Adiabatic quantum-flux-parametron (AQFP) circuits are known as extremely energy-efficient circuits due to adiabatic change of the energy potential during the switching process. They can operate at high clock frequencies of 5-10 GHz with power consumption about 5 orders of magnitude lower than CMOS circuits [1].
The gate-to-gate interconnections for AQFP logic circuits are formed by superconducting wires and are regarded as ideal inductance at present. So, in the current simulations of AQFP circuits, we model the gate-to-gate interconnection by pure inductance. In the long interconnection wires, however, the effect of the parasitic capacitance is not negligible, and the transmission line effect has to be considered. In our recent attempts to model the transmission line effect between AQFP gates, we found significant reflections of the signals resulting in functional errors in the logic circuits.
In this study, we investigated the methods to stabilize the reflection effect in the transmission line for the gate-to-gate interconnection in the AQFP circuits by circuit simulations. In the circuit simulations, we examined the operation of nine stages of AQFP buffers with a transmission line between the fifth and sixth buffer at the clock frequency of 5 GHz. We used a transmission line model characterized by the impedance Z0 and propagation delay τp. It was found that multiple reflections in the transmission line reduce the bias current margins and induce malfunction when the transmission length is longer than about 650 μm. We investigated the way to stabilize the reflection effect in a transmission line by adopting the damping resistance. It was found that a damping resistance connected in parallel to the transmission line efficiently reduces the reflections. We confirmed that the maximum transmission line length was increased to about 2000 µm when the 4 Ω damping resistance was used. We also examined the dependence of the energy dissipation in the damping resistance as a function of the transmission line length. Finally, we implemented a BER measurement circuit composed of a long transmission line with a 4 Ω parallel damping resistance by using the AIST HSTP Josephson process. The BER test results will be presented at the conference.
Reference
[1] N. Takeuchi, et al., Supercond. Sci. Technol. 28, 015003, 2015.
Keywords: Adiabatic quantum-flux-parametron, superconducting wires, transmission line effect, BER