Electronic Feedback System for Superconducting Quantum Interference Devices

We report on the development of a digital flux-locked loop (DFLL) feedback system for readout of superconducting quantum interference devices. The DFLL is based on a high-speed digital-to-analog converter/analog-to-digital converter and field programmable gate array (FPGA) that can be readily reconfigured for different types of sensors and applications. Our demonstrated DFLL currently supports 4 sensors but is expandable to 40, all of which can be used for real-time calculations. We describe the various system components and present the results of our FPGA algorithm, including a novel approach with flexible working points and a combination of locking, flux jumping, and further digital signal processing. The DFLL has several advantages to conventional systems particularly for multisensor applications.