High-T_cand low-T_cdc SQUID electronics

Abstract

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).

Linear magnetic flux amplifier

“…As the voltage -flux V (Φ) characteristic of SQUIDs is highly nonlinear, they are commonly used in the flux-locked loop, where a magnetically coupled feedback provides that the SQUID operates around the steepest part of the V (Φ) characteristic [1,3]. A lot of effort is focused on enhancing the sensitivity and minimizing the noise in SQUID systems (see [3] and references therein). Recently, π/2-and π-SQUIDs based on d-wave superconductors have been investigated as the possible candidates for SQUIDs without the external flux bias [4,5,6].…”

MgB2 SQUID for Magnetocardiography

“…Working Point In recent years, it has been shown by several groups that the electronics within the digital SQUID, where the flux-locked loop system is controlled by a digital circuit, can achieve a wide dynamic range [21][22][23]. These FLL systems can measure biomagnetic signals in a magnetically unshielded environment.…”

A planar picoamperemeter based on a superconducting quantum interferometer

“…Figure 2 shows the most important working characteristic of a SQUID: output voltage V versus current in input coil I in . The maximum peak-topeak voltage swing is 42 µ V. With the use of the simple approximation V pp / | V Φ | = Φ 0 / π ( Φ 0 is a unit of one flux quantum equal to 2.07 × 10 -15 Wb) [7], it is possible to obtain from the curve that dV / d Φ = 132 µ V/ Φ 0 . A highly precise value of current sensitivity in the input coil was measured with electronics in the flux-trapping mode, thereby yielding dI in / d Φ = 1.25 µ A/ Φ 0 .…”

“…In contrast to [2], the low-frequency noise was reduced by using modulation electronics with a 4-MHz modulation frequency. The advantages and disadvantages of such circuits utilizing a SQUID as a null detector are described in detail in [7]. The electronics' noise at the amplifier input was 1 nV/Hz 1/2 .…”