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Page 1: Digital-Analog Magnetometer Utilizing Superconducting Sensor

DigitalAnalog Magnetometer Utilizing Superconducting SensorR. L. Forgacs and A. Warnick Citation: Review of Scientific Instruments 38, 214 (1967); doi: 10.1063/1.1771358 View online: http://dx.doi.org/10.1063/1.1771358 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/38/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Delta-sigma digital magnetometer utilizing bistable spin-dependent-tunneling magnetic sensors J. Appl. Phys. 99, 08B320 (2006); 10.1063/1.2171942 A hybrid digital–analog long pulse integrator Rev. Sci. Instrum. 68, 381 (1997); 10.1063/1.1147835 Interfacing a 32 K commodore PET computer with digital/analog and analog/digital converters Rev. Sci. Instrum. 52, 614 (1981); 10.1063/1.1136649 A uhf superconducting magnetometer utilizing a new thin film sensor Rev. Sci. Instrum. 46, 474 (1975); 10.1063/1.1134245 Persistent Current Magnetometer Utilizing a Vibrating Superconducting Plane to Shuttle Flux J. Appl. Phys. 40, 2100 (1969); 10.1063/1.1657921

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Page 2: Digital-Analog Magnetometer Utilizing Superconducting Sensor

214 DYNE, GLASSER, AND KING

The large number of digits reported must not be taken to

mean that each of these parameters is known to such a high degree of precision. They are quoted to enable k to be cal­

culated to the precision of the present experiments. These parameters have been compared to other litera­

ture values in Fig. 6. The solid line represents the tempera­ture range of the present measurements, while the broken line is an extrapolation into a region of low temperature. The energy of activation, E, compares to reported values.

THE REVIEW OF SCIENTIFIC INSTRUMENTS

In dilute aqueous solutions, Rivett and Sidgwick obtain­ed 10.3 kcaljg-mole, while in aqueous acetic acid Marek16

reported the activation energy as 13.75 kcalj g-mole. Marmers17 found the activation energy for equimolar con­centrations to be 16.37 kcaljg-mole, which is somewhat higher than the present result.

16 J. Marek, Chemike Listy 48, 168 (1954); Collection Czech. Chern. Commun. 19, 621 (1954), quoted in Ref. 17.

17 H. Marmers, Ph.D. Thesis, University of Birmingham (October 1965).

VOLUME 38. NUMBER 2 FEBRUARY 1967

Digital-Analog Magnetometer Utilizing Superconducting Sensor

R. L. FORGACS AND A. WARNICK

Scientific Laboratory, Ford Motor Company, Dearborn, Michigan 48121

(Received 23 September 1966)

A magnetometer was developed which utilizes a superconducting quantum interference device (SQUID) as a sensor, and permits digital and analog measurements of magnetic fields. The digital mode permits measurement of large field changes with a constant accuracy of approximately 1/10 the natural periodicity of a SQUID. (The natural period is typically lO-L IQ-s G.) Thus, the precision of measurement of large field changes is high. The analog mode is advantageous for high precision measurement, of the order of 10-8 G, but for small field changes. Combined mode measurements permit the very high precision of the analog mode to be combined with the large range of the digital ~ode. The digital mode, which represents an improveme~t over the original lock-on mag­netometer, utilizes two synchronous detectors to provide signals which uniquely determine the instantaneous magnetic bias of the SQUID. The synchronous detector outputs drive a novel logic circuit which produces appro­priate add-subtract pulses to a digital counter, which reads out field changes in units equal to the periodicity of'a SQUID.

INTRODUCTION

RECENT investigationsl-6 in the area of superconduct­ing phenomena have resulted in the development of

devices which exhibit extreme sensitivity to magnetic fields. Such a device, called a superconducting quantum interference device, or SQUID, has been used as a sensor in an experimental lock-on magnetometer;7 magnetic field changes of the order of 1O-L 1Q-9 G were detectable with the lock-on magnetometer. The digital-analog mag­netometer to be described is an extension of the previous work in that digital measurements of field changes are now made possible, in addition to analog measurements. The general effect of the addition of the digital capability is to permit measurement of large field changes with in­creased precision. The explanation for this statement

1 B. D. Josephson, Phys. Letters 1, 251 (1962). 2 J. Lambe, A. H. Silver, J. E. Mercereau, and R. C. Jaklevic, Phys.

Letters 11, 16 (1964). 3 A. H. Silver, J. E. Mercereau, and J. E. Zimmerman, Bull. Am.

Phys. Soc., 10,318 (1965). • R. C. Jaklevic, J. Lambe, A. H. Silver, and J. E. Mercereau, Phys.

Rev. Letters 12, 159 (1964). 6 J. E. Zimmerman and A. H. Silver, Phys. Letters 10, 47 (1964). 6 J. E. Zimmerman and A. H. Silver, Phys. Rev. 141, 367 (1966).

becomes apparent from a comparison of the two modes of operation.

DIGITAL APPROACH COMPARED TO ANALOG APPROACH

SQUID Ch8!acteristics

Operating characteristics of the SQUID itself are covered in some detail in previous references,2-4.6 but will be outlined very briefly here. The SQUID (which is im· mersed in liquid helium in operation) consists of two parallel current paths constructed of superconducting type materials which enclose an aperture. In each path a weak link or area of small cross section is interposed, making it possible for relatively small currents, e. g., 1Q-6-10-4 A, to produce critical current densities in the links. When a direct current is passed through the parallel paths of an appropriate magnitude to bias the SQUID on the verge of superconductivity, the dc voltage across the SQUID is a function of the magnetic flux threading the sensitive aperture. The functional relationship between voltage and flux results from the presence of circulating currents in the SQUID which attempt to prevent magnetic field changes

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Page 3: Digital-Analog Magnetometer Utilizing Superconducting Sensor

MAGNETOMETER 215

in the aperture; the circulating currents then modiIy the current density in the links affecting their superconductive properties. The voltage across the SQUID varies periodi, cally with a periodicity equal to hl2e, equal numerically to 2.1 X 10-7 G-cm2 (h is Planck's constant, e is electronic charge).

Two different approaches for utilizing the sensitivity of the SQUID to magnetic fields in the design of a mag­netometer are to (1) count the number of periodic varia­tions in voltage which occur when the magnetic field encompassing the SQUID is varied, or, (2) use the SQUID as a null detector by providing a bucking field which automatically cancels any change in the environmental field being measured; the bucking field change is then a measure of environmental field change. The latter approach was used in the original lock-on magnetometer.? The former approach is added in the present magnetom­eter to yield a digital readout of field changes in units equal to one period of the SQUID employed. The combina­tion of the two approaches permits the high sensitivity of the analog approach to be attained, along with the high precision for large field changes of the digital approach.

Digital Approach

The periodic variations in SQUID voltage with flux q, or flux density B are essentially sinusoidal, as shown in Fig. 1, at top. Some method must be employed to algebrai­cally total the number of variations in SQUID voltage which occur when the field applied to the SQUID changes.

VSQUID t

2

o~----------------------~------e __

t'[ "'- .... e / 'v I

I O~----~----~----~-----+-----

FIG. 1. Upper waveforms: Effect of sinusoidal modulation of magnetic field applied to SQUID. Lower waveforms: Fundamental frequency and second harmonic content of SQUID voltage as a function of biasing flux density.

7 R. L. Forgacs and A. Warnick, IEEE Natl. Convention Record, Pt. 10, pp. 90-99 (March, 1966).

i'lfA OUT

I')fe OUT

FIG. 2. System for generating voltages identifying SQUID biasing field as a fraction of one period.

The sense of the B field change (increasing or decreasing) can be determined if two voltages are obtainable which vary periodically in a known manner with B, approxi­mately 90° out of phase with each other (referenced to one period of B). A method for obtaining two such voltages is described. Referring to Fig. 1, if the B field applied to the SQUID is sinusoidally modulated about point 1, with a field perturbation of frequency fo, the voltage out of the SQUID is in phase with the applied B modulation and at a frequency fo. When modulation about point 2 takes place, the output signal is doubled to 2fo. When modula­tion about point 3 takes place, the output frequency equals fo, but is 1800 out of phase with the modulation. When modulated about point 4, the output frequency is doubled, but 1800 out of phase with that obtained for point 2. Curve 'VA shows the amplitude of output at fo; curve 'lis shows the amplitude of output at 2fo. The system for generating VA and Vs is shown in Fig. 2. The dc biased SQUID is subjected to a sinusoidal modulation of its magnetic field by an oscillator of frequency, fo. The peak to peak amplitude of the modulation is approximately one half period. When the voltage out of the SQUID is amplified and passed to a synchronous detector gated with the frequency fo, detector output as a function of B is similar to 'VA shown in Fig. 1. The output of a synchronous detector receiving the Same input signal but gated with the frequency 2fowill produce the output shown as VB in Fig. 1.

If the two voltages VA and VB are applied to appropriate horizontal and vertical plates, respectively, of a cathode ray tube, the spot will move in a circular path, clockwise for increasing B and counterclockwise for decreasing B. The number of revolutions of the spot can be counted visually to determine t::..B in units of one period. Or, the number of periods traversed may be counted by passing the two signals VA and 'Vs into appropriate logic circuits which drive a reversible counter.

Analog Approach

The analog approach which was employed in the original lock-on magnetometer and is also incorporated into the present design is illustrated in Fig. 3. The de

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Page 4: Digital-Analog Magnetometer Utilizing Superconducting Sensor

216 R. L. FORGACS AND A. WARNICK

8IAS~ CURRENT~

AC AMPLIFIER

SYNCHRONOUS OPERATIONAL DETECTOR .--~v'V" ..... AMPLIFIER

SQUID"/

FIELD COIL

biased SQUID is again subjected to a sinusoidal perturba­tion of its magnetic field. The fundamental frequency com­ponent of the SQUID voltage is amplified, then detected in a synchronous detector gated at the fundamental fre­quency. The dc signal from the synchronous detector is filtered and amplified by an operational amplifier in an integrating configuration and the resulting direct current is introduced into the field coil to buck out any environ­mental field change which attempts to drive the SQUID off the valley of the VB curve which it is locked onto. The change in environmental field can then be measured by calibrating and measuring the bucking field current. In­strument sensitivity, in meter current per unit field change, is varied by shunting a known fraction of monitored integrator current past the field coil.

FIELD CHANGE READOUT

As mentioned previously, application of signals VA and VB from the fundamental frequency synchronous detector and the doubled frequency synchronous detector, respec­tively, to the horizontal and vertical plates of a CRT, permits visual determination of the number of periods traversed. For convenience with slow field changes, and of necessity for fast field changes, a reversible counter can be incorporated to algebraically total the number of revolu­tions of the hypothetical shaft upon whose rim the CRT spot resides. Visual observation of the CRT display permits determination of fractional period increments, whereas the counter records an integral number of period changes.

The circuitry which accepts VA and VB and delivers ap­propriate add and subtract pulses to a reversible counter has certain unique requirements. After any given count there is no certainty whether the next count will be add or subtract; therefore, difficulty would be encountered in at­tempting to reset a conventional flip-flop to the state de­sired to permit the next count to take place. A second prob­lem is that consecutive add and subtract counts might occur more closely spaced than the resolving time of the counter employed, yielding incorrect readout, when the shaft hovers at the triggering point and noise produces a· succession of closely spaced alternate add and subtract pulses. Circuitry which avoids these problems is shown in Fig. 4.

OUTPUT METER

FIG. 3. Analog or lock-on magnetometer.

The general approach used is to employ VB to control the state of a flip-flop circuit, and to use VA to supply col­lector voltage to the flip-flop. Pertinent waveforms are also presented in Fig. 4. V cs is an amplified, inverted, and clipped reproduction of the negative swing of VA; VBD is a dc offset comparable reproduction of the negative swing of VB. When B equals Bo, collector supply voltage Vcs is zero and base drive voltage VBD is negative. As B in­creases from Bo, VBD goes positive. Upon further increase of B, Vcs goes positive, thus supplying collector voltage to the flip-flop; T2 is conducting, so VC2 remains near zero. TI is biased off, therefore VCI rises with Vcs. When B=Bl, VBD

falls and regenerative switching of the flip-flop causes a rapid drop in VCI, which is differentiated to produce an add pulse. Further increase in B produces a drop in VC2 (which had risen during regenerative switching), but the drop is nonregenerative and therefore too slow to produce a sizeable differentiated pulse. For B decreasing from B 2,

when Vcs rises, VC2 rises, since VBD keeps Tz cut off. When VBD rises, VC2 falls regeneratively, producing by differ­entiation a subtract pulse. Further decrease in B causes a nonregenerative drop in VCl (which had risen during re­generative switching), but the drop is again too slow to produce a sizeable differentiated pulse.

If B is increasing, but changes to decreasing, or vice versa, at any point in the cycle other than Bl, no output pulse can occur, since regenerative switching cannot take place. If alternations in B take place about B I , alternate "add" and "subtract" pulses occur, but they cannot occur more closely spaced than the recovery time of the flip-flop. Thus, if the flip-flop is designed to have a recovery time longer than the minimum resolving time of the counter totaling the output pulses, no residual counting errors can occur.

Although the waveforms for VA and VB shown in Fig. 4 are sinusoidal, the circuit will operate satisfactorily for other waveshapes, providing only that the inputs each exhibit peaks which are displaced from each other by ap­proximately one half the width of the base of the peak.

The following circui t design considerations are pertinent: Referring to Fig. 4, the collector supply dc amplifier consists of a conventional common emitter amplifier direct coupled to an emitter follower. The component values are selected to permit the output level Vcs to swing between

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Page 5: Digital-Analog Magnetometer Utilizing Superconducting Sensor

MAGNETOMETER 217

... 12V

-12V

COLLECTOR SUPPLY DC AMPLIFIER

BASE ORIVE DC AMPLIFIER

ALL TRANSISTORS 2N 2714

330 IK IK

10K 10K

+12V

-FLIP-FLOP

1)fBD

2200

100

2200

40

+12 V

120K 2200 68K 18K

DR403 5000 ADD ...... OI--+--il-_-O PULSE

OUTPUT

100 120K 1500 18K 5600

ADO PULSE SHAPER

+12V

120K 2200 68K 18K

DR 403 5000 SUBTRACT PULSE

OUTPUT

10K 100 120K 1500 18K 5600

SUBTRACT PULSE SHAPER

o

Or---~--+---+---~

0

0

o B o B,

B INCREASING -

FIG. 4. Circuitry which converts synchronous detector output levels to add-subtract pulses.

zero V (with input zero or positive) and approximately +5 V (with input -0.4 V or more negative). A Zener diode limits the positive output to about +5 V. The base drive amplifier is of similar design, except tailored to yield output levels of -1.2 V (with input zero or positive) and +4.4 V (with input -0.4 V or more negative). With these output levels, the 2200 n resistance connected to one base of the flip-flop is small enough to insure that the flip-flop can be pulled to the toggling point to permit regenerative switching to occur, but large enough to prevent premature reswitching during the recovery time. The flip-flop re­sistors and capacitors are selected to give the desired minimum recovery time and an adequate collector level change on switching. Capacitors are connected to ground from each collector to prevent collector excursions other than regenerative ones from moving fast enough to produce excessive spurious outputs. The two IN626 diodes and the 39 kn resistor at the input to the add pulse shaper, the 40 pF differentiating capacitors at the input to both output pulse amplifiers, and the biasoo DR403 diodes in both out­put pulse amplifiers are incorporated to enhance the ratio of desired output pulse amplitude (on regeneration) to spurious output pulse amplitude. The remainder of the output pulse amplifier circuitry consists of conventional common emitter amplifiers ac coupled to emitter followers.

The reversible counter employed utilizes five Burroughs BIP-8054 modules rated at a maximum counting rate of 110 kHz and a maximum readout of 99 999. Paired pulse

resolution is 9 JLsec, compared to 15 JLsec recovery time for the flip-flop in the count pulse generating circuitry.

In measuring field changes with the digital counter aug­mented by the CRT for fractional period changes, the accu­racy of determining a fraction of a period depends on how sinusoidal the VB curve of the SQUID is. This in turn de­pends on the adjustment of the weak links in the SQUID. Present SQUIDs employ weak links which consist of fine, pointed columbium screws which join the two halves of the SQUID. Adjustment of the screws changes the cross sectional area of the weak link and affect the SQUID characteristics. The present SQUIDs' VB curves are rarely sinusoidal enough to permit much better than approximately one tenth period resolution of field changes by observing the angle of rotation of the "shaft" over a large number of rotations. A SQUID with a 0.2 cm2

aperture has a periodicity of 1 JLG, and would there­fore permit about 0.1 JLG resolution. For increased res­olution, it is possible to switch from digital to analog mode to measure the final fractional period portion of a field change. This is accomplished merely by activat­ing a switch which permits the integrator to develop a bucking current whose magnitude is determinable by analog meter readout.

An alternate method of measuring a large field change is to begin the measurement using the analog mode on a sensitive scale, say 1 JLG full scale. Assuming an in­creasing field, when I1B reaches 1 JLG, the meter deflects

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Page 6: Digital-Analog Magnetometer Utilizing Superconducting Sensor

218 R. L. FORGACS AND A. WARNICK

!m<

~_AOD_SlJBSTRAe" 81S'lloIILE

• I"tNNlJMBEJlSFOSlTlONE£l FOflDRAWtNG$IMPUClT'I'. ALL CAPACITORS IN IIIICRO FARADS UNLESS NOTED. RESISTORS IN OOOMS I'OWERSUPPL'I RECTIFlEIl$ ARE FW ooOM6519 LI UTCMQD"2 L21)1CTOA-1

FIG. 5. Circuit schematic of digital-analog magnetometer.

full scale. Further increase in field would be expected to cause the meter deflection to exceed full scale; however, the circuitry is designed to make full scale deflection of the meter occur when the integrator is saturated and deliver­ing the maximum current possible. Therefore, further increase in field will cause the SQUID to be pushed away

from the valley point on the VB curve where it has been locked onto; this is evidenced by rotation of the "shaft" on the CRT. Upon reaching! period field change, the system slips onto the next valley of the VB curve, and integrator current drops an amount corresponding to ! period. The digital readout counts this slipped period as

FIG. 6. Front view of magnetometer cabinet.

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Page 7: Digital-Analog Magnetometer Utilizing Superconducting Sensor

MAGNETOMETER 219

FIG. 7. Interior of magneto­meter cabinet.

one period change in field in the normal manner; thus the total field change can be continuously monitored with high precision by a combination of digital and analog readouts.

PERFORMANCE CHARACTERISTICS

The analog portion of the present magnetometer is similar to the original lock-on magnetometer, except that the field modulation frequency has been increased from 10 to 50 kHz. The higher modulation frequency permits faster response filters to be used at the output of the synchronous detector(s) in cases where the filtering required is deter­mined by the modulation frequency rather than the noise generated in the SQUID. The noise in the SQUID is very dependent on the contact adjustments and so, there­fore, is the attainable system' frequency response. It is expected that the noise level originating in the SQUID will be more reproducible in the thin film SQUIDs under development than in the mechanically adjustable contact SQUIDs employed thus far.

Tests conducted to date with the digital-analog mag-

netometer were performed using a SQUID with a periodi­city of approximately 500 jlG. A typical noise level over a 1 min interval is approximately! jlG peak to peak, about the same as obtained with the original lock-on magnetom­eter with the same probe. Use of a more sensitive SQUID, 8 jlG periodicity (and reduction of field coil turns from 240 to 4), reduced the noise level of the original magnetom­eter to approximately 0.2 jlG peak to peak, and should do the same for the present magnetometer in the analog mode.

The maximum rate of change of field which has been followed with the magnetometer in the digital mode was 2130 periods/sec.

The magnitude of the maximum allowable field change over which the system will operate is also a function of SQUID contact adjustment. Although periodic variations of SQUID critical current occur for very large changes in field, to at least 3000 G,5 other (longer wavelength) periodi­cities are superimposed on the periodicities determined by the main aperture, which are believed to be due to micro­scopic apertures under each screw point contact. The result

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Page 8: Digital-Analog Magnetometer Utilizing Superconducting Sensor

220 R. L. FORGACS AND A. WARNICK

FIG. 8. Magnetometer probe, cabinet, and digital readout.

is that the required direct current for a satisfactory VB curve varies with field for large field changes. Manual adjustment of bias current is possible for slowly varying fields. Although it may be feasible to incorporate an auto­matic tracking current control system to keep the bias current optimized, the expectation is that the development of thin film SQUIDs will eliminate multi-aperture con­tacts. The maximum number of consecutive periods counted in a field change to date, without readjustment of the SQUID bias current, is 675. This compares to a maxi­mum full scale deflection of approximately 340 periods for the least sensitive range in the analog mode. (The afore­mentioned figures were obtained using the probe described previously.) Assuming the problem of variable bias cur­rent requirements is eliminated with thin film SQUID's, a precision of 0.1 JLG in 3000 G is anticipated with the digital approach-equivalent to 1 part in 3X101o•

MAGNETOMETER DESCR'lPTION

Electrical

A complete schematic of the digital-analog magnetom­eter is shown in Fig. 5. Considerable use is made of integrated circuits in this instrument. The gain of the ac amplifier is approximately 2XI05 maximum, reducible by factors of 10 and 100. The basic oscillator in the system is a 100 kHz crystal controlled square wave oscillator. This output is fed directly to the 100 kHz synchronous detector reference input. In addition, the output's positive-going leading edges are differentiated and trigger variable time delay monos table multivibrator No. 1. The trailing edge of the pulses from this monos table multivibrator drives a bistable multivibrator whose output provides the refer­ence signal to the 50 kHz synchronous detector. The positive-going edges of the output of the 50 kHz bistable multivibrator trigger variable time delay monos table

multivibrator No.2. The trailing edges of output pulses from this multivibrator then trigger a 10 JLsec fixed pulse width monostable multivibrator, which drives a 50 kHz tuned amplifier to provide 50 kHz sinusoidal modulation of the field coil. Time delay No.2 permits optimization of the relative phase of the 50 kHz modulation and the 50 kHz synchronous detector reference. Time delay No. 1 can then be adjusted to optimize the relative phase of the 100 kHz synchronous detector reference signal. (The simplified block diagram of Fig. 2 shows a frequency doubling arrangement rather than the frequency halving arrange­ment used, but the net result is the same.)

Provisions are incorporated in the instrument for obser­vation of the VI curve of the SQUID at low frequency, 1 kHz, to facilitate contact adjustment.

Physical

Photographs of the instrument are presented in Figs. 6 and 7. The cabinet is approximately 34X22X24 cm. The complete system including probe, magnetometer cab­inet, and digital readout is shown in Fig. 8. The probe incorporates LC filters in the leads, capable of passing 100 kHz, but attenuating higher frequency components in electromagnetic disturbances, broadcast transmissions, and possible electrostatic discharges which may affect SQUID performance. In addition, a resistance of typically 40(}-1000 Q is inserted in series with the current leads to the SQUID to facilitate VI curve observation.

ACKNOWLEDGMENTS

Helpful discussions and ideas for the successful develop­ment of this magnetometer were provided by J. Hickmott and Dr. J. Zimmerman. Assistance in circuit development and construction were provided by C. Kukla.

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