Smart SQUIDs based on Relaxation Oscillation SQUIDs

Smart SQUIDs based on double Relaxation Oscillation SQUIDs (DROS) and a superconducting up-down counter have been developed. DROS and counter form a flux locked loop on one single chip. The DROS output consists of a series of pulses that controls the two up and down write gates of the counter. The pulsed output structure of the DROS constitutes the internal clock for this single-chip device. Several prototypes were built with a clock frequency of 100 MHz, a linear operation flux range of about 2.5 ¿0, and a white noise level of 6.5 ¿¿0/¿Hz. The smart SQUID is in principle a promising device for application in multichannel SQUID system

The use of (double) relaxation oscillation SQUIDs as a sensor

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 Φ, whereas the output of a DROS is a dc voltage, with a typical flux-to-voltage transfer of ∂V/∂Φ≈1 mV/Φ0. 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 analyze

High Sensitivity Magnetic Flux Sensors with Direct Voltage Readout Double Relaxation Oscillation SQUIDs

The experimental sensitivity of double relaxation oscillation SQUIDs (DROSs) has been compared with theory and with the results obtained by numerical simulations. The experimental sensitivity ranges from 60 to 13h, where h is Planck's constant, for relaxation frequencies from 0.4 up to 10 GHz. For low frequencies the DROS characteristics can be explained by thermal noise on the critical currents. For high frequencies, the voltage-flux characteristics and the sensitivity are limited by the plasma frequency. The cross-over frequency is at 2 GHz, which is about 2% of the plasma frequency of the DROS

A 1-MHz low noise preamlifier based on Double Relaxation Oscillation SQUIDs

A low noise and wideband preamplifier based on Double Relaxation Oscillation Superconducting Quantum Interference Devices (DROSs) has been realized. A major advantage of a DROS is that it can be operated in a simple flux modulation. So far, biomagnetic measurements performed in our group required only a limited bandwidth smaller than 100 kHz. Other applications, like for instance readout of radiation and particle detectors, demand a larger bandwidth. In this paper, we will discuss our efforts aimed at increasing the operational bandwidth of a DROS in flux locked loop. Presently, a flux locked loop scheme with a -3 dB bandwidth of 1.45 MHz has been built. With this system a white flux noise of 8 ¿¿0/¿Hz was measured with a 1/f-corner frequency of 10 Hz. The slew rate was 2.5·105 ¿0/s. With the mutual input inductance of 6.7 nH, an input current noise of the preamplifier of 2.5 pA/¿Hz was found and a current slew rate of 80 mA/s. We will discuss the suitability of our DROS-based preamplifier for readout of cryogenic particle detectors based on superconducting tunnel junction

Advanced Relaxation Oscillation SQUIDs

This thesis describes the development of low Tc superconducting quantum interference devices (SQUIDs) based on the relaxation oscillation principle. SQUIDs are the most sensitive magnetic flux sensors available nowadays. As they can detect very accurately any physical quantity that can be converted to a magnetic flux, their field of applications is wide-ranging. Examples of applications are the detection of the tiny magnetic fields generated by the human brain, the measurement of voltages in the picovolt range and the readout of cryogenic particle detectors