SFQ Pulse Transfer Circuits Using Inductive Coupling for Current Recycling

An increase in bias currents supplied to single-flux-quantum (SFQ) circuits is a rising problem in building large-scale integrated SFQ circuits, because the magnetic fields generated by bias currents and ground return currents affect the circuit operation. Current recycling technique, which can drastically reduce the total bias current of SFQ circuits, is expected to be a very effective solution for large-scale SFQ circuits. To realize the current recycling technique, signal transmission between SFQ logic circuits located on different ground planes is required. Two types of inductively coupled pulse transfer circuits were examined in this study. One is composed of a simple JTL driver and a DC-SQUID receiver whose junctions are slightly under-damped to enhance the magnetic sensitivity of the SQUID. The other circuit is composed of an SFQ driver based on a delay flip-flop (DFF) and a SQUID receiver. The advantage of the DFF-based circuit is that the coupling time between the driver and the receiver can be increased to ensure appropriate switching in the receiver junction. Two types of transfer circuits were implemented using the SRL 2.5 kA/cm² Nb process and their correct operation was experimentally confirmed. Circuit structures for effective coupling of the SFQ pulse transfer were also investigated. It was shown that a ground plane boundary should be separated from a ground plane hole underneath coupling inductors to obtain high coupling factors.