Experimental Evaluation of Signal-to-Noise Ratio of Sigma-Delta Modulator for Superconducting Analog-to-Digital Converter

摘要

We designed a superconducting front-end circuit for an analog-to-digital converter and experimentally evaluated its signal-to-noise-ratio $(SNR)$ at a sampling frequency of 10 GHz. The modulator was based on a first-order sigma-delta modulation with an LR integrator. Correct noise shaping was experimentally confirmed, and an $SNR$ of 70.9 dB at a bandwidth of 10 MHz was achieved. In the circuit design, we investigated the effect of leakage in the LR integrator and thermal noise on the $SNR$ by using transfer function analysis and circuit simulations with noise sources. Circuit parameters were designed to keep the discrepancy of the $SNR$ from the ideal value (77.6 dB) within 6 dB. In the experimental evaluation, the modulated data signal from the superconducting chip was accumulated with room-temperature electronics. A fast Fourier transform (FFT) calculation was performed to obtain power spectra and $SNR$ s. The power spectra and $SNR$ s were consistent with the predictions of the transfer function analysis and circuit simulations.