Superconducting high-order cascaded lowpass sigma-delta modulator

We propose a superconducting high-order cascaded lowpass sigma-delta modulator. The second-order cascaded modulator consists of two single-loop modulators. A part of an integrating inductor of the first-stage modulator forms a primary coil of a transformer. The secondary coil of the transformer forms an integrating inductor of the second-stage modulator. Output of the first integrator is thus magnetically fed to the second integrator through the transformer. Digital outputs from both modulators are input to a delay and a digital differentiator. The quantization noise of the first-stage modulator is canceled by choosing proper transfer functions for the delay and differentiator. This cascaded type modulator eliminates the difficulty of designing a feedback driver in a double-loop type modulator. Analytical calculation of the transfer function showed that the signal-to-noise ratio is comparable to an ideal second-order lowpass sigma-delta modulator at a given sampling frequency.