Impedance Design of Excitation Lines in Adiabatic Quantum-Flux-Parametron Logic Using InductEx

The adiabatic quantum-flux-parametron (AQFP) is an energy-efficient superconductor logic family that utilizes adiabatic switching. AQFP gates are powered and clocked by ac excitation current; thus, to operate AQFP circuits at high clock frequencies, it is required to carefully design the characteristic impedance of excitation lines (especially, above AQFP gates) so that the microwave excitation current can propagate without reflections in the entire circuit. In the present study, we design the characteristic impedance of the excitation line using InductEx, which is a three-dimensional parameter extractor for superconductor devices. We adjust the width of an excitation line using InductEx such that the characteristic impedance is maintained at 50 Ω even above an AQFP gate. Then, we fabricate test circuits to verify the impedance of the excitation line. We measure the impedance using time domain reflectometry (TDR). We also measure the S parameters of the excitation line to investigate the maximum possible clock frequency. Our experimental results indicate that the characteristic impedance of the excitation line agrees well with the design value even above AQFP gates, and that clock frequencies beyond 5 GHz are possible in large-scale AQFP circuits.