We studied the signal-to-noise ratios ( SNR s) of a superconducting first-order sigma–delta modulator with an LR integrator. Effects of leakage in the LR integrator and thermal noise on SNR were investigated analyzing a transfer function and simulating circuits with thermal noise sources. Leakage resulted in a decrease in the SNR of 1.5 dB and thermal noise in a decrease of 5.5 dB, at a sampling frequency of 20 GHz. We found that decreases in SNR due to leakage and thermal noise in a comparator were independent of the sampling frequency. Thermal noise in an integrating resistor on the other hand became dominant for a decrease in SNR at frequencies above 5 GHz. We also designed a modulator and evaluated its SNR s experimentally. An SNR of 71 dB was obtained at a sampling frequency of 10 GHz. The dependency of SNR on sampling frequency was consistent with the simulated one. The present results indicate that transfer function analysis and circuit simulations with thermal noise can provide practical estimates of SNR s for modulators.
