[Show abstract][Hide abstract] ABSTRACT: We analyzed the dc SQUID with voltage feedback circuit (VFC) and a low-noise room-temperature preamplifier to evaluate the feasibility of a low-noise SQUID direct-coupled readout scheme (DRS), possibly eliminating the need for a two-stage scheme employing a SQUID preamplifier. The passive VFC, connected in parallel to the SQUID, consists of a resistor Rs in series with an inductor L s. This inductor is coupled to the SQUID by a mutual inductance Ms. The purpose of the VFC is to increase the SQUID's flux-to-voltage transfer coefficient ∂V/∂Φ, thus reducing the preamplifier noise contribution δΦpreamp. However, at the same time, VFC introduces the thermal noise of Rs, δΦR, which may not be negligible. Generally, the noise of the readout scheme, δΦreadout, may thus include both δΦpreamp and δΦR, i.e., δΦreadout2 = δΦpreamp2 + δΦR2. To characterize the SQUID operation with VFC we introduced two dimensionless parameters, r = Rs/Rd and Δ = (M s/Mdyn) − (Rs/R d), where Rd and Mdyn = 1/(∂i/∂Φ) are dynamic properties of the SQUID itself. For assumed intrinsic SQUID parameters, we then numerically analyzed the dependence of δΦreadout noise components on r and Δ to determine their suitable ranges and the minimum of δΦreadout. To verify our analysis, we experimentally characterized, in liquid helium, three niobium SQUIDs with VFC, having suitably chosen r and Δ. The measured SQUID system flux noise was on the order of 1 μΦ0/√Hz, comparable to the intrinsic noise of the SQUID itself. The deduced equivalent voltage noise was comparable to that of a SQUID preamplifier in the two-stage readout. Simple single-stage ultra-low-noise SQUID DRS readout was thus demonstrated.
[Show abstract][Hide abstract] ABSTRACT: We experimentally studied weakly damped superconducting quantum interference devices (SQUIDs) shunted by an external resistor Rs and operated in either current- or voltage-bias mode. The SQUID parameters, such as the flux-to-voltage transfer coefficient @V/@U and the dynamic resistance Rd, are reduced due to Rs, while the SQUID intrinsic noise remains unchanged. The reduced parameters can be enhanced again by using voltage feedback circuitry. Furthermore, Rs can be used to damp the SQUID in order to avoid the appearance of hysteresis or oscillation in SQUID characteristics. SQUID shunted by small Rs is always operated in mixed-bias mode.
[Show abstract][Hide abstract] ABSTRACT: We investigated niobium thin film superconducting quantum interference devices (SQUIDs) with
large Steward-McCumber parameter (bc>1). No hysteresis was observed in the current-voltage (I-V)
characteristics of the SQUIDs, even for bc � 17. We attribute the absence of hysteresis to an excess
voltage noise of the junctions which increases the SQUID intrinsic noise dUs. It can be represented
by an effective noise temperature T* of the SQUID which is higher than the bath temperature T. We
simulated SQUID I-V characteristics using the measured device parameters and confirmed the
absence of hysteresis.
[Show abstract][Hide abstract] ABSTRACT: Recently, it has been shown that voltage-biased readout of SQUIDs with weakly damped junctions (large Stewart–McCumber parameter β c , due to high shunt resistance) is useful for suppression of preamplifier noise. We experimentally studied the characteristics of 53 planar niobium–SQUID magnetometers with junction shunt resistors R J nominally of 30 fabricated on 5 × 5 mm 2 chips. The field-to-flux transfer coefficient ∂B/∂ of the magnetometers was 1.5 nT/ 0 , with a SQUID loop inductance L s of about 350 pH. The distributions of important SQUID parameters, such as the current swing I swing , the dynamic resistance R d , and the flux-to-voltage transfer coefficient ∂V/∂ are given. Nearly all the SQUIDs could be stably operated in the voltage bias mode and their ∂V/∂ reached a large mean value of 380 µV/ 0 . In this case, the SQUIDs can be read out directly by a commercial operational amplifier without any additional means to suppress preamplifier noise. The mean flux noise of the SQUIDs was found to be 4.5 µµ 0 Hz −1/2 , corresponding to a field resolution of 7 fT Hz −1/2 . To demonstrate the applicability of these SQUIDs in the direct readout scheme, a simple four-channel SQUID gradiometer system was set up to perform magnetocardiography and magnetoencephalography measurements in a magnetically shielded room. (Some figures may appear in colour only in the online journal)
[Show abstract][Hide abstract] ABSTRACT: A planar superconducting quantum interference device (SQUID) magnetometer consisting of a parallel gradiometer SQUID with integrated input coils connected to an on-chip pickup loop was designed and fabricated in conventional niobium technology. SQUID bootstrap circuitry (SBC) incorporating suitable current and voltage feedbacks was also integrated into the design. For a SQUID inductance of Ls = 350 pH and a chip size of 5 × 5 mm2, the field resolution of the voltage-biased SQUID magnetometer reached <5 fT Hz−1/2 with the bootstrap circuit and an ordinary preamplifier. We also observed that the effective McCumber parameter βc of the junctions is influenced by the bias mode. Indeed, when the nominal junction βc was larger than unity, our SQUID magnetometer operated stably in the voltage bias mode. The device exhibited low noise even without SBC.
[Show abstract][Hide abstract] ABSTRACT: We experimentally studied two important parameters of helium-cooled superconducting quantum interference devices (SQUIDs) in the voltage bias mode: the dynamic resistance Rd and the flux-to-current transfer coefficient ∂i/∂Φ, with different junction shunt resistors RJ. We investigated a voltage-biased SQUID using the direct readout current-to-voltage converter scheme involving an operational amplifier. At higher RJ, the flux-to-voltage conversion coefficient ∂V/∂Φ becomes sufficiently large to effectively suppress the room-temperature amplifier's noise without any need for additional feedback circuits. The McCumber parameter limits the rise of ∂V/∂Φ. We discuss the performance of voltage-biased SQUIDs at different effective McCumber parameters.
[Show abstract][Hide abstract] ABSTRACT: We recently demonstrated and analysed the voltage-biased SQUID bootstrap circuit (SBC) conceived to suppress the preamplifier noise contribution in the absence of flux modulation readout. Our scheme contains both the additional voltage and current feedbacks. In this study, we analysed the tolerance of the SBC noise suppression performance to spreads in SQUID and SBC circuit parameters. Analytical results were confirmed by experiments. A one-time adjustable current feedback can be used to extend the tolerance to spreads such as those caused by the integrated circuit fabrication process. This should help to improve the fabrication yield of SBC devices integrated on one chip-as required for multi-channel SQUID systems.
[Show abstract][Hide abstract] ABSTRACT: The voltage-biased SQUID bootstrap circuit (SBC) is suitable for achieving simple and low-noise direct readout of dc SQUIDs. In practice, an ideal voltage bias is difficult to realize because of non-zero internal resistance Rin of the bias voltage source. In order to clearly observe the influence of Rin on the SBC parameters (namely the flux-to-current transfer coefficient (∂I/∂Φ)SBC and the dynamic resistance Rd(SBC)) and the noise performance, we introduced an additional adjustable resistor Rad at room temperature to simulate a variable Rin between the SQUID and the preamplifier. We found that the measured SQUID flux noise does not rise, even though Rad increases significantly. This result demonstrates that a highly resistive connection can be inserted between the liquid-helium-cooled SQUID and the room-temperature readout electronics in the SBC scheme, thus reducing the conductive heat loss of the system. This work will be significant for developing multichannel SBC readout systems, e.g. for biomagnetism, and systems using SQUIDs as amplifiers, for example, in TES-array readout.
[Show abstract][Hide abstract] ABSTRACT: The SQUID Bootstrap Circuit (SBC) for direct-coupled readout of SQUID signals in voltage bias mode was recently demonstrated. In addition to the conventional dc SQUID, the SBC incorporates a shunt resistor R <sub>s</sub>, and two coils coupled to the SQUID via mutual inductances M <sub>1</sub> and M <sub>2</sub> . In this paper, basic equations of SBC are formulated based on its equivalent circuit model. The expression of equivalent flux noise from the preamplifier is also given. The effect of the three adjustable parameters ( M <sub>1</sub>, M <sub>2</sub> and R <sub>s</sub>) on the characteristics of SBC and the preamplifier noise suppression are numerically simulated. The SBC combines current and voltage feedbacks in one circuit, allowing for an effective suppression of the preamplifier voltage noise through increased flux-current transfer coefficient and dynamic resistance. In contrast to other direct-coupled schemes, it offers not only a good noise performance, but also tolerance to a wide range of adjustable parameters.
[Show abstract][Hide abstract] ABSTRACT: We recently presented a direct readout technique for the dc Superconducting QUantum Interference Device (SQUID) without flux modulation (FM), operated in voltage bias mode, and named it the SQUID Bootstrap Circuit (SBC). The SBC combines additional voltage and current feedbacks to minimize the room-temperature preamplifier noise. The main point of this paper is to compare the flux noise performance of the SBC readout with that of the FM scheme using a sine wave modulation signal. Several liquid-helium-cooled SQUID magnetometers with different layouts and loop inductances were characterized using these two readout schemes. Measured noise was comparable to or even lower than that measured by FM electronics. Furthermore, the SBC noise performance was evaluated as function of resistance which, when properly adjusted, permits us to nearly fulfill the critical noise suppression condition. We believe SBC to be a promising candidate for multi-channel SQUID systems.
[Show abstract][Hide abstract] ABSTRACT: Recently, we demonstrated and analysed the superconducting quantum interference device (SQUID) bootstrap circuit (SBC). It is a direct readout scheme for dc SQUID in the voltage bias mode, permitting one to suppress the preamplifier noise. The SBC enables us to control the two key parameters of a voltage-biased SQUID: the flux-to-current transfer coefficient and the dynamic resistance. The flux-to-current, I-Phi, characteristics of SBC are made asymmetric by introducing the additional current feedback. Depending upon the choice of the working point, this feedback can be positive (working point W-2 on the steeper I-Phi slope) or negative (W-1 on the less steep slope). The dynamic resistance is controlled by the additional voltage feedback. In our publications to date we presented only the SBC operation at W-2, while in this paper we demonstrate operation at W-1 and show that also in this regime the preamplifier noise suppression is possible. We used a liquid-helium-cooled Nb SQUID with a loop inductance of 350 pH and attained white flux noise of 2.5 mu Phi(0) Hz(-1/2) both at W-2 and at W-1. In the latter case, the linear flux range exceeded one half-flux quantum Phi(0). This large linear range should lead to a significantly improved stability and slew rate of the system and also make the tolerable spread in circuit parameters much wider than in all SQUID direct readout schemes known to date. Consequently, operation in this regime opens a new path to possible SBC optimization.
[Show abstract][Hide abstract] ABSTRACT: Low-field (LF) liquid sample NMR measurements using different detectors are performed at Larmor frequencies fL ranging from 300 Hz to 35 kHz. Five different sensors cooled with liquid nitrogen are compared: a wire-wound coil, a printed planar copper coil, both resonant with capacitance C and read out by a room-temperature amplifier, a high critical temperature Tc thin film rf (bare) SQUID and two tuned SQUID circuits, one with a wire-wound input coil and another with a high-Tc superconducting tape coil. The resonant frequency of each LC circuit is adjusted to fL. The sensitivity of any LC circuit is determined by its inductance, quality factor, and fL. The magnetic field resolution of the sensors was evaluated with a homogenous magnetic field source at different frequencies and with a small source at different positions. We discuss the signal-to-noise ratio of the LF NMR signals recorded by these detectors and find the high-Tc tape coil to be unsuitable for LF NMR measurement. The homogeneity of the magnetic measurement field is deteriorated by this bulk superconducting object. That effect leads to a dramatic reduction of the decay time of the free induction decay (FID) signal. In contrast, the bare SQUID oriented parallel to the measurement field does not influence the FID significantly.
Journal of Physics Conference Series 07/2010; 234(4):042008. DOI:10.1088/1742-6596/234/4/042008
[Show abstract][Hide abstract] ABSTRACT: We present a dc superconducting quantum interference device (SQUID) readout circuit operating in the voltage bias mode and called a SQUID bootstrap circuit (SBC). The SBC is an alternative implementation of two existing methods for suppression of room-temperature amplifier noise: additional voltage feedback and current feedback. Two circuit branches are connected in parallel. In the dc SQUID branch, an inductively coupled coil connected in series provides the bias current feedback for enhancing the flux-to-current coefficient. The circuit branch parallel to the dc SQUID branch contains an inductively coupled voltage feedback coil with a shunt resistor in series for suppressing the preamplifier noise current by increasing the dynamic resistance. We show that the SBC effectively reduces the preamplifier noise to below the SQUID intrinsic noise. For a helium-cooled planar SQUID magnetometer with a SQUID inductance of 350 pH, a flux noise of about 3 mu phi(0) Hz(-1/2) and a magnetic field resolution of less than 3 fT Hz(-1/2) were obtained. The SBC leads to a convenient direct readout electronics for a dc SQUID with a wider adjustment tolerance than other feedback schemes.
[Show abstract][Hide abstract] ABSTRACT: In low-field NMR measurements, we employ a high temperature superconducting quantum interference device (SQUID) as a detector with an inductively coupled liquid-nitrogen-cooled LC tuned input circuit. However, ringing across the LC circuit appears after the sudden switch-off of the prepolarizing magnetic field. This ringing leads to instability of the SQUID readout and prevents the acquisition of short-relaxation-time signals. We developed and tested two simple and effective FET-based Q switch circuits with adjustable parameters which suppress the ringing. Each of these Q switches makes it possible to record free induction decay signals with a Larmor frequency of 1.2 kHz and an effective relaxation time constant of 30 ms. A gradually changing current caused by the release of charges stored in the p-n junction of the FET, which delays the Q value recovery of the LC circuit, can only be observed by the SQUID because of its frequency-independent sensitivity.
[Show abstract][Hide abstract] ABSTRACT: High-resolution low-field nuclear magnetic resonance (LF-NMR) investigations of liquid samples, recorded using a HTS radio frequency (rf) superconducting quantum interference device (SQUID) are overviewed in this paper. The measurements were performed either in a magnetically shielded room (MSR) or in the Earth's Magnetic Field (EMF). in MSR, measurements with Larmor frequencies (f(L)) ranging from 2 Hz to 40 kHz were demonstrated. The natural spectral linewidth of water and the scalar coupling spectra of 2,2,2-trifluoroethanol and of fluorobenzene were determined. An additional nitrogen-cooled resonant LC input circuit was also introduced in higher f(L) (>10 kHz) region to enhance the signal-to-noise ratio up to an order of magnitude, The influence of the speed of decay of the polarizing magnetic field (B-p) on the free-induction-decay (FID) signal was analyzed quantitatively. In EMF, NMR spectra of different liquid samples were recorded. To compensate the line broadening due to EMF fluctuations we used frequency-adjusted averaging. (C) 2009 Elsevier B.V. All rights reserved.
Physica C Superconductivity 10/2009; 469(15-15-20):1624-1629. DOI:10.1016/j.physc.2009.05.085 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The influence of low-frequency magnetic field disturbances on SQUID based low field (LF) nuclear magnetic resonance (NMR) measurements is investigated. Two types of sinusoidal fields, a homogenous field and a linear gradient field, were applied as artificial disturbance sources. The influences on the free induction decay (FID) signals as well as on the spectra are discussed. The homogeneous disturbance field caused a frequency modulation of the FID signal. The measured spectra were found to be in good agreement with calculated traces obtained from a solution to the Bloch equation. The gradient disturbance field yielded an amplitude-modulated FID signal. In both cases, frequency mixing lines were observed. It is shown that for disturbances at the power line frequency and harmonics, the influence on the NMR spectra is negligible.
[Show abstract][Hide abstract] ABSTRACT: High-resolution low-field nuclear magnetic resonance (NMR) signals of selected liquid samples were recorded using a nitrogen-cooled superconducting quantum interference device (SQUID). The NMR measurements were performed at Larmor frequencies (f L) from 2 Hz to 40 kHz. The natural spectral linewidth of tap water could be measured in magnetic fields below 7 microtesla. To demonstrate the measurement sensitivity and resolution, J-coupling spectra of 2,2,2-trifluoroethanol were recorded at different measurement fields, with signals separated by several hundreds of Hertz. An additional nitrogen-cooled tuned LC-circuit and a signal recovery procedure involving a pi/2 AC pulse were applied in the higher f L region ( >10 kHz) to enhance the signal-to-noise ratio up to one order of magnitude.
[Show abstract][Hide abstract] ABSTRACT: In reported low-field nuclear magnetic resonance (NMR) measurements using Superconducting Quantum Interference Device (SQUID) detection, the pre-polarizing magnetic field has been usually oriented orthogonal to the measuring field, B(p) perpendicular B(m). Melton et al. systematically analyzed the consequences of B(p) decay in time after turnoff and showed that this decay should be nonadiabatic. We evaluated our measuring procedure in the light of that analysis, and found good quantitative agreement. It was showed that, when the decay time constant is comparable to the precession period of the magnetization of the sample, M, the optimum procedure is to orient B(p) parallel to B(m) and to apply a pi/2 pulse to flip M, similar as in the case of conventional NMR.
Journal of Magnetic Resonance 10/2008; 196(2):101-4. DOI:10.1016/j.jmr.2008.09.009 · 2.51 Impact Factor