A design approach to passive interconnects for single flux quantum logic circuits

We developed a design approach for interface circuits to connect Single Flux Quantum (SFQ) cells by using passive transmission lines (PTL´s). In the approach, an interface circuit between a PTL and JTL is optimized to obtain a standard interface circuit, and then, modifications are made to previously designed SFQ cells and the standard interface circuit to connect the SFQ cells by using PTL´s. The key point is the use of approximately the same interface circuit with every SFQ cell to maintain the matching condition between the interface circuit and the PTL´s. Based on this approach, we designed an interface circuit and a test circuit composed of two D-flip-flops connected using 2-mm-long PTL´s via the interface circuits. The impedance of the PTL was 2 Ω. We achieved high-speed operation of the test circuit up to 35 GHz with a bias margin of -15/+30%.