Design and Operation of 64-Fold Variable Single-Flux-Quantum Pulse-Number Multiplier

One of the unique applications of the single-flux-quantum (SFQ) circuitry is a digital-to-analog converter (DAC) with fundamental accuracy based on the Josephson effect. Recently, we have proposed a DAC based on frequency modulation of the output SFQ pulse train. A variable SFQ pulse-number multiplier (PNM) is a key component for the frequency-modulation DAC. In this paper, we report our design and test of a variable PNM in which the multiplication factor is determined by the sum of selected SFQ trains coming from a series array of toggle flip-flops. A 64-fold variable PNM is designed and tested using chips based on Nb. The low-speed test shows its correct functions of variable multiplication of one- to 64-fold. The high-speed test demonstrates the operation of output SFQ repetition frequency up to 0.82 GHz for 64-fold multiplication.