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We designed a digital double relaxation oscillation SQUID (DROS) with an SFQ up/down counter. The DROS is connected to a feedback SQUID loop and a signal magnetic flux is input to the feedback SQUID loop. The DROS generates two kinds of voltage pulses corresponding to the total magnetic flux in the feedback SQUID loop. Two kind of voltage pulses are converted to SFQ and anti-SFQ pulses and input to the feedback SQUID loop to keep the magnetic flux in the SQUID loop constant. The number of the generated SFQ pulses is counted using the SFQ up/down counter. Thus, the count corresponds to the input magnetic flux. Because the SFQ up/down counter is operated in the pulse driven mode, the digital DROS needs only dc bias current for operation. We designed and fabricated a digital DROS with a relaxation oscillation frequency of 3 GHz and an 8-bit SFQ up/down counter. Numerical simulation suggested the designed digital DROS has a dynamic range exceeding \pm100\ \Phi0 and magnetic flux slew rate of 3\times10^9\ \Phi0/s. [ABSTRACT FROM AUTHOR]

Relaxation Oscillation SQUIDs (ROSs) and Double Relaxation Oscillation SQUIDs (DROSs) are based on relaxation oscillations that are induced in hysteretic dc SQUIDs by an external L-R shunt. The relaxation frequency of a ROS varies with the applied flux /spl Phi/, whereas the output of a DROS is a dc voltage, with a typical flux-to-voltage transfer of /spl part/V//spl part//spl Phi//spl ap/1 mV//spl Phi//sub 0/. The flux-to-frequency response of several ROSs has been measured and compared with theory for frequencies up to 7 GHz. Various DROS designs-a multi-loop direct coupling DROS, a DROS with a washer type signal SQUID and a DROS with gradiometric signal SQUID-will be discussed in this paper. The integration of a DROS with a digital flux locked loop ("Smart DROS") will also be analyzed. [ABSTRACT FROM PUBLISHER]

Under the low-damping limit, flux to voltage transfer function and classical intrinsic noise expressions for both thermal fluctuation and relaxation oscillation in double relaxation oscillation SQUID(DROS)s can be obtained with numerical calculation. The expressions show that the noise has T/sup 6/5/ temperature dependence and that at low relaxation oscillation frequencies, /spl omega//sub r/, the noise has a 1//spl omega//sub r/ behavior, and then turns up and gradually saturates at very high frequencies comparable with the plasma frequency. The upturn point, which gives the optimum energy resolution, might be near 10 GHz for typical DROS circuit parameters. [ABSTRACT FROM PUBLISHER]

Presents a compact and reliable planar gradiometer for a multichannel system measuring tangential components of biomagnetic fields. The readout electronics was simplified by using a double relaxation oscillation SQUID (DROS) which provides very large flux-to-voltage transfer of typically 3 mV//spl Phi//sub 0/, enabling direct readout by a DC preamplifier with modest voltage noise. The DROS was made from hysteretic Nb/AlO/sub x//Nb junctions. To realize a compact and reliable gradiometer, the planar pickup coil is integrated on the same wafer with the SQUID. The planar pickup coil consists of two 10 mm/spl times/10 mm coils connected in series to form a first-order gradiometer and has a baseline of 30 mm. The field gradient noise is 9 fT/cm/spl radic/Hz at 1 Hz inside a magnetically shielded room. The gradiometers were arranged to measure simultaneously x and y components of biomagnetic fields. [ABSTRACT FROM PUBLISHER]

We designed a digital superconducting quantum interference device (SQUID) system that contains a double relaxation oscillation SQUID (DROS) as a magnetic flux comparator that is connected to an additional relaxation oscillation (RO) circuit with mutually coupled inductances. In the designed digital SQUID, the RO circuit works as a resonator and increases the relaxation oscillation frequency without decreasing the input current of dc/SFQ converters. Numerical simulation shows that the digital SQUID system can operate up to 7 GHz. A fabricated digital SQUID using a DROS comparator with an RO circuit provides correct digital SQUID operation. For a magnetometer using the digital SQUID system, we can realize a magnetic field noise less than 1 pT/ \sqrt\mathrmHz with a small signal bandwidth of more than 10 MHz. [ABSTRACT FROM AUTHOR]

The MiniGRAIL is one of the three similar spherical gravitational wave detectors that are currently being developed. The detector has a resonant frequency of about 3 kHz and will be operated at 20 mK. The ultimate goal is to use a readout system consisting of six transducers coupled to nearly quantum limited two-stage SQUIDs. The two-stage SQUIDs are based on double relaxation oscillation SQUIDs, which enables a direct voltage readout scheme. We have developed nonintegrated two-stage SQUIDs and experimentally verified the proper operation of the system coupled to a capacitive transducer. Based on the results that were achieved, integrated two-stage SQUIDs were designed. Special attention has been paid to the sensor SQUID, the back action of the SQUID and the feedback scheme that is used for linearizing the SQUID output. [ABSTRACT FROM AUTHOR]

For reliable and long-term operation of superconducting quantum interference devices (SQUIDs), proper protection against mechanical damage and electrostatic discharge is required. In this study, we developed a simple and practical molding method using a polydimethylsiloxane–carbon black composite to protect SQUIDs. By simply dipping SQUIDs into the molding composite precursor, they could be molded reliably with no additional mechanical damage from the molding process itself. Additionally, the molded SQUIDs showed no degradation in the flux–voltage modulation curves for ten thermal cycles between 4.2 K and room temperature. We also compared the electrostatic discharge immune characteristics of bare and molded SQUIDs by applying air discharge voltages in the range of 0.1–4.0 kV. Among the 52 SQUIDs with or without molding, all the molded SQUIDs survived up to 1.2 kV. For higher discharge voltages, the molded SQUIDs have a lower likelihood of damage than the bare SQUIDs. This study also confirmed that neither the molding material nor process increase the flux noise, indicating that the developed molding method can be used reliably for multichannel SQUID systems. [ABSTRACT FROM AUTHOR]

The general-purpose circuit simulator analysis program for linear active circuits (APLAC) is not widely known within the superconductor circuit community, regardless of its built-in Josephson junction model and capability of modeling the superconductive phase transition with controlled sources. We review the use of APLAC to model, e.g., noisy dc SQUIDs and transition-edge sensors. Based on an APLAC simulation, we also comment on the preference of voltage bias over current bias in dc SQUIDs in applications where large dynamic range and high bandwidth are simultaneously required. [ABSTRACT FROM AUTHOR]

Adiabatic quantum-flux-parametron (AQFP) logic is an energy-efficient superconductor logic family. In this article, we design binary counters, namely an up counter, a down counter, and an up/down counter, based on AQFP logic. These binary counters are T-flip-flop-based and count logic 1s in the input signal that is synchronized with excitation currents, which clock the counters. The internal state of a T flip-flop changes when the internal state of the previous T flip-flop changes from logic 1 to 0 and from logic 0 to 1 for the count-up and count-down operations, respectively. Each internal state of a T flip-flop is read out serially by a buffer chain coupled to the T flip-flops. We fabricate 4-bit up, down, and up/down counters using the AIST 10 kA/cm2 Nb high-speed standard process and demonstrate their correct operation at low frequency. This is the first demonstration of binary counters based on AQFP logic. [ABSTRACT FROM AUTHOR]

Practical superconducting quantum interference devices (SQUIDs) have been developed into multiloop hybrid electric circuits with Josephson junctions and normal elements such as resistor, inductor and capacitor. To find a common circuit analysis and simulation method for different types of SQUID hybrid circuits consist of both superconducting and normal components, we introduce a mesh-current method, which can derive both the general system model and uniform circuit equations directly from the equivalent circuit. The mesh-current method is demonstrated in the simulation of a SQUID additional positive feedback circuit. The numerical simulation results are presented and verified with the measurements. Compared with the conventional nodal analysis method, the mesh-current method is excellent in dealing with the relations including external flux, self and mutual inductances in multiloop hybrid circuit analysis; its final circuit equations directly exhibit the physical theorems; its general system model reveals the quantum interference mechanism inside SQUID circuits, and is applicable for further dynamics studies. [ABSTRACT FROM AUTHOR]

Pulse tube (PT) cryocoolers can be used to cool SQUIDs, however their electromagnetic interference (EMI) through the conducted high frequency noise currents are normally too large to operate SQUIDs. In this paper, high frequency noise currents of a pulse tube cryocooler system are simultaneously observed at several critical points to analyze conducting paths of the noise currents and to find a solution for this problem. High frequency noise currents are detected by current sensors consisting of a current transformer and a terminating resistor at the secondary side. By covering three-phase power lines and a neutral line connecting the valve motor to the inverter with a copper mesh, and by using it as an additional neutral line, conducted EMI (noise currents) to the cryostat has bee reduced by one fifth. With this treatment, SQUIDs have been operated normally. [ABSTRACT FROM AUTHOR]

We designed two different sensor SQUIDs for the readout of the resonant mass gravitational wave detector Mini-GRAIL. Both designs have integrated input inductors in the order of 1.5 μH and are planned for operation in the mK temperature range. Cooling fins were added to the shunt resistors. The fabricated SQUIDs show a behavior that differs from standard dc-SQUIDs. We were able to operate a design with a parallel configuration of washers at reasonable sensitivities. The flux noise saturated to a value of 0.84 μΦ0/√Hz below a temperature of 200 mK. The equivalent noise referred to the current through the input coil is 155 fA/√Hz and the energy resolution yields 62 h. [ABSTRACT FROM AUTHOR]

We have developed several types of low-Tc first-order planar gradiometers with long baselines ranging from 3.6 cm to 4 cm. Thin-film planar gradiometers are very attractive for measurements of the off-diagonal components of the magnetic field gradient tensor in noisy environments because of the high intrinsic balance that can be achieved with the precision photolithographic techniques used to fabricate these devices. The gradiometer pickup loops are series configured and connected to the input circuit of a dual-washer gradiometric dc SQUID. Series and parallel SQUID washer configurations have been investigated. For a given chip size, model calculations using a magnetic dipole point source and a distributed current dipole source show that gradiometer performance can be improved by increasing the size of the pickup loops at th expense of a small reduction of the baseline length. Based on these results, an improved planar gradiometer has been developed with 1.175 cm x 1.175 cm square pickup loops integrated with a series-configured dual washer SQUID on a 1.2cm x 4.8cm chip. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 μΦ0/Hz1/2 (rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/Φ0. This results in a magnetic field gradient noise of 0.42 fT/cm - Hz1/2. The gradiometers are operable unshielded in typical laboratory environments without losing lock. [ABSTRACT FROM AUTHOR]