We have made accurate measurements of the noise and gain of heterodyne mixers employing small-area (1 μm2) Ta/Ta2O5/Pb0.9Bi0.1 superconductor-insulator-superconductor tunnel junctions. These junctions have very low subgap leakage current and an extremely sharp current rise at the sum gap. We have measured an added mixer noise of 0.61±0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta for a single-sideband mixer. Values of the imbedding admittances are deduced from the shapes of I-V curves pumped at the upper and lower sideband frequencies. Using these admittances, the mixer performance calculated from the quantum theory is in good agreement with the experiment.
A Single Flux Quantum (SFQ) demultiplexer and a cross-bar switch for a high speed data switch are described. Both circuits were error-free in simulation to 15 Gbps. The cross-bar switch was two input, two output (2x2) and used -400 pA control signals to set the data path. The switch has been demonstrated at over 15 Gbpshine using average voltage measurements. The correct routing of a bit stream was demonstrated at 2.488 Gbps.
Abstruct-The sigma-delta architecture is the method 0 f choice for analog-to-digital converters (ADCs) for high dynamic range applications. This architecture uses oversampling and precise feedback to generate a shaped spectral distribution of the quantization noise. Subsequent digital filtering suppresses out-of-band quantization noise, yielding a large signal to in-band noise ratio. A unique advantage of superconducting electronics is the availability o f the flux quantum to provide quantum mechanically accurate feedback at GHz rates. Josephson digital technology extends sigma-delta ADCs from MHz sampling rates to GHz sampling rates, from kHz signal bandwidths to MHz signal bandwidths, with comparable or better dynamic range when compared to semiconductor implementations. This paper presents circuits for Josephson sigma-delta ADCs, including single-loop and double-loop modulators operating at sampling rates up to 2 GHz, and circuits for quantized feedback. The first demonstration of double-loop noise shaping is also presented.
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