  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.
