Probing Magnetic Configurations in Co/Cu Multilayered Nanowires
Probing the Magnetic Field Probe
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Probing the Magnetic Field Probe By Roy Ediss, Philips Semiconductors, UK. Introduction Commercial and handcrafted probes similar to those shown in Fiure ! are commonly used in E"C dian ostic wor#, but ha$ e you e$er conside red how they ope rate % &he man etic field probes are made in the form of a loop with an inherent electrostatic shield, enerally from '( )hm semi*riid coa+ial cable. &hey $ary slihtly in confiuration and in characteristics, but essentially they are electrically small shielded loop antennas deri$ed from the antennas used since the !-(s for radio communication and direction findin /!,-0. Figure 1. Various shielded loops. How they work Refe r to the diar ams of the $ari ous 1*fiel d loop probes show n in Fiur e -. &he foll owin e+planation can be applied in eneral to all the probes, but the common probe type -2a3 will be considere d. &he e4ui$a lent circuit diaram is shown as Fiure 5, which has numbered locati on points correspon din to Fiure -2a3 /5,60. 7n eleant arran ement e+ists where electric fields may impine on the outer she ath but are shielded from the inn er sinal line. 7 small ap in the outer sheath is howe$er always included, pre$entin a shorted*turn to manetic fields. 7 ma netic field passin throuh th e probe loop enerate s a $oltae accor din to Fara days law, which states that the induced $oltae is proportional to the rate of chane of manetic flu+ throuh a circuit loop. 7t $ery low fre4uencies a $oltae would be induced directly in the internal loop conductor, but the copper sheath is 4uite a ood shield to manetic fields at fre4uencies e+ceedin the low K18 rane. So at hih fre4uency , a $oltae is then induced preferen tially in the outer sheath loop, and this appears across the sheath ap 2points ! 9 :3 /'0. &he metal sheath thic#ness is se$eral s#in depths, so this pre$ents direct interaction between currents on the e+ternal surfa ce and internal surfa ces of the shield. &he '( )hm tra nsmission line, formed by the inner surface of the sheath and the inner conductor, is then dri$en by this $oltae and is terminated by the '( )hm measurement system 2points 6 9 '3. 7n analo y to the effect described abo$e may be made, by consideri n a circuit loop arran ement where a coa+ial shield termina tion is unconnected at one end 2see Fiure 63. )r an impe rfec t coa+ ial shie ld terminat ion , wher e some ro und curren t may cre ate a $olta e across an inducti$e path such as a ;Pitail< round connection.
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Figure 2. Loop configurations.
Figure 3. Equivalent circuit diagram for type 2(a) 2(!).
Figure ". #ircuit analogy of pro!e construction.
Type variations
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Probe type 2-a3 is common and a commercial $ersion is a$ailable /:0. 7 '( )hm chip resistor termination is sometimes included 2at points - 9 5 as in -2b33, to terminate the short lenth or half loop of coa+ial transmission line in a balanced way and help pre$ent contributin effectsfrom any unbalanced cable currents. &his chip resistor will howe$er somewhat reduce the lowfre4uency sensiti$ity when the probe is used as a pic#*up.
=hen the outer sheath is formed in two hal$es as in Fiure -2b3, the outer surface of the shieldmay then be considered as a balanced two*conductor transmission line arranement />0. &hisarranement of the electrostatic shield was particularly important for direction findin as ithelped a$oid distortion of the loop antenna directional pattern. 7 "oebius $ersion of the balanced probe type -2b3 with both sinal lines connected to their opposite sheath at the central ap, is claimed to ha$e twice the output of the Kin probe type-2d3 and is noted in /?0. &he Kin type probe is named*after R. =. P. Kin who discusses loopantennas and probes in / 9 !(0. 7n interestin 5*layer Printed Circuit Board $ersion of theKin probe type -2d3, is described in /!!0.
Sensitivity and performance comparison
&he probe types shown in Fiure - are not normally utili8ed to determine circuit current by
application of a transfer impedance@ they are enerally used as a dianostic aid to detect thesource of sinals, or as local field*eneratin source. Performance of the probe types hastherefore been compared in a representati$e way, by usin the microstrip line techni4ue asdetailed in /!!0 but with the probe at a fi+ed distance of !(mm from the microstrip line, as shownin Fiure '.
Figure $. %ro!e sensitivity test arrangement. %d & 'dm.
&he transfer function of this arranement with $arious probe types may be compared in Fiures: to !(. 7 throuh*connection was normali8ed to the reference le$el at the top of the plot prior to measurement. 7ll the handcrafted probes were formed with 5.:mm )A semi*riid coa+.ote their uniform flat response abo$e !'("18 due to the low pass filter effect described below. Aue to the nature of the manetic field within it, a coa+ial cable transmission line can beconsidered to form a !! transformer with its shield and inner conductor. &herefore a linear impro$ement in the probe open circuit $oltae and low fre4uency sensiti$ity is created with more
inner sinal line turns. But since the probe inductance forms a low pass DR filter with the '()hm measurin instrument impedance, the loop current and the hih fre4uency beha$iour isthereby notably limited by Ds abo$e a corner fre4uency to a flat response. See Fiure !!,which shows the effect of two inner conductor turns on performance. &he probes are normally reconised to ha$e a self*resonant fre4uency due to the loopinductance and parasitic capacitance between the centre conductor and outer sheath. &hesmaller the probe loop diameter, the hiher this self*resonant fre4uency, but coa+ diameter andconstruction type also ha$e an effect. For a 5(mm diameter type -2a3 the resonance occurs ataround ??("18 2as seen in Fiure >3. =ith type -2b3, -2c3 and -2d3, no resonance was seen upto a fre4uency of !'(("18 2see Fiures ?, and !(3@ but type -2c3 had noted sensiti$ity due tocapaciti$e influence when handlin probe nec# and cable. From those assessed, types -2b3and -2d3 are therefore considered to be the preferred $ersions of manetic loop probe for hih
fre4uency circuit in$estiati$e use.
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Figure *. E+#, -* 3'mm dia. loop. Loop type 2(a)
(/n0non semirigid coa ,). #entral gap.
Figure 4. 3'mm diameter loop type 2(a) formed ith3.*mm , semirigid coa. #entral gap.
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Figure 5. 3'mm diameter loop type 2(!) formed ith 3.*mm , semirigid coa. #entral gap plus
terminating resistor.
Figure 6. 3'mm diameter loop type 2(c) formed ith 3.*mm , semirigid coa. 7ap at nec0.
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