Periodic-threshold amalgamated quasi-one-junction SQUID comparator

This article proposes a periodic-threshold superconductive comparator named amalgamated quasi-one-junction SQUID comparator (AQOS) for sensitivity enhancement. The comparator employs a pair of compound Josephson-junction SQUID with magnetically-controlled effective critical current in quantization to relieve the constraints on the period current without disrupting the non-hysteresis operation state. As a result, the AQOS comparator can achieve three times more comparing operations than the quasi-one-junction SQUID (QOS) comparators, corresponding to a 9 dB sensitivity improvement. The AQOS comparator also adopts the complementary structure and feedback-free topology for linearity and robustness. It can reach a 30 GS/s sampling rate, 0.5 mA period current, −25 dBm sensitivity, more than ±10 % bias current margin, deviation of quantization current peak-to-peak magnitude that is about 10 % of the mean, and the 4.4 dB average DC duty bias margin in simulation from 1 GHz to 30 GHz with a process of 6 kA/cm² Josephson critical current density. A 2-bit AQOS comparator has been fabricated with the SIMIT Nb03 process for the qualification test. It has passed functional tests at low frequency, and its performance metrics, including period current, sensitivity, and bias current margin, have also been proven.