Analysis and Stabilization of Signal Reflections in Gate-to-Gate Connections for AQFP Circuits

Abstract

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> increase in length for similar energy consumption is achieved over alternative solutions.