  Most active quench circuits used for single-photon avalanche detectors are
designed either with discrete components which lack the flexibility of
dynamically changing the control parameters, or with custom ASICs which require
a long development time and high cost. As an alternative, we present a
reconfigurable and robust hybrid design implemented using a System-on-Chip
(SoC), which integrates both an FPGA and a microcontroller. We take advantage
of the FPGA's speed and configuration capabilities to vary the quench and reset
parameters dynamically over a large range, thus allowing our circuit to operate
with a wide variety of APDs without having to re-design the system. The
microcontroller enables the remote adjustment of control parameters and
re-calibration of APDs in the field. The ruggedized design uses components with
space heritage, thus making it suitable for space-based applications in the
fields of telecommunications and quantum key distribution (QKD). We
characterize our circuit with a commercial APD cooled to 253K, and obtain a
deadtime of 35ns while maintaining the after-pulsing probability at close to
3%. We also demonstrate versatility of the circuit by directly testing custom
fabricated chip-scale APDs, which paves the way for automated wafer-scale
testing and characterization.
