Abstract
We present theory and performance data on oscillator/counter A/D converters fabricated in TRW's Nb process. Circuits with junction critical current density of 2 kA/cm/sup 2/ are discussed. This simple, low power A/D converter architecture is uniquely enabled by the wide bandwidth voltage-controlled oscillator (VCO) and compatible, ultra-fast superconductor single-flux-quantum (SFQ) divide-by-two flip-flops. The measured signal-to-noise ratio (SNR), frequency response, and signal distortion are discussed within the framework of a basic theory of performance as well as time-domain simulations. Noise contributions from quantization error, aperture jitter, and thermal noise are included. The measured signal-to-noise ratio (SNR) is shown to be within a few dB of theoretical performance over more than two decades of frequency. The frequency response is shown to fit well to the expected function through 1 GHz of signal frequency. Harmonic distortion is shown to be consistent with the non-linearity in the front-end's DC I-V characteristic which is subject to design improvement. This architecture extends to higher performance using a multi-junction VCO. Measured two-junction VCO SNR shows essentially the theoretical improvement over that of a single junction.
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