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IEEE Transactions on Applied Superconductivity

Experimental demonstration of superconducting bandpass delta-sigma modulator

John F. Bulzacchelli, Hae-Seung Lee, James A. Misewich, M. B. Ketchen

Published Jun 1, 2003• 11 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2003.813903

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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.
Superconducting bandpass delta-sigma ( ) mod- ulators are being developed for direct analog-to-digital conversion of radio frequency signals in the GHz range. Recently we demon- strated operation of a superconducting bandpass 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 im- plementation details of this complex test chip, which contains 4065 Josephson junctions. The paper also explains how magnetic cou- pling 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 sam- pling rate are presented.
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.
We have successfully fabricated and tested a high temperature superconducting (HTS) sigma-delta modulator for analogue-to-digital conversion. This is the first demonstration of a GHz sampling A-to-D in HTS. The 15-junction single-flux-quantum (SFQ) circuit, fabricated using an epitaxial multilayer HTS process with YBCO/Co-YBCO/YBCO edge junctions, was internally clocked at 27 GHz and used to convert a 5.01 MHz signal. The modulator demonstrated a spur-free dynamic range of more than 75 dB. Two-tone measurements with 5.01 MHz and 5.51 MHz signals demonstrated third-order intermodulation products to be lower than -59 dBc. Demonstration of a functional HTS modulator represents a significant milestone in the development of high dynamic range ADCs suitable for such applications as surveillance radar.