A dc superconducting quantum interference device (SQUID) magnetometer has been integrated on a 9×9 mm2 chip with eight pick‐up loops in parallel to directly form a SQUID inductance of about 0.5 nH. Very simple feedback electronics have been developed which do not require liquid‐helium temperature impedance matching circuits or flux modulation techniques. The magnetometer has a typical white noise of 8 fT/(Hz)1/2 and a 1/f  corner frequency below 3.5 Hz. With an additional positive feedback circuit at 4.2 K the white noise level has been further reduced to 4.5 fT/(Hz)1/2. Using a two‐pole integrator, a 3 dB bandwidth around 0.5 MHz and a maximum slew rate of 3 mT/s at 1.3 kHz have been attained with a ±0.4 μT feedback range.

Low‐noise high‐speed dc superconducting quantum interference device magnetometer with simplified feedback electronics

An overview of the current status of DC and RF SQUIDs (superconducting quantum interference devices) is presented. Using Nb-based thin-film technology, a number of groups can now routinely fabricate DC SQUIDS with input circuits integrated on the same chip. At 4.2 K, the DC SQUID, which operates with a constant bias current, can detect signals with an equivalent energy per unit bandwidth of a few hundred h2 pi . RF SQUIDs, which operate with an RF flux bias, are usually machined from bulk Nb, and have a typical energy sensitivity per unit bandwidth of 10/sup 5/ to 10/sup 6/h2 pi . Both types of SQUID can be used in a wide variety of applications, including magnetometers, gradiometers, susceptometers, voltmeters, RF amplifiers, gravity-wave antennas, and gravity gradiometers. Both DC and RF SQUIDs have been fabricated from high-temperature superconductors and operated at temperatures of up to 77 K. These devices exhibit high levels of 1/f noise, which generally arises from the motion of trapped magnetic flux. >

Principles and applications of SQUIDs

Superconducting quantum interference devices (SQUIDs) are commonly operated in a flux-locked loop (FLL). The SQUID electronics amplifies the small SQUID signal to an acceptable level without adding noise, and it linearizes the transfer function of the SQUID in order to provide sufficient dynamic range. In this paper, the fundamentals of SQUID readout are reviewed including a discussion of preamplifier noise. The basic FLL concepts, direct readout and flux modulation readout, are discussed both with dc bias and bias reversal. Alternative readout concepts such as additional positive feedback (APF), two-stage SQUIDs, SQUID series arrays, relaxation oscillation SQUIDs and digital SQUIDs are briefly described. The FLL dynamics are discussed on the basis of a simple model with finite loop delay. It is shown that with optimized SQUID electronics a system bandwidth of ≈18 MHz and a corresponding slew rate of ≈8 Φ0 µs−1 are possible. A novel FLL scheme involving a Smith predictor is presented which allows one to increase the FLL bandwidth to about 100 MHz. The theoretical predictions are experimentally checked using a high-speed SQUID electronics prototype with a small-signal bandwidth of 300 MHz. Methods for increasing the dynamic range of SQUID systems are described: flux-quanta counting and dynamic field compensation (DFC). With DFC, the residual magnetic field at the SQUID can be kept close to zero even if the device is moved in the Earth's field. Therefore, the noise level of a high-Tc magnetometer measured inside a magnetically shielded room (60 fT Hz−1/2 with a 1/f corner at 2 Hz) remained unchanged after moving the device in the magnetic field outside the room (60 µT dc plus 0.8 µT peak-to-peak power line interference).

https://iopscience.iop.org/article/10.1088/0953-2048/16/12/002/pdf

High-Tc and low-Tc dc SQUID electronics

Superconducting quantum interference devices (SQUIDs) are the most sensitive devices for measuring weak magnetic fields. Beside the SQUID itself, the design of the read-out electronics decisively determines the performance of the whole sensor. In standard read-out electronics, a cooled impedance-matching transformer between SQUID and preamplifier, and a flux modulation technique are employed. Recently, several novel SQUID read-out concepts without flux modulation have been developed, mainly for biomagnetic multichannel systems. This chapter gives a description and comparison of the most important ones: concepts using multiple SQUIDs, SQUIDs with additional positive feedback, relaxation oscillation SQUIDs, and digital SQUIDs. Both the noise and the dynamic behavior are discussed, and a simple model for the achievable speed of a directly-coupled feedback loop without flux modulation is presented, which allows one to estimate the dynamic merits of the various SQUID concepts.

Advanced Squid Read-Out Electronics

There is provided an apparatus for measuring magnetic sources that exist in an object and produce magnetic fields outside the object. The apparatus comprises an element for detecting the magnetic fields which has a sensing portion placed in proximity with the object and adapted to incorporate an observing plane. The observing plane consists of a plurality of sensing blocks each containing at least three sensing planes on which a magnetic sensor is each mounted. The normal directions of the sensing planes are different from each other. The apparatus further comprises an element for evaluating three-dimensionally the intensities of other magnetic fields at given evaluation positions on the basis of values of the magnetic fields at the detecting element, and an element for obtaining data of the magnetic sources on the basis of the intensities of the other magnetic fields evaluated by the evaluation element.

Apparatus for measuring magnetic sources

Nanobridges with hysteretic I-V (current-voltage) characteristics can be operated as relaxation oscillators when connected to an R-L shunt. A DC SQUID (superconducting quantum interference device) configuration results if two such oscillators are incorporated in a superconducting ring. Operating such a self-oscillating SQUID (SOS) with a suitable bias current leads to an output signal of the order of 10 mV, whose frequency depends on the magnetic flux through the SQUID ring modulo Phi /sub 0/. Due to this large output voltage, no impedance matching or special low-noise preamplifier is needed for the readout scheme. Standard integrated circuits for FM (frequency modulated) signal processing can be used, leading to matchbox-size electronics. Such DC SQUIDs have been made of thin Nb, NbN, and Nb/sub 3/Ge films. The highest sensitivity achieved so far is Nb devices with a white flux noise below 10/sup -5/ Phi /sub 0// square root Hz at 4.2 K. >

M. Muck

Papers

Simple DC-SQUID system based on a frequency modulated relaxation oscillator