CA2035726A1 - Circuit arrangement for superconductive quantum interference detectors (squids) - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A sensor system for magnetic signals, especially for biomagnetic signals such as signals from the brain or the heart in the form of a SQUID has two superconducting magnetic field detectors connected in parallel and a circuit including a correlator connected to the sensors for eliminating from the output uncorrelated noise.

Description

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CIRCUI~ ARRAMGEMENT FOR SUPERCONDUCTIVE QUANTUM
INTERFERENCE DETECTORS (SQUIDs) SPECIFICATION
Field of the Invention ..
My present invention relates to a circuit arrangement -for superconductive quantum interference detectors (SQUIDs) with the electronic circuitry for generating outputs therefrom and, more particularly/ to improved magnetic sensors utilizing superconductive detectors, especiallv for low frequency operation, for example, to detect biomagnetic signals such as heart and brain activity. .

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Wi h the develop~rlt o~ hlgh temperature ~uperconductore ( see High-~e~p~rature Superaonduator~ On th~ Road to Applic~at~on~, Chan~i¢~l and Englnesring Newo, 27 November lg89, page~ 9~E~), lt h~ become posf;lble to pr~id superconductive quantum inter~er0n e d~t~ctor~
~SRUIl)~) with op~rating temperature~ ln the, temparatur~3 range of ~he boilin51 point o~ llquid nitrogen and higher temperature~.
Such sQuIn& are provlded in various embodiment~ and con~i~aurations, including ~ingla-hole 8QVIDs or two-hole SQUID~ in gradiom2tsr6, both in ~a~8ive bodies or by thin teahr~ ues. ~hen the ~nh~rent nois~ o~ ~uch ~sansors at ~requencie~ abov~ about 100 Hz has attraatively low ~15 valua~, the ~SQUID can be u~0d at low-r ~ignal treclu~3n~::ies .
: :~ in: tha ranye oP ~oYeral HZ and le~ Por avaluating bio~agnQtic ~ignals ~uch as blomagn~t~c ~gnal~ o~ th~
haart~ or brain activlty. Wi~h ~u~h applications it ha~
be~n ~oundf however, that decr~a~in~ *requ~3n¢i~ ar~
ao ~ as-oc1ated with inareasing levell6 of~ ~xtxin~i~ noi~e.

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~: ; It i~ herqfore , an ob~ ect oP the ir~vention to :: :
prov1de a aircuit arrangement which ~xpands th~ Pield of appllcation o~ ~nagn~t:l:a detectora ae descrlbed ln th~ .
25 ~ ~bove-idonti~ied ;application.
An~equal.1y important ob~e~t i~ to prov:lde a magn~ic sor ~yet~3m uti11s~ g 8QUID~ and whioh i~ oharacterized .
by a reduced 1e~Y~1 0~ noi~e, i . a. has an output wh1ch 19 :~ ~: not mat~ria11y e~Qc:t~3d by noi~
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It i~ still anoth~r ob~ect o~ the lnvention tc) provlde a circuit arr2~rlgem0nt :eor SQUIl)~ which reduc~s the e~ct o~ noi~ on ~ny output ~ignal~ produced therQby.

~1~ .
The~e ob~ect~ are attained, in ~cc~rdance with the invention by prc~Yidin~ two SQUID detel::tors, e . g . ~n~ore oi~
the typ~ described in the above~ nti~i~d ¢opending application irl parall~31 with one another i~nd providing the ;~
~3lectcric airouiit:ry with whi~h the~e deteators ~re ¢r~rln~cted, with a aorrelation c:lrcuit having th~ e~Pe¢t o~
removlrlg nonaorrQ~ated sxtrin~ noise ~rom thQ nutput ~ignal~. ~ta~ed oth~ e the aorrelation ¢ixcult i6 able ; ~ .
~o d~t~ extrinsl~: noi~ o~ the ~wo detector~; which cannot . -be cbrrelat~d arl~C thu~ re~ove~ thi~ noncorralated extrln~
:
~ noi~ o thalt ths re~naining ~ignal-to-tloiss r~tio can be co~paratively high.
or~ p~rticularly, the ~agn~tic ~en~or SQUID ~y~tem o~ the lnvent:lor~ can ¢omprl~a two superconducting qtuantum inter~er~noe detectors ~ .
:~ 20 ¢onra@ct~ad. in parallel and having re/lpective output ~ignalF3 ~; inaluding measurement ~ignals, inharant nol~ and, upo~
: ~ operation at cartain ~re~encie~, f3xtxin~;1a noiset and :: :
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circuit mea~ inoluding a correl~tion electronic circuit connected to ~oth o~ tha l~uperconducting quantum interferenc~ d~tector~ ~orm~ng an output ~or o~tput ~ignal~
Pro~ the measureme~t signal~ o~ t]he parall~l ~onnected ~uperconducting quantu~ ~nter~erence detector~ for eli~nating ~r~m the output, ~lgnal~ reipre~énting uncorrQlated noi~e ~ignal~ ~rom tbe ~uperoo~ducting quantum intar~erence deteotor~.
Th~ principlQ~ o~ correlation el~ctronlc~ and ~::
~iorr~latlon analy i~ ar~ wall known tse~ Engineering ,: .
AppIlcations o~ Correlation and 8p~atral Analy~
Jullu~ S. Bendat and Allan G. Piarsol, John Wiloy and So~
Naw YorkO 1980). Utiliz~ing theso prinaiplee ~nd ~uoh circuitry, I am abl~ to extract the uncorrelated n~i~e ~5 whil~ leaving the u~eSul compsn~nt o~ the output signal .
lntaot. Thli~ prinolpl~ oan b~ ~ployed in variou~i ways in accordanc~ wlth oorr~lation analy~l~, e.g~ ~y a time-based ~ -; extraction o~ the nonoorrolated noise utiliiæing ;~.
utocorrQlation and crosis corr~lation principles, or ~y 20~ ;~ ensemble an~lysis in a multi-channel system. The latter :
meth~d i5 0~ greàte~t advan~age wh~n th~ time con~itant ~or , th~ analy~ ust be i~all ~y ao~parl~on with the ~ignal tim~ constant ~r ln the aa~e o~ nonpsriodia ~ignal~.

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18004 ~ 3 For high t~n~paratllr~ ~up~rconductor~3 the sen~itivity can be e~pecially increa~ed by thla ~ e of th~ correla~cion proc~:~s c)f the invention. The higher operatlng temp~3r~ture~3 o HTSC S5~UID~3 (hiyh temp~ratur~
6up~rconduct~ng SQUIl)a~ are~ a~sooiated with higher th3rmal inh~rent noi6e and, when correlation analysit3 i6 u~d as descrlbed to reduce th~ e~fsc:t Qf th~ noiF~6~ on the output lgnal and h~nc~ to inaxea6~ the ~ignal ~o no:Lsel ratio, ~uch H~SC ~QUID~ aan be employed everl ~or tha detection of brain ~ignal~ to which the high th~rmal noise lev~l has bsen a barrier heretofore.
The use o~ the c:orrel~tis:~n technlqu~ o~ the invention al~o represent~ a ~igni~icarlt improvement for æQUID~ of t3~ more cla~sical ~uperconductor~, being . .
a8~30aiated wlth an improve~ent in 8en ivlty to leval~
which could not be attainable hereto~ore.
Pre~erably the SQUID~3 whic:h are uF.13d are thos~e : ~ ~ de~;cribed in t~ above-iderlt:l~Eied application and (;erman Patent Document 39 31 441. q!he~;a SQUID-type ~3en~0r~
provide uperconductlns~ rlng~ in a tank airauit which can include a Jo~3ph~30n junotlon or element.
The tank c~irc::uit C~ aompri~s a strip reso3~ator in the ~orm oP a ~uperc:onducting ~tr~p havinçl an opanlng which ~OrmB the superco~duclting rlng in which th~ Jo~eph~on ~5 ~ elem~nt i~ incorporated. ~ore ~peai~ically, the SQUID
oo~npri~ a ~up~rcorlduativl3 ring having a Jo~3eph~0n :~ ~l6smenl~ or a p:Lurality oi~ Jo~eph90n ~unction~ ~orming a Jo~3eph~on QleT~er~t incorps~rated th~rein and which i~ coupled l~o an electric o~ atory or tz~n~C circuit.

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1 3 004 ~ 3 Advantag~ou~sly, ac:cording to thç~ invention the electr~ c o~cillatory circu~t cDmp;ri~e~ a p:Lece o~ a etrlp conduGtor ~orming a re~orla'cor ~or s~anding elec:tr~ cal waves on an appropriate aubatrat~ and w~hich iR ~orlD~d with an opening traver6ed by lthe magnetlc ~lux to be measur~d, the opening being bounded by z~t lea~t one Josephson elemerlt ~30 that the~ relulting ~uper¢ollduotiv~ ring ~ ~or~ed around thi~
opening ansl inoludin5~ the Joseph;0r3 21ement~ i~ integrated in th~ strip r880n21tOr ~ormed by that condu¢tor. The ~02~ephson ~lQm~nl: i8 ori~nted ~30 lthat it exterld~3 als:~ng an axl~ oP th~ ~trip re~onator, na~ely, lt~ cent~r l ine or middla axis or a ls~ngltudinal axis thereo~ and th~ couplirlg o~ th~ ~trip resonator to th~ evaluatia~g electronic ciroll~try, i~e. tha o~cillation da~ping or voltage drop : 15 ~ d~t~tor i.8 capacig~ve and ~ e~fect~d through a further - .
piec~ D~E a etrip conductor lying along th~ ~trip re~onator.
~dvantag~aouE~ly to the invQntion, there~ore, the ~ nl3or has it~ fiuperconductlve ring and tank circuit ~ormad a~ a ~3lngle ~tructural unit and con~t ~ tuted by the ~trip 20~ re~o3~ator E~O that the 6trip re~onator itsE31~ ~orm9 a tank clrcuit and it~ boundary arouncl th~a operling c:on~titutes the . ~
uperconductiv~ rirlg in whlch the Jo ekh~-:>n ~1l3men~ iE~ ..
incorporated. : :.
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Since the Josephson element lies on a longitudinal a~i~3 of the ~strip re~30nator, ~ l3u~eZiciently hlgh ~reguency current can b~ fed thereto to 2nsur~ that tl~er~ will flow in tha Joseph~on ~uns:tion th~ requislte current ~or the measurement of the magnetis: flux.
For operation o~ tho Gen~Or, o~ cour~, it i6 nec:essary to ~:upply the ~trip re60nator with a resorl~nt ~reguency which can genarate E~tanding wave~; in the ~trip re~30n2ltor. For a given length of ths strlp reson~tor, this ~0 can be ea~3ily done by ad~u~;ting the applied r2~;0nant ~requerlay until ~tanding wave~3 d~3velop in the ~trip re80nator, utillzlng, ~or exampl~, a variable ~requency ~ourca ~
However, it is also po~sibla 'co operat~ with a fisc~d frequency ~ource and to ad~ust the ~ engt~ oP the strip re~;onator 50 that ~ita slectrical length 1 wiLll correspond -;.
to an odcl number multiple o~ th~ halP wavelength s:~f th~
oporating ~requenay .
To in~orporat~ the Josephson ~unotion ira the 20 ~ ~uperconductiv~ ring E;O it will be maximally e~ctive, it i~ po~3itloned in the ~;trip resonator at a locatlon c~f maxi~num current ~low in the strip x ssonator .
It ha~3 bl~en Pound to bs de~irable, ~or good coupling or th3 ~trip re~onator to the ~lsctronia circuitry ror deteating damplng and henc:e outputtlng th~ me~ur~ment of magn~tio i~lux and which operate8 at xoom temperatur~, ~r-inaglk~r re~rred to a~ the room tempe:ratur~ eleotrs>nia alrc:ultry, to utlllze ~or thi~ coupling the ic'urthl3r pi~ce Or ~tr~p oondllcltor and to loca~e ik in a r~ion c: ~ ~he strip re~;onator at which a ~tanding wav-3 antinode i8 ::
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lB004 ~ 3 ~
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A nu~ber o~ piece~ o~ the strip conduator ~erving ~or coupling purpoee~ oan be ~pplled to th0 strip r~sonator at ~ ~ultipliaity o~ 8uch ~tanding w~va antinode~.
The ~en~or ~y~t~m o~ the invention, by comparl~on with sarll~r seneor~ utllizing a rlng ~nd a JosQph~on ~unotion or ~le~snt incorpor~t~d therein, ha~ hlgh~r :~
sen~itivity to ~easurem~nt~ o~ chang~ in the magnetic ~lux. As a rule, th~ mea~urem~nt i~ ~ot a meaaure~Qnt o~
absolute values o~ magnetic flux.
Io More ~pecifically, eaah d~teotor o~ the pre~e~t By~t~ can comprl~es a ~trip ~upercon~ctor having a longitudinal axis .
and for~ing a strip re~onator ~or standlng electriaal wavss , . -:~ ~ at;an operatlng fr~uency, thc ~trip r~sonator ~eing formed ~wlth an openlng and being ~ravar~able by the magnetic ~lux nd~bounded ~y ~ supsroonductive ring o~ the ~trip -~
up-roondu~tor having a Jo~eph~oD ele~nt incorporat~d .
tha~e~in and ~t a boundary o~ the opening BO thàt the ring ~ 1ntegrated in the ~trip resonator, th2 JQ6ephson ~lement :~ ~20 lying along the axl~ oP s~rip r~onator, and at least on~
further ~trlp conductor ly~ng Gn tha ~trlp re~onator and ; ~ aapaoitively coupl~d therete ~or oonnecting the re~pective detaQ~or to the clrauit mean~. .

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U~;ually ~en~ors of this type ars use~ul only for determining c:hange~ $n the magnetlc ~lux by moving th~3 ~en~or along a body or ~y6tem along which th~ magnetic ~ield varie~ or by leaving the s~nsor in placa in a cas~
whlsre magn~atic variability r88ult~ ~rola the mag~3'cic Xleld source. In the mea~ur~3ment rang~, therePore, a homogeneous magnetic f ~ eld can ba detected by a lack o~ relative change o~ the magnetic Plux by ~he ~ea~urQment.
When a nonhomoqeneou~ magnet1G ~Eluac i~ to be detec:t~d, i.e. evaluated by mea~uring it, and it 18 .
de~irable to ~liminata the e~f~t o~ a ~uperimposed hf:>mogeneous magnetic ~lux, ~or exampl~, in the region o~ a 60urce o~ ~lux dl~turbance, I can u~e a versi4n o~ the sen~or of the invent~an in which along the center line or lS ~ long~tudinal axi~ o~ th~ ~triE; re onator ther~3 are form~d a plurality oP openlnstfi~ or holes in ~uac2s~3ion and bstwe~3n wh~ch th~ Jo~ph~on element or el~ments can be prc~vid~d.
~he ~trllp re~onatl:>r in the region Qi~ the opening as well as with re~poat to thl3 openlng and with r~paat to lts canter .
lirle or lon~itudinal aacis ~hould than b3 ~ormed a~ymmetricall~f .
Tbe asymmatry o~ the 0trip re,sollator in the region o~ ~he opening~ a~3rVe# to gen~rate a hlgh ~requency current in the Jo~3eph~0n ~lem~3nt. The asymmetry can be realiz~3d by ~ providlng the l rlp re~onator at one o~ tha openirigs with a lateral outout or re~ o l:h~k a oon~trict~on i~ ~ormed ~ at thiE~ loaation. The aurrent i~low ln this aonstrlction o:e , . .
lthe ~trip res~nator, ther~Pore, i~ erenk ~rom th~
curr~3nt ~low at th2 corre~ponding location in th~ viainity o~ tha other opening, i . e. lthe current di~trlbution along th~ two o~enlng~ diPPers~ Thls ensure~ a resultant current through the ~ro~eph~on ~unation.
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It 1~ al~o po~;iblQ to detect or to ~ea~iure an ~nhomoqeneou~ maqnetic 1ux in the region o~ both openingF: : :
or a di~erently changlrlg magnetic ~lux irl thi~ meaJ3uring rang~3 with re~renc~ to th~ two op~nlngEi, ~or example, in the ca~ of changing braln ourrent~
~ccording to this a~p~at ,ie the inv~nt~on, gurthar, the opening~ can be arrangeid ~y~metrically o~ the ceinter ::
linei or longitudinal axi~ o~ tha 6trlp. A~ ntioned ;
above, moroOvQr~ t;he~ lungth 1 o~ tbe f~trip re~sonator can be an odd number ~ultiple o~ th~ hal~ wavelength o~ ths opera~ing Prequency, thl3 Josephsoll element c~n be di~3po~ad at a ~axi~u~ curr~nt~ ~low loc~ on oP th~ ~trip re~onator :
and th~ strip condua~or ~or capacitive coupl ing o~ the trip ro30nat~r can ~bei di~po~ed in a regiosl O:e a curren1: ;:
~ ant:inode of the standin~ wav~
Nor~ ~peoi~i~ally, one or both o~ the detactors can ~ ;
aompri~e a strip ~uperoonduc~or having a longitudinal axis and~f~ormlng a ~tr~p resonator ~or ~itanding el~ctrical waves at:~an operating ~requency, the ~trip re~onator being ~ormed : with two eguial ~z~d opening~ ~ormed in ~iucce~ion in the strip re~on~bor along thei axi~ and h~ving a ~o~eph~on elam-nt p~ovidedl thsr~betw0en BO th~t tha openin~ de~ine :
uparsondu~tive rlngs~ int~grated in ths ~trip resonator ; with the Joseph~on ~ ment lylng generally along the axl~, ;
: 25 ~ th- strip r~sonator in ~he region o~ the openings and with r~p~at:to the axi~ being ~ormed ~ym~etrically, and a ;:
urth~r :~trip aondua~or lying on the Btrip resonator and ; capacitively coupled thareto ~or oonnectlng the respective detec~or to khe c~rcui~ m~ans.
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Accordlng to a Purther ~eature Or the invention, ~he Jo~3phson ~lem~nt 1~ ~orm6~d a~3 ~ ~icrobridg~ and the part o~ the oubstra~e b~n~aath th~ Jo~3eph~0n el~3ment 1 ~on~tltuted o~ tarlal h~3vln~ th~ lle~t po~l3ible th~rmal conduGtivity, l.e~ a low thelrmal c:onductiYlty alnd a thermal conductivlty that iEI l ~awer tharl that o~ the strip condu¢tor~3. In th1s manner, the he~t g~nerated in the Jo~eph~on elem~nt by ~ ;ipation at th~ r:ritic:al c:urrent o~
th~ ~oseph~;on ~lement can be r~duc~d in ~ particularly l3~Pectiva mann~r wh~n the period dl ration of th~ operatinr3 fre~uency of th~ ~;en~or i~ 6mall by s;:o~nparilsorl witb th~ -tl~ ov~r whis::h th~ h~at arisin5~ ~n th~ Jc~6eph~n ele~nent : mu~;t be carri~d away by thea~mal conductiv1ty .

15 ~ ~Q ahove and~other ob~ect~ eature nd advantagss ~: , o~ my inv~ntio3l will becoma more r~adily apparent ~ro~ the rollowirlg de~cription, re~erence beirlg made ~o ~he ac¢ompanying highly diagrammatio drawing in which:
FIG. 1a i8 a platl viaw o~ one ver~3ion o~ ~ d~tector 20 ~ ~ wh~ah can ~ u~ed in aa¢ordan~e with the invention having a n~le operling in th~ superaonduct~v~ ~trip xesonator and showing~ th~ Purt:her ~trip conductor a~ ~lightly og~sst fro : th0 ~trip re~orleltor ~lthough it normally will be ~o t::lo8ely uxtapo~ed ther~wl~ll a~ to b~ aapaaitlYely coupleà

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~ : , ', FIG. lb is a section taken gene ~ ~y along the line A B of FIG. la, i.e. is a longitudinal section through the strip resonator thereof;
FIG. 2a is a view similar to FIG. la but in a version wherein the strip resonator has two holes or openings;
FIG. 2b is a section along the line A'-B' of the strip resonator of FIG. 2a;
FIG~ 2aa is a view similar to FIG. 2a but showing a slightly modified version;
FIG. 2bb is a section along line A"-B" of the strip resonator of FIG. 2aa;
FIG. 3 is a view similar to FIG. la but showing the meahanical support for the sensor and its electrical connections;
FIG. 4 is a block circuit diagram for the circuit ;means associated with each detector or sensor;
FIG. 5 is a block diagram of the o~erall circuitry ~ ~
of the SQUID of the invention; and ;~ ;
FIG. 6 is a block diagram of the correlation circuit.
Specific Description ~ In accordance with the principles set out in the :, . -above-ldenti~ied United States application, the sensor shown in a~slmpllfied form in FIG. la comprises a strip resonator 1 having an opening or hole 2 integrated therein and ~orming ~around that opening a superconducting ring. The resonant current paths are represented at R, Rl in FIG. la. The closed-,, path~low traverses a Josephson element 3 disposed along alongitudinal a~is LA which is parallel to the centerline CL of the strip resonator 1. What is important, of course, is that the~portion ll of t:he strip resonator opposite the Josephson elemen~ 3 is wlder than the Josephson element.

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1~004 The ring and the Jo~3~ph~0rl ~lemsn~ 3 ~or~ a tank ¢irauit in which an o~cillation i.E~ genor~ted with thl3 oscillat10n aurrent R, R~L A maglnet~c ~lux through the hole 3 or even ad~acent th~: seneor will aau~3i3 damping o~
tha o~oiLllation a~ desaribed abov~ and this e~ect, coupled to the E~t:andir;lg wav~ in the ~trlp 3:e8011atOX', ::an be :
capacitiv~3ly detecl:ed in another ~3trip conduotor 4 arranged parallel to the strip resonator 1 and ~;erving ~or the oapacltive coupling o~ the ~;triLp resonator to the circuit means repre~erlted in FIGS. 4 and 5.
StrIp re~onator and the ~urlth~r ~trip conductor ~
are applied to a substrate 6 which~ ~n turr~ ; aarried by a ~ub~trate holder 7 th~ length 1 o~ the 6trip resonator i~ ~
~o dl~ 3n~iDned 'chat it c:c7rrespond~ to an odd ~ultiple o;
. . ~
th~ hal~ wavelength of th~ operating ~r~quency ~. ~he Jos~ph~on ele2a~rlt i~ loc~t~d at A raglon oie ~axl~nur~ curra~t flow of tha Ætrip resonator.
FIC;. 1~ ~how~3 the ~en~or o~ FIG. 1 in longitudinal ction.
.
In thlE; ~IGUP~ the ~;ubstrate holder 7 is shown in qreater d01:ail and can be bo h highly e1Q~triCa11Y ~nd hlghly th~rmally ~onductlv~3, i . e~ compc~sed olE a mat~3rial copper. q'~Q 0ub~3trate ~ i~ a T~atf~rial alP the ~malle~t pos3sibl~3 diç~lec:trio lo~ type a~nd oan be compo~;e::l, Por :
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The version of the sensor illustrated in FIGS. 2a and 2b also makes use of a strip resonator 1 in which, however, two openings 2a and 2b are provided with the Josephson element 3 disposed between these openings. The other strip conductor 4 providing the capacitive pickup, has also been illustrated.
In the region of the gradiometer opening 2_, the strip resonator 1 is formed with a cutout or recess 5.
Alternatively, as indicated in FIG~ ~aa r the strip resonator is formed with a cutout or recess 5 in the region of opening 2a as well as 2b so that the strip resonator is only point symmetrical with respect to a point formed b~ the Josephson element 3. As has been developed above, this version of the sensor is particularly efective for the measurement of non-homogeneous magnetic fluxes.
FIG. 3 illustrates the mechanical support and the ;
electrlc~al connection to the sensox.

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'' ' f~ , 180~4 on tha ~ubstrata holder 7, a Purther !3ubE~trate 6~, $~;
provld~d And ç~rrle~ urth~r pil~G~ ~ Or ea ~trip condu~ or (5 ohm~) whiah ia c:e~entad to the ~ub~trat~ The 13trip condus::tor piec~ 4 i~ rlc:ally c:onneoted wl'sh the ~3trip S aonduatQr 8 by ~hort ~1l uminum wire~ 9 ~nd the sheath oP ~
coaxial oabl~s 10 i~ aold~red to th~ holder 7 while the ¢ore wire o~ the coaxial cabl~3 iB ~oldered to the Btr:lp conduotor 8 . The aahle 10 run~ to th~ ~lec:troni ::
evaluation circuitry.
Io FIG. 4 lllu~tr~ts~ thia clrcuitry in the c:ontext o~
thla lnvent ~on.
For the operation O~e th-3 l3en~30r ot t21~ inYention ~ a high ~ ~r~quency curren~ 1~ g~3n~rated, e.g. a a ~ruquency o~

3 . 5 GHz. For thl~ puxpof3e a hlgh ~requency generator ~o i~
;- 15 : ~ p~ovided~ It~ a~plitude ia controlled by a variable a~ttanuator PIN to ~t t:h~ optl~ur~ lavel ~or the re~p~Sative nsor.
: Thl~ current 1~ ~up~rimpo~ed upon tha Btrip ;re30nator via a directional coupl~r DC. ~h~ voltage acros~ -; 20 ~ ~tbo~ r~sonator i~ 1ni~1ally ampl~led ln a preampl~er ~MP
and ~ubj~atad to a~plitude demodulation at SA.
: : ::, : :
~: ~ rh~ r~ulting low frequency voltage whioh aontains tha lnrormation a~ to ~hang~ in tha magnetic ~lux .~ ~
travqr~ing~tha:sensor aan be d~playad on an o~CillosCopQ :, 25 ~ whioh;c~n~b~ optloDal ~ reprasented ~y th~ dot-dash 1ines whon~th oorre1atio~ ctronice oP the lnventio~ aro employe~ In that C~5~ t~ output o~ the aorr~lation leatronlo~ can~b~ connected to the SQUID di~play, e.g. an ~;~
oso1110scope or~any other analysi~ or display d~vic~. ;

, .: .

1800~

Th~ en~o~ pr~erably oparat~d in a ~lux-lock~d loop sylatam and ~or that purpo~o a low Pre~ ncy generator 11 hav~ ng an output of approximal;ç~ly 2 kHz 1~ connect~d to a coil 12 dl~pQsed i~ th~ regioll o~ the ~en~or to provide a magn~ti¢ alternating ielux feedbac~ with a ~req[uency ot' 2 k~Z c ,,.
Vla th~ loak in ampli~ler LKI and integrator INT, -~
th~ n~agnetic flux ~uperimpo~ed on the ~en~or oan be malntained con~tant. A switch 13 in E;Qries with a resi~ter ~L4 aan cut in th~ ~lux locked loop c:~ rcuitry.
Fl~;. S ~hov~ the interconnectlon o~ two ~y~te~s o~
the lty~?e illuo r~t~d i3~ ~IGS. 3 and 4 to ~orm a SQUID with ~orralation alectronic~ XE ~or al imillating uncorrelated noi~a sl~nalc.
15: Th~ two block~ S ~ A sh~wn in FTG~ 5 each c:orraspond to th~ ~n~or 8 oP ~I&. 4 plu~ the ra6peCtivQ oircuitry A : :
`
f~or valuating the sen~or output . Thi a circuitry has a lte~inal: ~ which 1~ oonneated to a r~pective input o~ the corralat~cn cirou~ltry KE. AB ind$cat~d prllaviou~ly, that . ~: 2 o :~: c:irc:uitry can utilize autocorr~alation oircuitry (sea chapt~er 25, pag~ e and 15 ~ o~ P~adar Handbook, Merrlll : I . Bkolnilc, HcGraw-Hill Book Company~ ~ew Yorlc, 1970 or : :
ahapter 20 page~ 21 ~ th~reo~ 86~e al~o c:hapter 20 pages 3 fe~ ~ cros~3 oorrelation circultry o~ c:ol3ventional design or :
~5 : ~n~mble proo0e~ g clrouitry. The output Q can b~ ::
applled to a aG~putar ~valu~tlng the output o~ th~ S52UID
and aontrolling th~ display on an oaaillo~cope or other C:RT
yater~ of the blomas3 netis:~ brain or heart ~;ignal s or the .
l ike .

.
,, ' For ~ensor~ oP 1:21e type ~hown in FIGS. la ~nd 1l3 the ~ollowing parameters c:an be u~ed~
The ~3ub~r~te 6 can consi~t o~ ~apphire with dlmen~lon~ o~ 25 x 5 x 1 mm3 (widlth x length x thicknes~3. The niobiu~ appli~d ln a thick7le~s o~
100 n~ by cathodia sputtering, util~zing a ma~k or the like to 3tructure~ lts ~11~ and ~orm th~ opaning ~ th~r~in. Th2 ~trip conductor re~onator 1 can have a length cP 2 0 ~n and a width of 1 ~. Tha op~ning 2 o~ tho ~en6~0r ~a~
dl~ensions o~ 40 x 40 ~icrs:~Meter~. Th~ Jo eph~on ele~e3~t 3 18 tormed a~ a aicrobridg~ with ~ wldth oP 150 ~icrometer8 ansl ~ lerlgth o~ 3 m~cromatarsO It al~o ¢aE~ c:on3i;t o~ a Jo~eph~c~n tunnel ~unction. Tha ~ub~trata hold~r 7 wa~ a copp~3r blcsak. The appl ~ ~d high ~ra~uenoy ourr~nt had a 15: ~ fregu~ncy o~ 3.5 GHz with a pow-r o~ 40 d~. The PluY-d~pendont voltag~3 chang~ d~tect~d by the tank clrcui1:
carl aD~ount to ~aver~l hundred 2~lcrovolt~ pe~k-to- peak. q`ha measurements were carried out at a temperatur~ o~ tha sen~or o~ 4 . 2k.
~ ~ ~irl~ilar paraD~ter~ . can b-3 u~ed wlth th~ sansor o~
FIG6. ~3a and ~
ln ~IG/ 6 I h~e ~how~ a corr~lation airauit I~E
:
wl~ich ma~ bo u~ad a~ the correla'clon oirau~'cry ~or ellmlriatin5f unoorr01ated noise ~rom tha output and which ~S : ba~ically i~ ~ xoot ~nean equara d~vica whosQ amplî~ier~ 20 nd ~1 ar~ aonnec:t~d to th~ two SQUID ~3en~0rs and .

~ : ..

18004 ~ i 7 .~ ~

......... .. . .
'':' respectively ~eed a ~ultiplier 22 ~ollowed by ~rl av~rager 23 ~nd a root-taking unit 24, all o~ which ara Or conventional s:ircult de~ign. Whil~ the E~ensor e,y8'Cerrl oP
the~ lnventlon ha~3 been ~hown with only two SS2UID~, it will be under~tood that additional air~ of SQllIDs ~ay al~o be used ~30 that th~ sy8tem can operate with ~our or eix SQUIDs working into the correlation c ircuitryO

i: : : ~ :

::: : : :

. ,. . ......... ... ., . , ". ,, . ,~ , . . .. .. .

Claims (11)

1. A sensor for magnetic signals comprising:
two superconducting quantum interference detectors connected in parallel and having respective output signals including measurement signals, inherent noise and, upon operatlon at certain frequencies, extrinsic noise; and circult means including a correlation electronic circuit connected to both of said superconducting quantum interference detectors forming an output for output signals from tha measurement signals of the parallel connected superconducting quantum interference detectors for eliminating from said output, signals representing uncorrelated noise signals from said superconducting quantum interference.

2. The sensor system defined in claim 1 wherein each of 2 said devices comprises a strip superconductor having a longitudinal axis; and forming a strip resonator for standing electrical waves at an operating frequency, said strip resonator being formed with an opening and being traversable by said magnetic flux and bounded by a superconductive ring of said strip superconductor having a Josephson element incorporated therein and at a boundary of said opening so that said ring is integrated in said strip resonator, said Josephson element lying along said axis strip resonator, and at least one further strip conductor lying on said strip resonator and capacitively coupled thereto for connecting the respective detector to said circuit means.

3. The sensor system defined in claim 2 wherein the electrical length of said strip resonator is an odd number multiple of a half wavelength of said frequency.

4. The sensor system defined in claim 2 wherein said Josephson element is disposed at a location of maximum current flow in said strip resonator.

5. The sensor system defined in claim 3 wherein said further strip conductor is disposed at a standing wave voltage antinode of said strip resonator.

6. The sensor system defined in claim 1 wherein at least one of said detectors includes a strip superconductor having a longitudinal axis and forming a strip resonator for standing electrical waves at an operating frequency, said strip resonator being formed with two equal sized openings formed in succession in said strip resonator along said axis and having a Josephson element provided therebetween so that said openings define superconductive rings integrated in said strip resonator with said Josephson element lying generally along said axis, said strip resonator in the region of said openings and with respect to said axis being formed asymmetrically, and a further strip conductor lying on said strip resonator and capacitively coupled thereto for connecting the respective detector to said circuit means.

7. The sensor system defined in claim 6 wherein E;aid openings are symmetrical to a longitudinal axis of said strip resonator.

8. The sensor system defined in claim 6 wherein the electrical length of said strip resonator is an odd number multiple of a half wavallangth of said frequency.

9. The sensor system defined in claim 6 wherein said Josephson element is disposed at a location of maximum current flow in said strip resonator.

10. The sensor system defined in claim 6 wherein said further strip conductor is disposed at a standing wave voltage antinode of said strip resonator.

11. The sensor system defined in clam 6 wherein said openings and said strip resonator is point symmetrical about: a point at sald Josephson element.