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Japanese Journal of Applied Physics

Noise-Shaping Characteristics of Superconducting Band-Pass Sigma-Delta Modulators

Tatsunori Hashimoto, H. Hasegawa, Shuichi Nagasawa, Hideo Suzuki, Kazunori Miyahara, Youichi Enomoto

Published Oct 1, 2001• 5 citations
Publisher

Abstract

The noise-shaping characteristics of superconducting band-pass single-loop sigma-delta modulators were clearly observed for the first time at 4 K using a 2.86 GHz sampling clock and a 0.69 GHz signal. The modulator consisted of a Josephson comparator, a driver, and a lumped-element-circuit superconducting resonator. The modulator chips were fabricated using NEC's Nb standard process. Digital data from the modulator were consecutively acquired using a semiconductor data acquisition system. By using a fast Fourier transform (FFT) software, the output spectrum of the noise-shaping characteristics was calculated from the measured data. Also, the input-output characteristics were measured experimentally, and the modulator's linearity was confirmed.

DOI
10.1143/jjap.40.l1032

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The noise-shaping characteristics of superconducting band-pass single-loop sigma-delta modulators were clearly observed for the first time at 4 K using a 2.86 GHz sampling clock and a 0.69 GHz signal. The modulator consisted of a Josephson comparator, a driver, and a lumped-element-circuit superconducting resonator. The modulator chips were fabricated using NEC's Nb standard process. Digital data from the modulator were consecutively acquired using a semiconductor data acquisition system. By using a fast Fourier transform (FFT) software, the output spectrum of the noise-shaping characteristics was calculated from the measured data. Also, the input-output characteristics were measured experimentally, and the modulator's linearity was confirmed.
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 SNRs. The power spectra and SNRs were consistent with the predictions of the transfer function analysis and circuit simulations.
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This paper presents a superconducting bandpass /spl Delta//spl Sigma/ modulator for direct analog-to-digital conversion of radio frequency signals in the gigahertz range. The design, based on a 2.23-GHz microstrip resonator and a single flux quantum comparator, exploits several advantages of superconducting electronics: the high quality factor of resonators, the fast switching speed of the Josephson junction, natural quantization of voltage pulses, and high circuit sensitivity. The modulator test chip includes an integrated acquisition memory for capturing output data at sampling rates up to 45 GHz. The small size (256 b) of the acquisition memory limits the frequency resolution of spectra based on standard fast Fourier transforms. Output spectra with enhanced resolution are obtained with a segmented correlation method. At a 42.6-GHz sampling rate, the measured SNR is 49 dB over a 20.8-MHz bandwidth, and a full-scale (FS) input is -17.4 dBm. At a 40.2-GHz sampling rate, the measured in-band noise is -57 dBFS over a 19.6-MHz bandwidth. The modulator test chip contains 4065 Josephson junctions and dissipates 1.9 mW at T=4.2 K.
Bandpass delta-sigma modulators digitize narrowband signals with high dynamic range and linearity. The required sampling rate is only a few times higher than the centre frequency of the input. This paper presents a superconducting bandpass delta-sigma modulator for direct analogue-to-digital conversion of RF signals in the GHz range. The input signal is capacitively coupled to one end of a microstrip transmission line, and a single flux quantum balanced comparator quantizes the current flowing out of the other end. Quantization noise is suppressed at the quarter-wave resonance of the transmission line (about 2 GHz in our design). Circuit performance at a 20 GHz sampling rate has been studied with several long JSIM simulations. Full-scale (FS) input sensitivity is 20 mV (rms), and in-band noise is -53 dBFS and -57 dBFS over bandwidths of 39 MHz and 19.5 MHz, respectively. In-band intermodulation distortion is better than -69 dBFS.
High performance oversampling delta-sigma data converters require high clock rates and accurate feedback of the digital output to the analog regime. Superconductor modulators offer the unique advantages of clock rates in the tens-of-gigahertz, comparator switching energy below attoJoules, and quantum-accurate digital-to-analog feedback. We have designed a two-loop baseband modulator whose noise transfer function was calculated using a simple, linear circuit model. The circuit was fabricated in a commercial superconductor IC process with 4.5 kA-per-square cm Josephson junctions and 1.5 micron minimum feature size. The circuit was tested at a derated sample rate of 5 GHz. Measured signal-to-noise dynamic range was 81 dB over a 10 MHz band. Third-order intercept was measured to be +24 dBsat in two-tone test. The measured noise transfer function was in excellent agreement with our linear model. The linear model indicates that with appropriate parameter values, this family of modulators extrapolates to significantly higher performance.
Superconducting bandpass delta-sigma (/spl Delta//spl Sigma/) modulators are being developed for direct analog-to-digital conversion of radio frequency signals in the GHz range. Recently we demonstrated operation of a superconducting bandpass /spl Delta//spl Sigma/ modulator with the highest center frequency (2.23 GHz) and sampling rate (up to 45 GHz) reported to date. The test chip used in that experiment includes an integrated acquisition memory, which eliminates the need for very high speed interfacing. This paper describes key implementation details of this complex test chip, which contains 4065 Josephson junctions. The paper also explains how magnetic coupling from large power supply currents can be reduced through the use of optically-isolated current sources with independent grounds. The measured output spectra of the modulator at a 45 GHz sampling rate are presented.