a problem because the gap will be closed by the underlyinglayers.

The characteristic impedance of the coa0 formed by the foil and sti= wireis probably not + Ohms like the feed cable likely is. But/ it does notmatter much since the two parasitic transmission lines formed by thetwo halves of the loop and their shields are un(terminated at the gap inany event.

Step &6 Put a second layer of heat shrink tubing over the assembly.

Step )6 The loop is then bent and one end inserted into a B5C connector.

Both the other end of the wire and copper tape are soldered to the sideof the B5C connector.

Step *6 5e0t/ the end of the B5C connector and two ends of the loop arecovered in copper tape which is soldered to the copper foil of the loop.

The added copper tape may also be soldered to the B5C connector aswell.

Step +6 3inally/ the assembly is covered with heat shrink tubing to make

the "nal product. The position of the gap is very important to theperformance of the shield. - shielded loop that is not symmetric !e.g.gap at base of loop# will be somewhat sensitive to E "elds. E "eldinduces current in shield so there is voltage drop across shield that willbe induced into center conductor loop %D'.

Better shielding e=ectiveness is obtained if the gap is located in themiddle of the loop as shown in 3igure *. 2n that way/ electric"eld symmetry is obtained.
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Figure (: #uare loop rom a coa$ cable unshielded %le t& and

shielded %right&.

The loop is formed by making a square from the coa0 with a gap placedsymmetrically in the middle of the loop as in 3igure *. The center wire isconnected again to shield in point !-#.

The loopAs shielding may be tested as described in %D' by applying an

electric "eld source to the loop. The loop should be least sensitive overthe gap with the ma0imum sensitivity $ust o= the gap in either direction.

The sensitivity should gradually fall o= as the electric "eld source ismoved toward the side of the loop !at the B5C connector# opposite thegap.

#uare hielded Loop )ith emi*rigid +oa$

One can minimi1e the work required to build a shielded loop by buying ashort length of small semi(rigid coa0ial cable with S - connectorsalready mounted on each end %F'. The assembly can be cut in half tomake two shielded loops saving the trouble of mounting the connectorson the semi(rigid cable.

?se a small diameter semi(rigid cable as the smaller the semi(rigid coa0diameter/ the better coupling between the center conductor of the coa0and the ad$acent circuit.


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3or our purposes/ one can think of the loop as starting with a straightlength of semi(rigid coa0 of small diameter with an S - connector onone end and shorting the center conductor to the shield with solder atthe other end. Then the loop is bent around to form a square !beingcareful not to bend the coa0 too sharply at the corners# and the soldershorted end is soldered back on the coa0 so as to form a squaresymmetric loop. - small gap is made in the shield in the middle of theside opposite the feed line. 3igure + shows a square shielded loop built%D' into a plastic housing which has been split to show the loop inside.

Figure ,: +uta)ay -ie) o a #uare hielded Loop

Shielding against electric "elds is best achieved if the "eld is symmetricaround a line from the solder $unction to the gap in the shield/ acondition that is met when a shielded loop is used to measure a "eldmuch further from a source than the si1e of the source itself.

To help ensure electric "eld symmetry when the loop is used on the

surface of a circuit board in the near "eld/ the gapped side should beheld against the board with the loop itself perpendicular to the board.-nd therein lies the main reason for using a square loop/ most circuitboards are >at and one side of the loop can be held directly against acircuit board resulting in better coupling to the circuit than a round loopof the same si1e would give.

The performance of a loop constructed in the previous section !3igure )#should compare favorably with one made from semi(rigid coa0ial cableup to the "rst resonance of the loop. The "rst resonance occurs at the


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frequency where the circumference of the loop is one half wavelength. -tthis frequency/ the parasitic transmission lines formed by the two shieldsand the underlying sti= wire are one quarter wavelength un(terminatedstubs.

Square shielded loops can be used both to measure many kinds ofsignals and to in$ect small :3 signals !G dBm # into circuits. Some of thetechniques involve coupling high voltage current short pulses into acircuit. 3or that application shielded loops made from small semi(rigidcoa0 are not a good option for this purpose because of possible voltagebreakdown in the coa0 and even heating under some conditions. 3orlarge pulses/ use unshielded sti= wire loops.

!arasitic +oupling Bet)een Unshielded ire Loops

?nshielded wire loops are ine0pensive and easy to build but capacitive!electric "eld# coupling has always been a concern when using thesesimple loops. Coupling between unshielded wire loops is investigated/included parasitic capacitive coupling.

3igure , shows an overall view of the test setup comprised of a pair ofsquare wire unshielded loops connected to an -gilent 54FF,- spectrumanaly1er set up to perform a two port insertion loss measurement. Thetwo loops are about one inch/ a few cm/ on a side and they arepositioned end(to(end.
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Figure /: Measuring loop to loop coupling or a pair o s#uareshielded loops %end*to*end& in re0ersed direction.

2n 3igure ,/ the loops are reversed in position/ that is the 9 side of oneloop !center conductor of B5C# is opposite the H of the other loop!ground side of B5C#. This will be referred to as the IreversedJ direction.By inverting one of the loops the InormalJ direction is obtained !9 sideto 9 side of the loops#. Kata is presented for both directions.

2f only magnetic "eld coupling e0isted between the loops/ the changeresulting when one of the loops is reversed would be a 4D degree phaseshift in the output/ which would not change the spectrum analy1er plot.

2f signi"cant capacitive coupling e0isted between the loops/ the output of the receiving loop would be the combination of the inductive andcapacitive components. Since the phase of the inductive component isreversed when one loop is reversed but the phase of the capacitivecomponent is not/ the spectrum analy1er plot will be di=erent.
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Figure : 2esponse end*to*end loops re0ersed and normal

directions.

3igure D shows the overview of a related case where the loops areoverlapped to insure ma0imum coupling. 5otice that the loops arearranged in the normal position as opposed to reversed in 3igure ,/ thatis the sides of the loops connected to the B5C center pin !or shieldconnection# are both on the same side.


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Figure 3: Measuring loop to loop coupling %loops o0erlapped&.

3igures F!a# and F!b# !page D # show the coupling from 4


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Figure 4: 2esponse o o0erlapped loops re0ersed and normal

directions.

3requency response plots of the coupling between small simple wireloops are reasonably >at.

Parasitic coupling electric "eld Shielding in square shielded magneticloops.


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@e have introduced before that shielded magnetic loops are used toreduce electric "eld coupling to the loop and square magnetic loops areuseful for coupling signals into a PCB or measuring noise in a circuit.


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3igure 44 shows the resulting plot of insertion loss !un(normali1ed#between 4


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for the total capacitance in the tuned circuit of 3igure 4&. -dding ferriteto the coa0 cables feeding the loops did not change the characteristics of 3igure 44 so the current loop of 3igure 4& is the controlling feature.

Current >owing in the center conductor of the driven loop generatesinductive voltage drop !8di dt# around the loop. - shielded cable is nearlyan ideal transformer / so the voltage drop on the center conductor undereach of the two shield segments is magnetically coupled into the shieldsegments as di dt. The mutual inductance/ / between the centerconductor and the shield/ is the inductance of the shield itself.

This driving voltage on the two shield segments causes current to >owaround the four shield segments coupled by the parasiticcapacitance between the loop shields as shown in 3igure 4& thus drivingthe resonant circuit. 2n this discussion/ we are treating the circuit ascomposed of lumped elements since each segment and the loopsthemselves are small compared to a wavelength at *


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Figure 1': Measuring loop to loop coupling or a pair o s#uareshielded loops at one cm spacing including a close*up.

The resulting two port insertion loss plot is shown in 3igure 4*. 5ote thatthe resonant dip has moved to about *F+


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Figure 1/: 6n7ected signal or unshielded loop normal orientation

and re0ersed 1358

Contrast the responses for the unshielded loop to the responses in3igures 4 !a# and 4 !b# for the shielded loop. 2n both 3igures 4 !a# and4 !b#/ a resonant dip in the response is seen similar to that shown forcoupling between shielded loops in part M. 2n this case/ the resonance isdue to the sum of the inductance of the shields of the loop and the

inductance around the split in the ground plane interacting with thecapacitance between the shields and the ground plane of the board. -s


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one would e0pect for a shielded loop/ the plots in 3igures 4 !a# and 4 !b#are not very sensitive to the normal and rotated positions of the loop.

Figure 13: Measuring the coupled signal into a circuit in the timedomain

One can conclude from the above plots that the unshielded loop worksbetter for in$ecting signals into a path crossing a ground plane split thandoes the shielded loop. Surely this result holds in general for in$ectingsignals into circuit boards with ground and power planes.

+A E ": Measurements in the 9ime omain

- case is shown where using both unshielded and shielded magneticloops to in$ect signals into a path on a circuit board results in an in$ectedsignal that is about the same for both loops. 3igure 4D shows a squareunshielded loop held ne0t to a path crossing a break in the ground planeof a test board.

The in$ected signal from a 3ischer Custom Communications TL(E3T pulsegenerator connected to the loop was measured at the B5C connector onthe board !left side# using an oscilloscope for cases where the loop ispositioned as shown and for a 4D degree rotation of the


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loop. Bandwidth of the oscilloscope used was +


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the plots is not signi"cant enough to make much di=erence when usingpulse in$ection for troubleshooting designs.

The plots in 3igures & !a# and & !b# for the shielded square loop are also

very similar as well as having about the same rise time for both plots. The amplitude of the in$ected pulse is about & less because thedistance between the center conductor of the semi(rigid coa0 formingthe loop is further from the path on the circuit board due to the diameterof the coa0 and the thickness of the plastic housing. The slightimprovement in matching of rise times is not signi"cant enough towarrant the e0tra complication and cost of shielded loops. 2n addition/ ifthe scope had greater bandwidth/ the resonance at about ,


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+onclusion

Capacitive coupling from an unshielded loop is not always a problem thatrequires the use of shielded loops to solve. On the contrary/ unshielded

loops often work as well as shielded loops as was demonstrated by pulsein$ection in this e0ample.

This article on square shielded loops has shown that unshielded loops areuseful in many cases and for in$ecting signals into circuit boardsspeci"cally. ?nshielded loops can even outperform shielded loops insome applications. Liven the ease of constructing an unshielded loopand its low cost/ this is an important result.

2e erences

4. Smith/ Kouglas C. 4FF&. High Frequency Measurements and NoiseIn Electronic Circuits. 5ew ork6 Man 5ostrand :einhold. 2SB5 Q (**&(,),(+.

&. Ott/


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Douglas C. SmithMr. Smith held an FCC First Class Radiotelephone license by age 16and a General Class amateur radio license at age 12. He received a.!.!.!. degree "rom #anderbilt $niversity in 1%6% and an M.S.!.!.degree "rom the Cali"ornia &nstitute o" 'echnology in 1%(). &n 1%()* he

+oined ,'-' ell aboratories as a Member o" 'echnical Sta"". He retiredin 1%%6 as a /istinguished Member o" 'echnical Sta"". From February1%%6 to ,pril 2))) he 0as Manager o" !MC /evelopment and 'est at

,uspe Systems in Santa Clara* C,. Mr. Smith currently is anindependent consultant speciali ing in high "re3uency measurements*circuit4system design and veri"ication* s0itching po0er supply noise andspeci"ications* !MC* and immunity to transient noise. He is a SeniorMember o" the &!!! and a "ormer member o" the &!!! !MC Societyoard o" /irectors. His t !"#i!$% i#t & sts i#!%'( "i)"

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Shielded vs. Unshielded Square Magnetic Field Loops for EMI-ESD Design and Troubleshooting