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Superconductor Science and Technology

Analogue demonstration of a high temperature superconducting sigma-delta modulator with 27 GHz sampling

Martin G. Forrester, B. D. Hunt, Donald L. Miller, John Talvacchio, Robert M. Young

Published Nov 1, 1999• 11 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/12/11/302

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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.
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.
Superconducting digital circuits are capable of producing high dynamic range analog-to-digital converters (ADCs). We present sigma-delta architectures using large (>100) oversampling ratios to give signal-to-noise ratios of greater than 100 dB in simulation. Three distinct designs, using two distinct mechanisms for feedback, are presented. All of the designs use only shunted junctions, and are therefore compatible with HTS SNS junctions. Simulations are presented for a modulator which creates feedback pulses by SQUID lobe-crossings, a modulator which uses the transresistance of a SQUID for feedback, and a modulator which uses threshold crossings in both the first and second loops for feedback.
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.
We have fabricated and measured high T/sub c/ superconductor single flux quantum 1-bit flux-counting analog-to-digital converters (ADCs). The ADCs were made with a multilayer all-epitaxial process which incorporates 10 edge SNS (superconductor-normal-superconductor) or step-edge grain boundary (SEGB) Josephson junctions with a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// groundplane. The ADC consists of a quantizer connected to a Toggle flip-flop through a buffer-like stage. Direct readout of the flux state of the T flip-flop was made with a Read SQUID inductively coupled through a hole in the groundplane. The circuits were operated at 65 K and low speeds. The SNS circuits outperformed the SEGB circuits because of their higher readout voltages and tighter critical current spreads.
The sigma-delta architecture is the method of choice for analog-to-digital converters (ADCs) for high dynamic range applications. This architecture uses oversampling and precise feedback to generate a shaped spectral distribution of the quantization noise. Subsequent digital filtering suppresses out-of-band quantization noise, yielding a large signal to in-band noise ratio. A unique advantage of superconducting electronics is the availability of the flux quantum to provide quantum mechanically accurate feedback at GHz rates. Josephson digital technology extends sigma-delta ADCs from MHz sampling rates to GHz sampling rates, from kHz signal bandwidths to MHz signal bandwidths, with comparable or better dynamic range when compared to semiconductor implementations. This paper presents circuits for Josephson sigma-delta ADCs, including single-loop and double-loop modulators operating at sampling rates up to 2 GHz, and circuits for quantized feedback. The first demonstration of double-loop noise shaping is also presented.