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Signal integrity of transmitted data is critical for achieving highly dense, large-scale superconductor circuits. Transmission line effects, such as signal reflections, are investigated and concepts from microwave circuit theory and conventional digital logic circuits are applied to AQFP circuits. The lossless transmission line model is used for interconnection modelling. The frequency content and timing characteristics of data signals are used to analytically predict a maximum propagation delay of 6.1 ps before failure due to reflections. This coincides well with our own results, and with those obtained in a separate study using simulation techniques. The investigation then extends towards the design of an impedance matching network with the aim of reducing reflections. The input impedance of a standard buffer gate is derived, and then matched to an 8 Ω transmission line. The proposed impedance matching network maintains data signal integrity up to 3 millimetres. Energy analysis of the matching network is also performed and a 36 % increase in length for similar energy consumption is achieved over alternative solutions.

Adiabatic quantum-flux-parametron (AQFP) logic is an energy-efficient superconductor logic family that utilizes adiabatic switching of logic gates. At present, a pure inductance is used to model a gate-to-gate interconnection in AQFP logic circuits. However, it is necessary to consider the transmission line effects when the interconnection length becomes long to take into account the propagation delay. This study investigated the transmission line effects of a long gate-to-gate interconnection in AQFP logic circuits. We observed reflection effects in a long transmission line between AQFP gates. These effects induced errors when the interconnection length exceeded several hundred micrometers. We adopted a damping resistance to stabilize the reflection effect to realize an error-free long gate-to-gate interconnection. The circuit simulations confirmed that a damping resistance connected in parallel to the transmission line effectively stabilizes the reflection effect, enabling an interconnection length that is longer than 1 mm. We fabricated AQFP buffer chains that included a long transmission line with and without a parallel damping resistance, and measured their bit error rates. The results showed that the parallel damping resistance effectively improves the bit error rate at high frequencies.