High-Speed Operation of a Single Flux Quantum Multiple Input Merger Using a Magnetically Coupled SQUID Stack

We investigated a single flux quantum (SFQ) multiinput merger composed of Josephson transmission lines (JTLs), a dc-SQUID stack magnetically coupled to the JTLs, and a dc/SFQ converter. The new merger can more efficiently merge many input signals than a conventional merger circuit, which is a two-input SFQ confluence buffer (CB). In this paper, we optimized and designed the multi-input merger according to an analog circuit simulation. The circuit simulation results show that the merger using up to 16 inputs can correctly operate. We implemented the test circuit and demonstrated a high-speed operation of a four-input merger at an input frequency of up to 23.3 GHz. We evaluated the delay time and the circuit scale of a practical multi-input merging circuit using the newly designed merger and the conventional merging circuit for an SFQ memory system. If we design a 4096-input merging circuit using a 16-input merger circuit tree, we can reduce the delay time, the number of Josephson junctions (JJs), and the total power dissipation of the merging circuit compared with the merging circuit based on a conventional CB tree. The reduction rates of the delay, the JJs, and the total power are approximately 11%, 35%, and 53%, respectively.