Digital signal processor-based dc superconducting quantum interference device controller

We report on the design and evaluation of a convenient digital signal processor-based (DSP) controller for a dc superconducting quantum interference device (SQUID). Program algorithms for the DSP conveniently replace major functions of an analog flux-locked loop: current bias, modulation signal, demodulation, filtering, integration, feedback, and reset. For the controller described here, the entire system was built using commercially available electronics and sensor units with the addition of a simple operational amplifier circuit for the required gain. The noise level of the nonoptimized dc SQUID controller system at 1 Hz was similar to 50 mu Phi (0)/root Hz. Application of the system was demonstrated as a readout system for a low-temperature magnetic susceptibility thermometer that required flux counting. (C) 2001 American Institute of Physics.