rf SQUID magnetometer with simplified circuitryI. Simacek, V. Zrubec, and M. Skakala Citation: Review of Scientific Instruments 64, 2401 (1993); doi: 10.1063/1.1143895 View online: http://dx.doi.org/10.1063/1.1143895 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/64/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A simplified HTc rf SQUID to analyze the human cardiac magnetic field AIP Advances 4, 127131 (2014); 10.1063/1.4904429 Johnson noise in SQUID magnetometers Rev. Sci. Instrum. 57, 490 (1986); 10.1063/1.1138914 rfSQUID magnetometer biased by harmonic J. Appl. Phys. 50, 4503 (1979); 10.1063/1.326556 Effect of noise on the performance of rf SQUID magnetometers J. Appl. Phys. 50, 521 (1979); 10.1063/1.325645 Optimization of SQUID differential magnetometers AIP Conf. Proc. 44, 145 (1978); 10.1063/1.31331

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ti SQUID magnetometer with simplified circuitry 1. Simacek, V. Zhbec, and M. Skakala Institute of Measurement Science, Slovak Academy of Sciences, 842 I9 Bratfitava, Slovakiq

’ (Received 30 December 1992; accepted for publication 26 April 1993)

This note contains a brief description of a portable type of magnetometer which uses an rf SQUID. In the simplified electronic circuit no low-frequency modulation is used.

To detect weak magnetic fields a portable type of mag- netometer has been constructed, whose modular version enables us to realize multichannel magnetometer systems while keeping good performance specifications. In the elec- tronic circuitry a simpler system was used than usual, re- sulting in a substantial reduction of the dimensions of the apparatus. The aim was to simplify the electronics and at the same time to eliminate possible interference between the modulating signals in multichannel systems.

The maytometer works on the principle of a flux- locked loop, *2 with the rf superconducting quantum inter- ference device (SQUID), biased by a 30-MHz rf signal. In comparison with the classical system,’ no additional low- frequency flux modulation is used, thus eliminating the lock-m detector. This is the same principle as that used by Drung et aL3 for dc SQUID magnetometers. A block dia- gram for the magnetometer is shown in Fig. 1. The input cryogenic part is represented by the rf SQUID and pickup coils. A variety of coil configurations, including magneto- metric or gradiometric systems, can be applied. The output signal of the SQUID is coupled from the resonant circuit directly to the rf preamplifier whose circuits ensure optimal operation conditions, amplification of the signal, tuning of rf circuits, and regulation of the amplitude of the bias sig- nal. The preamplifier amplifies the input signal - 100 dB, and is followed by amplitude detection. The remainder of the magnetometer, which is separated from the preamp- aer, consists of an amplifier, an integrator with an op- tional time constant determined by capacitances Cl or C2, and a manual reset of the signal in the feedback loop. The integrated signal is fed through another amplifier which also functions as an amplifier with a switchable three-level gain of the output signal. A low-frequency signal is con-

FIG. 1. Block diagram for the magnetometer.

netted from the output of the voltage/current converter to the bias resonance circuit whose coil is inductively con- nected to the SQUID. The power line frequency of 50 Hz can be filtered from the output signal by a narrow band rejection filter. An auxiliary triangular shape waveform with a frequency of 100 Hz is used for tuning of the rf circuit and for adjusting the optimum operation of the SQUID. The complete magnetometer has been realized us- ing standard electronic components.

Although this is a simplified version of the original magnetometer using flux modulation, e.g., SKM-2,4 the technical specifications of the system remained practically unchanged and are comparable with more complex and more expensive rf SQUID systems. For example, the rms flux noise (see Fig. 2), is below 10h4 &,/Hz’” down to 1 Hz, the 3-dB bandwidth of the closed loop is 15 kHz, and the maximum slew rate is lo4 &‘s. The slope of the intrin- sic noise spectral density is higher than the usual l/f re- sponse. It complies with the upper tolerance limit of the intrinsic noise frequency response determined by the oper- ational amplifiers which are built in the magnetometer. We have not yet analyzed this problem in more detail. Three sensitivity ranges, 10 mV/&, 100 mV/&, and 1 V/&, can be selected by pushbuttons. Elimination of the noise at 50 Hz is better than 32 dB. Power requirements are * 15 V (70 mA) . We did not observe any significant deterioration of the l/f noise in a practical application with a gradiom- eter system located in a magnetically unprotected environ- ment.

.I 1 2 '10'

Fr&nc:"(Hz) 10'

FJIG. 2. rms flux noise vs frequency.

2401 Rev. Sci. Instrum. 64 (8), August 1993 0034-8748/93/64(8)/2401/2/$6.QQ @ 1993 Amerlcsn institute of Physics 2401 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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’ R. L. Forgacs and A. Warnick, IEEE. Trans. Instrum. Meaa. XM-15, 113 (1966).

3D. Drung, R. Cantor, M. Peters, H. J. Scheer, and H. Koch, Appl.

*F. Wellstood, C. Heiden, and J. Clarke, Rev. Sci. Instrum. $5, Phys. Lett. 57, 406 (1990).

952 ( 1984). 4A. Urban, K. Bartok, L. Podolan, R. Skrucany, and A. Cigan, Proceed-

ings ofthe Confereence DNT (T&a WST, Prague, 1983), p. 253.

2402 Rev. Sci. Instrum., Vol. 64, No. 8, August 1993 Notes 2402 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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