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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. I I , NO. I , MARCH 2001
Electronic Gradiometer using HTc SQUIDS with Fast Feedback Electronics A n d r e i Matlashov, Michelle Espy, Robert H. Kraus, Jr., Kenneth R. Ganther, Jr., Lowell D. S n a p p during 1990s: direct-coupled one-chip gradiometers [ 1-31, Abstract-An electronic gradiometer was built using a HTS flip-chip gradiometers [3-61, and electronic gradiometers SQUID magnetometer array. A SQUID magnetometer in the center of the array was used to pick up background noise, and based on two or more magnetometers [7-101. the output signal fed back to other magnetometers to cancel Direct-coupled HTS gradiometers consist of small SQUID background noise. Fast feedback electronics were built for the loops and at least one larger pickup coil(s) that is directly background channel with a slew rate about lo7 @,c/secand 10 connected to the SQUID loop [ 1-31. This type of gradiometer MHz small signal bandwidth. Two other magnetometers of the has several disadvantages. First, they have low sensitivity array were connected to pcSQUIDTMelectronics with 5 ~ 1 0 ~ resulting from the low mutual inductance between SQUID @,,/sec slew rate using the AC bias mode to decrease l/f noise. loop and pickup coils. These devices also disturb the magThe output signals from these two magnetometers were input to netic field being measured because of large shielding currents a summing amplifier resulting in a gradiometric output signal without background channel noise. Constructing the gradiome- in superconducting pickup loops. The maximum baseline for these gradiometers is also severely limited by substrate size, ter with different magnetometers on the array enables us to vary the baseline from 0.75 mm to 7.5 mm with 2 ~ 1 0 TdHz '~ field typically to smaller then 1 cm. Finally, this style of gradiresolution in an unshielded laboratory environment. This variometer is poorly matched to bicrystal junction technology able-baseline gradiometer can be used for NDE, biomagnetism because it is impossible to avoid crossing of the bicrystal and other applications. boundary by the pickup coil leads. Index Terms-biomagnetism, high slew rate SQUID readout, non-destructive evaluation, superconducting gradiometer.
I. INTRODUCTION
P
erforming high-resolution magnetic measurements in unshielded or lightly shielded environments is typically not feasible with magnetometer pickup coils coupled to SQUID sensors because ambient magnetic noise will usually overwhelm the signals of interest. Wire-wound lowtemperature superconducting (LTS) gradiometers were developed in the early 1970s that enabled measuring extremely weak biomagnetic signals in unshielded environments. One can not yet implement the same concept in high-temperature superconducting (HTS) technology because of the lack of reliable HTS wire. Consequently, new gradiometer concepts and techniques are needed for HTS technology. At least three different gradiometer concepts were developed for HTS SQUID systems Manuscript received September 18, 2000. This work was supported by the Department of Energy/OBER and NIH Grant ROl-NS31052. Andrei Matlashov is with the Los Alamos National Laboratory, Los Alamos, NM 87545 USA (telephone: 505-665-6183, e-mail: matlachov @lanl.gov). Michelle Espy is with the Los Alamos National Laboratory, Los Alamos, NM 87545 USA (telephone: 505-665-621 8,e-mail:
[email protected]) Robert H. Kraus, Jr. is with the Los Alamos National Laboratory, Los Alamos, NM 87545 USA (telephone: 505-665-1938, e-mail: rkraus @ IanLgov). Kenneth K. Ganther, Jr. is with Honeywell Federal Manufacturing & Technologies, Kansas City, MO 64141 USA, operated for the DOE under contract # DE-AC04-76-DP00613 (telephone: 816-997-5954, e-mail:
[email protected]). Lowell D. Snapp is with Honeywell Federal Manufacturing & Technologies, Kansas City, MO 64141 USA (telephone: 816-997-5970, e-mail: lsnapp @ kcp.com).
Some of the disadvantages noted above can be avoided using a flip-chip gradiometer design. Flip-chip gradiometers consist of at least two separately manufactured parts, a small chip with the HTS SQUID, and much bigger chip with a flux transformer [3-61. The patterned sides of the two chips are pressed together, inductively coupling the SQUID loop and the flux transformer. This design enables larger gradiometer baseline (more than 5 cm has been demonstrated) and much higher field resolution than single-chip directly coupled gradiometers. Both first and second order planar gradiometers with very high balance level have been demonstrated using this design [5, 61. Flip-chip gradiometers, however, also have some inherent disadvantages. They still produce large disturbances to the external magnetic field being measured. In addition, only planar gradiometers can be built using this technology (i.e. these devices cannot measure diagonal gradienttensor components). Finally, the relative position of the two chips requires precise mechanical adjustment to attain high balance level [SI. Electronic gradiometer methods have been extensively investigated for LTS neuromagnetic systems in the 1980s and 1990s [7]. Similar technique can also be used for HTS measuring systems, and several simple versions of HTS electronic gradiometers have been successfully demonstrated for MCG and NDE measurement [8- 101. One significant advantage to electronic gradiometers is the ability to build them with virtually any size baseline and capable of measuring of any of the gradient-tensor components, providing greater flexibility than directly coupled or flip-chip gradiometers. Ref. 8 demonstrated that precise low-temperature mechanical adjustments are needed in order to attain high sensitivity and high balance levels for a HTS second-order gradiometer. However, by using a larger number of reference channels, one can avoid
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the need for precise cold mechanical adjustments [7]. Additionally, fast and accurate design of the magnetometer fluxlock feedback circuit will minimize or even eliminate perturbing the magnetic field of interest. Total noise of an electronic gradiometer depends entirely on intrinsic noise of the magnetometers used (using sum-of-squares rule). Reference [ I l l shows a first-order gradiometer design based on three magnetometers where the signal from the third magnetometer is used to cancel ambient background noise in the other magnetometers. By properly adjusting the two background cancellation signals when constructing the gradiometer, the intrinsic noise of the system does not include the intrinsic noise of the background magnetometer. Higher order gradiometers can also be built using this concept. Two different basic approaches (and variations) can be used to build electronic gradiometers. Either the signals from “primary” and “background” sensors can bc subtracted at a differential amplifier (parallel technique) or the signal from the background sensor can be inverted and convertcd to a magnetic field at the primary SQUID sensor (series technique). These two methods have been compared in [12]. To balance an electronic gradiometer both amplitude and phase of the reference and sensor output signals must be precisely adjusted. An important benefit to using an electronic gradiometer design is that the amplitudes can be easily adjusted and matched using simple electronic circuits. The relative phase of two signals depends on both the input signal frequency and amplitudes and the maximum slew rates o i each channel [ 131. Consequently, the phases cannot be easily or precisely adjusted, and limits the achievable balance level of electronic gradiometers. The solution is to use a very fast reference (or background) channel, i.e. with very high slew rate, and serics subtraction technique. There have been several published fast readout electronics methods (i.e. refs. 14-16). The highest slew rate reported to date is 3x107@,dsecwith a 5 MHz small signal bandwidth and demonstrated using direct readout electronics [ 161. Modulation readout electronics have demonstrated a slew rate up to 1xlO6@,dsec and 2.5 MHz bandwidth [15]. In this paper we present modulation SQUID electronics with slew rate up to 1.2x107@,dsec and about 10 MHz small signal bandwidth, These electronics were coupled to an HTS SQUID magnetometer to measure the background magnetic noise and subtract that noise from two other magnetometers operating at a lower slew rate. The output from the low-spced SQUIDmagnetometers were then used to form a first-order electronic gradiometer with high dynamic range and no appreciable noise contribution from the background channel. An elevenchannel HTS SQUID array was used in the final design of the electronic gradiometer.
nections, and matching circuit are shown in Fig. I . The matching circuit consists of cold elements placed at 75K close to thc SQUID sensor array and a room temperature matching transformer connected by a 56 cm, 50 Ohm cable. The electronics were designed using the traditional modulation-demodulation technique at a modulation frequency of 33 MHz and a two-pole integrator. The electronics consist of a low-noise prcamplifier, a mixer, modulation generator, highfrequency modulation signal attenuator, and two-pole integrator. Modulation signal together with a feedback signal and a test signal are fed to the SQUID feedback loop through 56 cm, 50 Ohm cable. Three different types of HTS dc SQUID magnetometers were tested with the electronics. We used commercial HTS SQUID magnetometers (Mr.SQUIDTMand miniMAGTMfrom Star Cryoelectronics 1171 ), and an eleven-SQUID array manufacturcd by IPHT, Jena [ 181.
I I I I
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11. FASTFEEDBACK ELECTRONICS We designed fast feedback electronics (patcnt pending) using a modulation technique with a two-transformer matching circuit between the SQUID and input low-noise preamplifier. A block diagram of the electronics, HTS SQUID con-
Fig. I . A block diagram of the high slew-rate SQUID electronics: A - lownoise preamplifier, G - inodulalion generator, M - mixer, AT - high frequcncy attenuator, I - two-pole integrator. Modulation frequency is 33 MHz. HTc SQUID and a cold matching circuit (MC) arc at T=75K and connected to rooin temperature electronics wing two 56 cm, SO Ohin cables.
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TABLE I FAST SQUID ELECTRONICS SLEW RATE
Field Sensitivity
Flux Sensitivity Flux Resolution Slew Rate at 1 0 0 Slew Rate
IPHT SQUID Array 100 nT/O(]
35 mV/%
Mr.SQUIDTM
Mini-MAGTM
700 nT/@o
25 nT/@[)
30 mV/%
55 mV/@O
?-N 10
20 p@ddHz
6.5 FO(r/dHz
5.0 p@cr/dHz
5x10' @{ds
6~ I 0' @.ds
3x106 W s
1 1 ~ 1 OCr/s 0~
12~10~@~r/s 6~10~%fs
k Y
Table I presents the electronics slew rates at 1 0 0 and 10000 input signals, field sensitivity, flux sensitivity and flux resolution at 10 kHz for the three HTS SQUID magnetometers. DC bias mode was used in all cases. The input signal dynamic range is about 150 dB for 1Hz band.
01
1
111. ELECTRONIC GRADIOMETER
I
I
I
10
100
Frequency [Hz]
The HTS SQUID array, consisting of eleven SQUIDs with 30" bicrystal Josephson junctions, was used to build an electronic gradiometer. Each SQUID has square 0 . 4 ~ 0 . 4mm2 loop and about 100 nT/Oo field sensitivity. The center-tocenter distance between neighboring magnetometers is 0.75 mm. Three of the eleven SQUIDs, the two outermost ones and a central one, were used to make an electronic gradiometer following the scheme proposed in [ I l l . A planar SQUID array was used for our electronic gradiometer to assure all SQUIDs were as similar as possible and placed on the same plane.
M2
M3
I
T
/ M :i
1623
Fig. 2. A block diagram of the first-order electronic gradiometer with backMa, M1 - magnetometers (JJ - Joground noise subtraction channel: MI, sephson junctions); CHI, CH3 - pcSQUID (DI, D 2 , RFB-feedback differential drivers and resistors); CH2 - fast electronics; B 12, B23 - balancing resistors for fine-tuning compensation signals (D3, D4, R C O M-~ compensation signals differential drivers and resistors); SA - a summing amplifier to form the first-order gradient GI3 by summing signals G I 2 and (323.
Fig. 3. HTS SQUID magnetometers noise spectra in shielded conditions. The bottom spectrum corresponds to the magnetometers connected to pcSQUID electronics and using AC bias inode (MI or M3). The top (noisiest) spectrum corresponds to the inagnetometer M2 connected to the fast electronics and using DC bias mode. The intermediate spectrum corresponds to the first-order electronic gradient signal G I (see ~ the text for details).
The connection scheme for the gradiometer described above is shown in Fig. 2. Feedback signals are directly coupled to the SQUID loops, thus minimizing crosstalk. The central background-sensing SQUID-magnetometer (M2) was connected to the fast electronics (Ch2) and measured to have 35 mV/Q0 flux sensitivity. The two outermost SQUIDs are connected to two channels (Chl and Ch3) of the commercial pcSQUIDTMelectronics [ 171. The center-to-center distancc between these magnetometers is 7.5 111111. Both pcSQUIDTM ~ ec channels havc 300 mV/Oo flux sensitivity and - 5 ~ 1 00 & slew rate. AC bias mode was used in pcSQUIDTMelectronics to reduce the low-frequency shot noise of the HTS SQUIDs. The shielded noise spectra of different channels are shown on Fig. 3. The central magnetometer (M2) has a flat noise spectrum about 2 0 ~ 1 0 O . ~d d H z at or above 30-60 Hz, corre'~ The lowsponding to a field resolution of 2 ~ 1 0 - TIdHz. frequency noise increases below 30 Hz, increasing to about 2 0 ~ 1 0 -T' d~H z at 1 Hz. W e observed a white noise spectrum at - 2 ~ 1 0l 2 TIdHz down to about 2 Hz for the other two magnetometers (MI and M3) using pcSQUIDTMelcctronics and AC biasing mode. Two first-order gradients were constructed by adding a properly scaled output signal from the central channel, M2, to both of the outermost magnetometers M I and M3. These gradient signals are represented as, G I ~ = M I - ~and M ~G23=PM2M 3 , where a and P are scaling factors. Two balancing resistors, B12and Bz3,were used to scale the background signal Mz after it was amplified by 10 times. Each of the gradient sig-
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magnetic measurements in unshielded laboratory environments. W c plan to build HTS measuring systcms for nondestructive evaluation (NDE), biomagnetism and other applications using this concept.
1o5
1o4
REFERENCES [1]
104 [2]
1o2
[3]
[4]
1o1 [SI
1 1o1
1o2
1o3
[6]
Frequency [Hz] [7] Fig. 4. Magnetic field spectra recorded in unshieltled laboratory environment. Dashed line shows ambient magnetic noise spectrum recorded by the magnetometer M1. Solid line corresponds to the gradient signal G13 formed by summing signals G12 and G23.
nals, Glz and Gz3,includes a large noise component from the reference signal, Mz. The final gradient signal is constructed by summing GI2and GZ3in a summing amplifier (SA) giving the final gradient GI3= Glz + GZ3= M I - M3+ (P-a)Mz. For scaling factors a and p nearly equal the noisy reference signal Mz can be cancelled out from the final gradient signal GI3 (see Fig. 3) [ 1 I]. In our particular case (P-a) 5 10.‘. This is why has a flat noise spectrum even at low frequency. Fig. 3 shows the magnetometer signals spectrum M I (magnetometer M3 has similar spectrum) as well as electronically derived gradient spectrum GI3 recorded in a shielded environment. Fig. 4 shows the same signals recorded in unshielded laboratory environment. W e observed that the external uniform test magnetic field is reduced by the factor up to about lo4 times and the power line noise, typically - I x ~ O -T~ in our laboratory, approaches the magnetometer whitc noise. IV. SUMMARY W e have built an HTS first-order gradiometer with a baseline that can vary from 0.75 mm to 7.5 mm using an elevenSQUID array. One of the array SQUIDS was used to cancel a uniform background magnetic noise from two other magnetometers (that comprised the gradiometer) without increasing the intrinsic noise level. Our gradiometer has a magnetic field resolution of 2x10-” TIdHz and can be used for precision
[8]
[9]
[lo]
[I I]
[I21
[I31
[ 141
[IS]
[ 161
[I71 [ 181
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