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High-Tc superconductor dc SQUIDs for unshielded operation and their applications
Physica C 378–381 (2002) 1378–1384 www.elsevier.com/locate/physc
High-Tc superconductor dc SQUIDs for unshielded operation and their applications Thomas Schurig b
a,*
, Dietmar Drung a, Sylke Bechstein a, J€ orn Beyer a, Frank Ludwig b
a Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, D-10587 Berlin, Germany Technical University Braunschweig, H.-Sommer-Str. 66, D-38106 Braunschweig, Germany
Received 29 September 2001; accepted 22 November 2001
Abstract Operating high critical temperature superconducting quantum interference device (high-Tc SQUID) systems in moderately shielded or unshielded environment requires SQUID magnetometers and read-out electronics that fulfill a number of stringent requirements. So, they have to have a low noise level even when exposing the magnetometers to high ac and dc magnetic fields, a large bandwidth and dynamic range to guarantee a stable operation in presence of large interferences pffiffiffiffiffiffiffi and a sufficient linearity. Recently developed single-layer high-Tc dc SQUIDs with a noise level of about 50 fT/ Hz seem to be well suited for unshielded operation. An improved directly coupled SQUID read-out electronics with a microcontroller on board enables computer control, large bandwidth and high slew rate operation of the SQUID sensors. The effectiveness of a combination of hardware and software gradiometry for suppression of environmental noise has been demonstrated for a biomagnetic multisensor high-Tc SQUID system. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: SQUID; Read-out electronics; Gradiometry
1. Introduction In recent years it has been demonstrated by several groups that high critical temperature superconducting quantum interference devices (highTc SQUIDs) are useful devices for a number of applications in geophysical sounding, biomagnetic research and diagnostics as well as nondestructive evaluation (NDE) [1,2]. The reduced cooling requirements in comparison to low-Tc SQUIDs
*
Corresponding author. Fax: +49-30-3481-490. E-mail address: [email protected] (T. Schurig).
which can provide ultimate sensitivity makes them very attractive for portable systems. Reliable cryocoolers addressing the temperature range from 60 to 80 K where high-Tc SQUIDs can be operated are already available. They have been successfully implemented in NDE systems and SQUID microscopes. It has been demonstrated that low pffiffiffiffiffiffivery ffi noise levels around and below 10 fT/ Hz can be achieved with high-Tc SQUID magnetometers in magnetically shielded rooms [3,4,8]. However, for practical applications, high-Tc SQUID sensors usually have to be operated in moderately shielded or completely unshielded environment. The SQUID sensors consisting of SQUID magnetometer and
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 7 2 4 - 0
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read-out electronics have to have a low noise level even when exposing the magnetometer to high ac and dc magnetic fields, a large bandwidth and dynamic range to guarantee a stable operation in presence of large interferences and a sufficient linearity. Even if only one signal channel is required the system has to be equipped with a number of sensors in order to enable noise suppression techniques. From a commercial point of view additional requirements have to be considered. The SQUID fabrication technology should enable a reproducible manufacture of the sensors and the sensor electronics should ensure a stable operation and simple handling of the SQUID system which requires the use of digital components in the fluxlocked loop (FLL) read-out electronics. Unfortunately, the high-Tc fabrication technique is still immature and so, sophisticated multilayer SQUID designs seem to be unsuitable for commercial products. The aim of this paper is to show that state-of-the-art single-layer dc SQUIDs in combination with the newest generation of SQUID sensor electronics can fulfill the above mentioned demands.
2. Single-layer SQUID magnetometer A simple and reproducible fabrication technology for single-layer high-Tc SQUID magnetometers can be based on a grain boundary junction technology using 30° SrTiO3 bicrystal substrates with outer dimensions of 10 mm 10 mm. For applications in unshielded environment the magnetometer is exposed to large environmental magnetic fields. It has been demonstrated, that magnetometers having a reduced total area of superconducting material without large connected superconducting areas [5] can be cooled in static magnetic fields in the order of that of the earth and exposed to ac fields with amplitudes in the lT range without increasing the noise down to 1 Hz. This is in particular important for applications like magnetocardiography. Fig. 1(a) shows a directcoupled SQUID magnetometer fabricated from 200-nm-thick YBa2 Cu3 O7x films deposited by hollow-cathode discharge sputtering. The magne-
Fig. 1. Single-layer direct-coupled SQUID magnetometer. (a) Photograph of the SQUID magnetometer. (b) Photograph of the open SQUID capsule with the SQUID chip mounted on the ceramic chip holder with thick-film resistive heater. The chip holder is glued on a ceramic ground plane with printed feedback coil, resistors and bond pads.
tometer is composed of a 100 pH SQUID loop and a slotted pickup loop consisting of 16 loops in parallel each crossing twice the substrate grain
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boundary [6]. The resulting grain boundary junctions in the loops act as flux dams which are supposed to have a critical current so that they remain closed for field amplitudes up to a few lT. The layout is not fully optimized because the current induced by a magnetic field applied to the pickup coil is not the same in each of the 16 loops. But nevertheless, these magnetometers have been successfully used for unshielded operation. To prevent the SQUID chip from moisture and to allow one to heat them above Tc for releasing trapped flux the SQUID chips are encapsulated in a ceramic housing with integrated heater and feedback coils in thick-film technology. The open capsule is shown in Fig. 1(b). The magnetic field noise of a typical magnetometer described above is presented in Fig. 2. When cooling the device in a magnetically shielded pffiffiffiffiffiffiffi room a white noise level of p about ffiffiffiffiffiffiffi 40 fT/ Hz is achieved increasing to 65 fT/ Hz at 1 Hz as represented by the dashed line. The magnetometer has a field sensitivity B=U ¼ 5:2 nT/U0 . When operating the magnetometer in unshielded environment it has to be cooled usually in the magnetic field of the Earth. The solid line gives the noise spectrum measured in the shielded room after cooling the
device in a static magnetic field of 64 lT which is comparable to the Earth’s field. Whereas the white noise level remains nearly unchanged the low frequency noise below 1 Hz has been found to increase with increasing cooling field. This excess noise is caused by thermal fluctuations and related changes of the London penetration depth which causes changes of the sensing area of the SQUID magnetometer. A temperature coefficient of the sensing area of these magnetometers of about 2 102 K1 has been found [6].
3. SQUID read-out electronics The overall performance of a SQUID system is not only determined by the SQUID magnetometer but depends significantly on the quality of the SQUID electronics. Whereas most signals measured with SQUIDs require small slew rate and bandwidth as for example in magnetocardiography, the SQUID system has to track interference signals that demand a high slew rate when it is operated in unshielded environment. A large dynamic range is often required in particular for mobile systems (e.g. airborne systems for geophysical survey). Furthermore, modern SQUID systems should be fully computer-controlled which means that digital circuits such as microcontrollers have to be operated on the read-out electronics boards without interfering with the SQUID. For mobile systems battery-powered operation and low power consumption are substantial. 3.1. Analog read-out electronics
Fig. 2. Magnetic field noise spectra of a direct-coupled SQUID magnetometer measured in the Berlin magnetically shielded room (BMSR). The dashed line represents a spectrum obtained after cooling the magnetometer in zero field. The low-frequency 1/f part is determined by the BMSR. The solid line is the spectrum measured after cooling the device in a static magnetic field of 64 lT.
As an example of state-of-the-art SQUID electronics with analog signal output our latest directcoupled FLL electronics type HF1A/0201 will be described here. This electronics is designed for both low-Tc and high-Tc dc SQUIDs and is described in more detail in Ref. [7]. It combines low noise pffiffiffiffiffiffiffi (preamplifier white noise level of 0.4 nV/ Hz) with high bandwidth (maximum 6 MHz) and slew rate (maximum 2.3 U0 /ls with the magnetometer described above). The setup of this electronics is shown in Fig. 3. Up to three FLL channels are housed in a small Al box which is
T. Schurig et al. / Physica C 378–381 (2002) 1378–1384
Fig. 3. Photograph of the components of the SQUID read-out electronics type HF1A/0201.
usually placed on top of the cryostat with the SQUIDs. This FLL unit contains all circuits needed to operate the SQUIDs. The bias signals of the SQUIDs are generated using in-system programmable microcontrollers and low-noise digital-toanalog (D/A) converters. The analog output signals (nominal voltage range 13 V) are transmitted to a connector box via a shielded cable with a maximum length of about 20 m. The connector box distributes the signals and provides the system power. A nominal power consumption of 1.3 W/ channel is adequate for battery-powered applications. For indoor applications an inexpensive switching power supply is used. In addition antialias filters are also available in the connector box, which can be optionally switched into the output path. All functions of the electronics can be controlled by computer connected to the connector box via an optically isolated RS-232 interface and using a LabVIEWâ program. It should be mentioned here that the FLL boards are manufactured using surface-mount components. The excellent high-frequency performance of surface-mount devices together with a careful board layout made it possible to suppress the interference produced by the on-board microcontroller to a level where it is not visible in the output spectrum. The system has two basic modes of operation: the normal FLL mode used in a measurement and
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the amplifier (AMP) mode which allows for the complete characterization of the SQUID and a convenient adjustment of its working point. Furthermore, a very short reset time of minimum 0.3 ls allows one to increase the dynamic range utilizing the periodicity of the SQUID V–U characteristic. This can be done by simply resetting the system each time the feedback limits are reached and counting the corresponding number of flux quanta. A novel feature of our read-out electronics is automatic bias voltage tuning in the bias reversal mode. Automatic bias voltage tuning improves the system stability and noise in case of large critical currents fluctuations caused by changes in the operation temperature of the SQUID or large magnetic fields penetrating the Josephson junctions. This was experimentally demonstrated with a high-Tc SQUID operated with a cryocooler exhibiting temperature fluctuations of about 0.3 K around the working temperature T ¼ 77 K. Without automatic bias voltage tuning the FLL typically unlocked twice per hour but with automatic tuning it remained locked even in over-night experiments. As auxiliary components the electronics involves a heater circuit to provide the power for the heater integrated into the magnetometer package and an on-board current source which is a very attractive feature intended to supply field coils compensating the Earth’s magnetic field in case of magnetically unshielded systems. With field coils designed to produce 60 pffiffiffiffiffiffi ffi lT, its noise contribution pffiffiffiffiffiffiffi is as low as 17 fT/ Hz at 1 kHz and 41 fT/ Hz at 1 Hz, respectively. Compensating the background field is expected to improve the performance of unshielded high-Tc SQUID systems. Furthermore, the short rise time of the current source allows dynamic field compensation in slowly moving systems. 3.2. Digital read-out electronics An interesting alternative to analog SQUID read-out is digital read-out. Here, the analog output of the preamplifier is digitized, integrated using a digital signal processor, and fed back to the SQUID via a fast D/A converter. The digital
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Fig. 4. Single-channel board with separately shielded analog and digital sections and 3-channel unit of the STL GradMag digital SQUID electronics (with courtesy of Systemtechnik Ludwig GmbH, see www.stl-gmbh.de).
feedback loop is highly suited to increase the dynamic range by flux quanta counting, i.e., every time the flux fed back to the SQUID exceeds 1 U0 the integrator is reset and the corresponding U0 steps are counted. Furthermore, higher order integrators can easily be implemented to improve the slew rate at low frequencies. A digital SQUID electronics that allows the operation of high-Tc dc SQUIDs with bias reversal is for example commercially available from the German manufacturer STL Systemtechnik Ludwig. This electronics is very flexible and versions for both high-Tc and low-Tc SQUIDs are existing. The concept of the preamplifier is similar to that of the analog electronics described above and a similar noise level is achieved. The power consumption of the whole electronics is about 1 W/channel. The electronics is connected to a computer interface card via optical fibers and the functions of the electronics are controlled by a software that enables among other features automated calibration for setting the working point which makes a SQUID system equipped with this electronics more user-friendly. The STL electronics is shown
in Fig. 4. Test measurements have been performed by STL in the magnetically shielded rooms at PTB. For a 16 mm 16 mm size pickup loop SQUID manufactured by M Faley (Forschungszentrum J€ ulich, p Germany) a very low white noise level of ffiffiffiffiffiffiffi 6.5 fT/ Hz was measured under shielded condition [8]. Using such a digital read-out concept allows one to achieve a very large dynamic range. However, even if the SQUID is kept locked during operation in large varying fields, large currents can circulate in the pickup coil giving raise to degraded linearity of the device. This behavior has to be investigated in more detail in the future.
4. A second-order gradiometer system for magnetocardiography The biomagnetic field of the human heart has been investigated intensively using multichannel low-Tc SQUID systems [9]. High-Tc SQUID systems with only a few channels operated in unshielded environment are well suited to evaluate
T. Schurig et al. / Physica C 378–381 (2002) 1378–1384
the efficiency of noise suppression techniques and the potential of high-Tc SQUID devices. We have built and tested a second-order gradiometer system for biomagnetic measurements based on five SQUID magnetometers of the type described in Section 2. A 45° inclined dewar was chosen so that the magnetocardiograms (MCGs) can be recorded in the sitting position as it is shown in Fig. 5. That means, the signal sensor detects the horizontal magnetic field component (z direction). Gradiometry is performed by combining electronic gradiometers and software gradiometry. This is an effective approach for relaxing the requirements on the A/D conversion of the data acquisition system. Two first-order electronic gradiometers, having a baseline of 75 mm, are performed with the three z magnetometers. A balance of 1:100 is achieved. The second-order gradiometer is performed by software after 16 bit A/D conversion using the two electronic first-order gradiometers, the center z magnetometer and the x and y magnetometers as inputs. Adaptive noise cancellation is used to adjust the system to a given noise environment. This technique is in particular useful for portable systems or mechanically complex systems operated in
Fig. 5. Photograph of a portable second-order gradiometer system for magnetocardiography.
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a noise environment that is relatively stable in time. In contrast to systems which have to be balanced in calibration coil systems the demands on mechanical system stability and parameter stability of the sensors are significantly reduced. The idea of adaptive noise cancellation is to express the noise of the signal sensor by the noise of the reference sensors. Applying the time-domain (TD) noise cancellation algorithm, as described by Woeltgens and Koch [10] the corrected signal is given by X Bgrad ðtÞ ¼ Bsig ðtÞ ai Bref;i ðtÞ; where ai are the calibration coefficients, determined by the algorithm, of the individual signals from the reference sensors. A better environmental noise suppression can be achieved by using frequency domain (FD) balancing procedures, i.e., each reference signal is not weighted by a single coefficient ai but by a frequency dependent vector hj ðf Þ [10]. The coefficients are obtained by recording the environmental noise without signal over a time period followed by a calculation procedure based on the minimization of the total noise energy of the system. After calculating the coefficients the system is ready for signal recording. With the TD balanced system the signal can be displayed easily on the computer in real-time. FD balancing requires much more computational power. So we have corrected the measured signal
Fig. 6. Magnetic field noise spectra for z magnetometer, TD and FD balanced second-order gradiometer measured in unshielded environment. SQUIDs were operated with 100 kHz bias reversal.
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tronics has lead to powerful, flexible and userfriendly instruments. In addition, features like automatic bias voltage tuning or background field cancellation using implemented low-noise current sources help to improve the system stability and noise under unfavorable measurement conditions. Environmental noise suppression by combination of electronic and software gradiometry and in particular adaptive noise cancellation has been demonstrated to be effective for unshielded systems.
Fig. 7. Averaged MCGs obtained from the TD and FD balanced system. The lower MCGs are obtained by applying a software 50 Hz notch filter and reducing the bandwidth to 100 Hz. The MCGs are shifted against each other for clarity.
after recording it. Fig. 6 shows the magnetic field noise spectra of the center z magnetometer, the TD balanced second-order gradiometer, and the FD balanced second-order gradiometer measured in a PTB laboratory environment. Whereas the TD corrected signal is still dominated by certain frequency components (16 23 Hz, 50 Hz, or 150 Hz) these interferences are further suppressed by FD balancing. In Fig. 7 averaged MCGs are presented. For future improvements of the signalto-noise ratio additional reference sensors have to be implemented in the system.
5. Conclusion State-of-the-arts single-layer high-Tc SQUIDs are already well suited for unshielded operation. These relatively simple devices can be manufactured with a fairly good reproducibility so that in combination with appropriate read-out electronics systems with a moderate number of SQUID channels can be built. The use of surface-mount devices and microcontrollers for SQUID read-out elec-
Acknowledgements The authors thank W. Ludwig from STL for providing material about the GradMAGâ electronics, and K.-P. Franke and M. Scheiner for help with the electronics design. This work was funded in part by the German BMBF under grant no. 13N7326.
References [1] A. Braginski, H.-J. Krause, J. Vrba, in: M.H. Fracombe (Ed.), Handbook of Thin Film Devices, vol. 3, 2000, pp. 149–225. [2] H. Itozaki, Physica C 357–360 (2001) 7–10. [3] D. Drung, F. Ludwig, W. M€ uller, U. Steinhoff, L. Trahms, Y.Q. Shen, M.B. Jensen, P. Vase, T. Holst, T. Freltoft, G. Curion, Appl. Phys. Lett. 68 (1996) 1421–1423. [4] E. Dantsker, F. Ludwig, R. Kleiner, J. Clarke, M. Teepe, L.P. Lee, N.McN. Acford, T. Button, Appl. Phys. Lett. 67 (1995) 725–726. [5] E. Dantsker, S. Tanaka, P.-A. Nilson, R. Kleiner, J. Clarke, Appl. Phys. Lett. 69 (1996) 4099–4101. [6] F. Ludwig, D. Drung, Appl. Phys. Lett. 75 (1999) 2821– 2823. [7] D. Drung, Physica C 368 (2002) 134–140. [8] C. Ludwig, C. Kessler, A.J. Steinfort, W. Ludwig, IEEE Trans. Appl. Supercond. 11 (2001) 1122–1125. [9] H. Koch, IEEE Trans. Appl. Supercond. 11 (2001) 49– 59. [10] P.J.M. Woeltgens, R.H. Koch, Rev. Sci. Instrum. 71 (2000) 1529–1533.
High-Tc superconductor dc SQUIDs for unshielded operation and their applications Thomas Schurig b
a,*
, Dietmar Drung a, Sylke Bechstein a, J€ orn Beyer a, Frank Ludwig b
a Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, D-10587 Berlin, Germany Technical University Braunschweig, H.-Sommer-Str. 66, D-38106 Braunschweig, Germany
Received 29 September 2001; accepted 22 November 2001
Abstract Operating high critical temperature superconducting quantum interference device (high-Tc SQUID) systems in moderately shielded or unshielded environment requires SQUID magnetometers and read-out electronics that fulfill a number of stringent requirements. So, they have to have a low noise level even when exposing the magnetometers to high ac and dc magnetic fields, a large bandwidth and dynamic range to guarantee a stable operation in presence of large interferences pffiffiffiffiffiffiffi and a sufficient linearity. Recently developed single-layer high-Tc dc SQUIDs with a noise level of about 50 fT/ Hz seem to be well suited for unshielded operation. An improved directly coupled SQUID read-out electronics with a microcontroller on board enables computer control, large bandwidth and high slew rate operation of the SQUID sensors. The effectiveness of a combination of hardware and software gradiometry for suppression of environmental noise has been demonstrated for a biomagnetic multisensor high-Tc SQUID system. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: SQUID; Read-out electronics; Gradiometry
1. Introduction In recent years it has been demonstrated by several groups that high critical temperature superconducting quantum interference devices (highTc SQUIDs) are useful devices for a number of applications in geophysical sounding, biomagnetic research and diagnostics as well as nondestructive evaluation (NDE) [1,2]. The reduced cooling requirements in comparison to low-Tc SQUIDs
*
Corresponding author. Fax: +49-30-3481-490. E-mail address: [email protected] (T. Schurig).
which can provide ultimate sensitivity makes them very attractive for portable systems. Reliable cryocoolers addressing the temperature range from 60 to 80 K where high-Tc SQUIDs can be operated are already available. They have been successfully implemented in NDE systems and SQUID microscopes. It has been demonstrated that low pffiffiffiffiffiffivery ffi noise levels around and below 10 fT/ Hz can be achieved with high-Tc SQUID magnetometers in magnetically shielded rooms [3,4,8]. However, for practical applications, high-Tc SQUID sensors usually have to be operated in moderately shielded or completely unshielded environment. The SQUID sensors consisting of SQUID magnetometer and
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 7 2 4 - 0
T. Schurig et al. / Physica C 378–381 (2002) 1378–1384
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read-out electronics have to have a low noise level even when exposing the magnetometer to high ac and dc magnetic fields, a large bandwidth and dynamic range to guarantee a stable operation in presence of large interferences and a sufficient linearity. Even if only one signal channel is required the system has to be equipped with a number of sensors in order to enable noise suppression techniques. From a commercial point of view additional requirements have to be considered. The SQUID fabrication technology should enable a reproducible manufacture of the sensors and the sensor electronics should ensure a stable operation and simple handling of the SQUID system which requires the use of digital components in the fluxlocked loop (FLL) read-out electronics. Unfortunately, the high-Tc fabrication technique is still immature and so, sophisticated multilayer SQUID designs seem to be unsuitable for commercial products. The aim of this paper is to show that state-of-the-art single-layer dc SQUIDs in combination with the newest generation of SQUID sensor electronics can fulfill the above mentioned demands.
2. Single-layer SQUID magnetometer A simple and reproducible fabrication technology for single-layer high-Tc SQUID magnetometers can be based on a grain boundary junction technology using 30° SrTiO3 bicrystal substrates with outer dimensions of 10 mm 10 mm. For applications in unshielded environment the magnetometer is exposed to large environmental magnetic fields. It has been demonstrated, that magnetometers having a reduced total area of superconducting material without large connected superconducting areas [5] can be cooled in static magnetic fields in the order of that of the earth and exposed to ac fields with amplitudes in the lT range without increasing the noise down to 1 Hz. This is in particular important for applications like magnetocardiography. Fig. 1(a) shows a directcoupled SQUID magnetometer fabricated from 200-nm-thick YBa2 Cu3 O7x films deposited by hollow-cathode discharge sputtering. The magne-
Fig. 1. Single-layer direct-coupled SQUID magnetometer. (a) Photograph of the SQUID magnetometer. (b) Photograph of the open SQUID capsule with the SQUID chip mounted on the ceramic chip holder with thick-film resistive heater. The chip holder is glued on a ceramic ground plane with printed feedback coil, resistors and bond pads.
tometer is composed of a 100 pH SQUID loop and a slotted pickup loop consisting of 16 loops in parallel each crossing twice the substrate grain
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boundary [6]. The resulting grain boundary junctions in the loops act as flux dams which are supposed to have a critical current so that they remain closed for field amplitudes up to a few lT. The layout is not fully optimized because the current induced by a magnetic field applied to the pickup coil is not the same in each of the 16 loops. But nevertheless, these magnetometers have been successfully used for unshielded operation. To prevent the SQUID chip from moisture and to allow one to heat them above Tc for releasing trapped flux the SQUID chips are encapsulated in a ceramic housing with integrated heater and feedback coils in thick-film technology. The open capsule is shown in Fig. 1(b). The magnetic field noise of a typical magnetometer described above is presented in Fig. 2. When cooling the device in a magnetically shielded pffiffiffiffiffiffiffi room a white noise level of p about ffiffiffiffiffiffiffi 40 fT/ Hz is achieved increasing to 65 fT/ Hz at 1 Hz as represented by the dashed line. The magnetometer has a field sensitivity B=U ¼ 5:2 nT/U0 . When operating the magnetometer in unshielded environment it has to be cooled usually in the magnetic field of the Earth. The solid line gives the noise spectrum measured in the shielded room after cooling the
device in a static magnetic field of 64 lT which is comparable to the Earth’s field. Whereas the white noise level remains nearly unchanged the low frequency noise below 1 Hz has been found to increase with increasing cooling field. This excess noise is caused by thermal fluctuations and related changes of the London penetration depth which causes changes of the sensing area of the SQUID magnetometer. A temperature coefficient of the sensing area of these magnetometers of about 2 102 K1 has been found [6].
3. SQUID read-out electronics The overall performance of a SQUID system is not only determined by the SQUID magnetometer but depends significantly on the quality of the SQUID electronics. Whereas most signals measured with SQUIDs require small slew rate and bandwidth as for example in magnetocardiography, the SQUID system has to track interference signals that demand a high slew rate when it is operated in unshielded environment. A large dynamic range is often required in particular for mobile systems (e.g. airborne systems for geophysical survey). Furthermore, modern SQUID systems should be fully computer-controlled which means that digital circuits such as microcontrollers have to be operated on the read-out electronics boards without interfering with the SQUID. For mobile systems battery-powered operation and low power consumption are substantial. 3.1. Analog read-out electronics
Fig. 2. Magnetic field noise spectra of a direct-coupled SQUID magnetometer measured in the Berlin magnetically shielded room (BMSR). The dashed line represents a spectrum obtained after cooling the magnetometer in zero field. The low-frequency 1/f part is determined by the BMSR. The solid line is the spectrum measured after cooling the device in a static magnetic field of 64 lT.
As an example of state-of-the-art SQUID electronics with analog signal output our latest directcoupled FLL electronics type HF1A/0201 will be described here. This electronics is designed for both low-Tc and high-Tc dc SQUIDs and is described in more detail in Ref. [7]. It combines low noise pffiffiffiffiffiffiffi (preamplifier white noise level of 0.4 nV/ Hz) with high bandwidth (maximum 6 MHz) and slew rate (maximum 2.3 U0 /ls with the magnetometer described above). The setup of this electronics is shown in Fig. 3. Up to three FLL channels are housed in a small Al box which is
T. Schurig et al. / Physica C 378–381 (2002) 1378–1384
Fig. 3. Photograph of the components of the SQUID read-out electronics type HF1A/0201.
usually placed on top of the cryostat with the SQUIDs. This FLL unit contains all circuits needed to operate the SQUIDs. The bias signals of the SQUIDs are generated using in-system programmable microcontrollers and low-noise digital-toanalog (D/A) converters. The analog output signals (nominal voltage range 13 V) are transmitted to a connector box via a shielded cable with a maximum length of about 20 m. The connector box distributes the signals and provides the system power. A nominal power consumption of 1.3 W/ channel is adequate for battery-powered applications. For indoor applications an inexpensive switching power supply is used. In addition antialias filters are also available in the connector box, which can be optionally switched into the output path. All functions of the electronics can be controlled by computer connected to the connector box via an optically isolated RS-232 interface and using a LabVIEWâ program. It should be mentioned here that the FLL boards are manufactured using surface-mount components. The excellent high-frequency performance of surface-mount devices together with a careful board layout made it possible to suppress the interference produced by the on-board microcontroller to a level where it is not visible in the output spectrum. The system has two basic modes of operation: the normal FLL mode used in a measurement and
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the amplifier (AMP) mode which allows for the complete characterization of the SQUID and a convenient adjustment of its working point. Furthermore, a very short reset time of minimum 0.3 ls allows one to increase the dynamic range utilizing the periodicity of the SQUID V–U characteristic. This can be done by simply resetting the system each time the feedback limits are reached and counting the corresponding number of flux quanta. A novel feature of our read-out electronics is automatic bias voltage tuning in the bias reversal mode. Automatic bias voltage tuning improves the system stability and noise in case of large critical currents fluctuations caused by changes in the operation temperature of the SQUID or large magnetic fields penetrating the Josephson junctions. This was experimentally demonstrated with a high-Tc SQUID operated with a cryocooler exhibiting temperature fluctuations of about 0.3 K around the working temperature T ¼ 77 K. Without automatic bias voltage tuning the FLL typically unlocked twice per hour but with automatic tuning it remained locked even in over-night experiments. As auxiliary components the electronics involves a heater circuit to provide the power for the heater integrated into the magnetometer package and an on-board current source which is a very attractive feature intended to supply field coils compensating the Earth’s magnetic field in case of magnetically unshielded systems. With field coils designed to produce 60 pffiffiffiffiffiffi ffi lT, its noise contribution pffiffiffiffiffiffiffi is as low as 17 fT/ Hz at 1 kHz and 41 fT/ Hz at 1 Hz, respectively. Compensating the background field is expected to improve the performance of unshielded high-Tc SQUID systems. Furthermore, the short rise time of the current source allows dynamic field compensation in slowly moving systems. 3.2. Digital read-out electronics An interesting alternative to analog SQUID read-out is digital read-out. Here, the analog output of the preamplifier is digitized, integrated using a digital signal processor, and fed back to the SQUID via a fast D/A converter. The digital
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Fig. 4. Single-channel board with separately shielded analog and digital sections and 3-channel unit of the STL GradMag digital SQUID electronics (with courtesy of Systemtechnik Ludwig GmbH, see www.stl-gmbh.de).
feedback loop is highly suited to increase the dynamic range by flux quanta counting, i.e., every time the flux fed back to the SQUID exceeds 1 U0 the integrator is reset and the corresponding U0 steps are counted. Furthermore, higher order integrators can easily be implemented to improve the slew rate at low frequencies. A digital SQUID electronics that allows the operation of high-Tc dc SQUIDs with bias reversal is for example commercially available from the German manufacturer STL Systemtechnik Ludwig. This electronics is very flexible and versions for both high-Tc and low-Tc SQUIDs are existing. The concept of the preamplifier is similar to that of the analog electronics described above and a similar noise level is achieved. The power consumption of the whole electronics is about 1 W/channel. The electronics is connected to a computer interface card via optical fibers and the functions of the electronics are controlled by a software that enables among other features automated calibration for setting the working point which makes a SQUID system equipped with this electronics more user-friendly. The STL electronics is shown
in Fig. 4. Test measurements have been performed by STL in the magnetically shielded rooms at PTB. For a 16 mm 16 mm size pickup loop SQUID manufactured by M Faley (Forschungszentrum J€ ulich, p Germany) a very low white noise level of ffiffiffiffiffiffiffi 6.5 fT/ Hz was measured under shielded condition [8]. Using such a digital read-out concept allows one to achieve a very large dynamic range. However, even if the SQUID is kept locked during operation in large varying fields, large currents can circulate in the pickup coil giving raise to degraded linearity of the device. This behavior has to be investigated in more detail in the future.
4. A second-order gradiometer system for magnetocardiography The biomagnetic field of the human heart has been investigated intensively using multichannel low-Tc SQUID systems [9]. High-Tc SQUID systems with only a few channels operated in unshielded environment are well suited to evaluate
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the efficiency of noise suppression techniques and the potential of high-Tc SQUID devices. We have built and tested a second-order gradiometer system for biomagnetic measurements based on five SQUID magnetometers of the type described in Section 2. A 45° inclined dewar was chosen so that the magnetocardiograms (MCGs) can be recorded in the sitting position as it is shown in Fig. 5. That means, the signal sensor detects the horizontal magnetic field component (z direction). Gradiometry is performed by combining electronic gradiometers and software gradiometry. This is an effective approach for relaxing the requirements on the A/D conversion of the data acquisition system. Two first-order electronic gradiometers, having a baseline of 75 mm, are performed with the three z magnetometers. A balance of 1:100 is achieved. The second-order gradiometer is performed by software after 16 bit A/D conversion using the two electronic first-order gradiometers, the center z magnetometer and the x and y magnetometers as inputs. Adaptive noise cancellation is used to adjust the system to a given noise environment. This technique is in particular useful for portable systems or mechanically complex systems operated in
Fig. 5. Photograph of a portable second-order gradiometer system for magnetocardiography.
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a noise environment that is relatively stable in time. In contrast to systems which have to be balanced in calibration coil systems the demands on mechanical system stability and parameter stability of the sensors are significantly reduced. The idea of adaptive noise cancellation is to express the noise of the signal sensor by the noise of the reference sensors. Applying the time-domain (TD) noise cancellation algorithm, as described by Woeltgens and Koch [10] the corrected signal is given by X Bgrad ðtÞ ¼ Bsig ðtÞ ai Bref;i ðtÞ; where ai are the calibration coefficients, determined by the algorithm, of the individual signals from the reference sensors. A better environmental noise suppression can be achieved by using frequency domain (FD) balancing procedures, i.e., each reference signal is not weighted by a single coefficient ai but by a frequency dependent vector hj ðf Þ [10]. The coefficients are obtained by recording the environmental noise without signal over a time period followed by a calculation procedure based on the minimization of the total noise energy of the system. After calculating the coefficients the system is ready for signal recording. With the TD balanced system the signal can be displayed easily on the computer in real-time. FD balancing requires much more computational power. So we have corrected the measured signal
Fig. 6. Magnetic field noise spectra for z magnetometer, TD and FD balanced second-order gradiometer measured in unshielded environment. SQUIDs were operated with 100 kHz bias reversal.
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tronics has lead to powerful, flexible and userfriendly instruments. In addition, features like automatic bias voltage tuning or background field cancellation using implemented low-noise current sources help to improve the system stability and noise under unfavorable measurement conditions. Environmental noise suppression by combination of electronic and software gradiometry and in particular adaptive noise cancellation has been demonstrated to be effective for unshielded systems.
Fig. 7. Averaged MCGs obtained from the TD and FD balanced system. The lower MCGs are obtained by applying a software 50 Hz notch filter and reducing the bandwidth to 100 Hz. The MCGs are shifted against each other for clarity.
after recording it. Fig. 6 shows the magnetic field noise spectra of the center z magnetometer, the TD balanced second-order gradiometer, and the FD balanced second-order gradiometer measured in a PTB laboratory environment. Whereas the TD corrected signal is still dominated by certain frequency components (16 23 Hz, 50 Hz, or 150 Hz) these interferences are further suppressed by FD balancing. In Fig. 7 averaged MCGs are presented. For future improvements of the signalto-noise ratio additional reference sensors have to be implemented in the system.
5. Conclusion State-of-the-arts single-layer high-Tc SQUIDs are already well suited for unshielded operation. These relatively simple devices can be manufactured with a fairly good reproducibility so that in combination with appropriate read-out electronics systems with a moderate number of SQUID channels can be built. The use of surface-mount devices and microcontrollers for SQUID read-out elec-
Acknowledgements The authors thank W. Ludwig from STL for providing material about the GradMAGâ electronics, and K.-P. Franke and M. Scheiner for help with the electronics design. This work was funded in part by the German BMBF under grant no. 13N7326.
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