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

TL;DR

This paper presents a method to design and verify the impedance of excitation lines in AQFP circuits using InductEx, enabling high-frequency operation above 5 GHz with minimal reflections.

Contribution

It introduces a design approach for excitation line impedance in AQFP circuits using InductEx and validates it through fabrication and measurements.

Findings

Impedance of excitation lines matches design values.

Clock frequencies beyond 5 GHz are achievable.

Impedance design reduces microwave reflection issues.

Abstract

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 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 becomes 50 {\Omega} even above an AQFP gate. Then, we fabricate test circuits to verify the impedance of the excitation line. We measure the impedance using the time domain reflectometry (TDR). We also measure the S parameters of the excitation line to investigate the maximum available 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 available in large-scale AQFP circuits.