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IEEE Transactions on Applied Superconductivity

Digital to analog conversion based on processing of the SFQ pulses

Vasili K. Semenov

Published Mar 1, 1993• 67 citations
Publisher

Abstract

Two novel circuits of DC voltage multipliers have been designed, fabricated using a 3.5 mu m 1000 A/cm/sup 2/ niobium trilayer technology, and successfully tested at low frequencies. A novel primary digital-to-analog (D/A) converter based on the processing of single-flux-quantum (SFQ) pulses has been suggested. The performance of analog-to-digital (A/D) converters combined from such primary D/A converters and voltage multipliers is discussed.>

DOI
10.1109/77.233969

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•46 citations
Two novel circuits of DC voltage multipliers have been designed, fabricated using a 3.5 mu m 1000 A/cm/sup 2/ niobium trilayer technology, and successfully tested at low frequencies. A novel primary digital-to-analog (D/A) converter based on the processing of single-flux-quantum (SFQ) pulses has been suggested. The performance of analog-to-digital (A/D) converters combined from such primary D/A converters and voltage multipliers is discussed.>
Precise voltage generation is a unique feature of superconducting single-flux-quantum (SFQ) circuits, and hence, several SFQ-based digital-to-analog converters (DACs) have been designed for metrological applications. In this letter, we propose a dual double-flux-quantum amplifier (dual-DFQA) that extends DAC output voltage from unipolar to bipolar. An SFQ-based DAC comprising a 4-bit variable pulse number multiplier and a ±20-fold dual-DFQA is fabricated using a niobium integration technology. A 1-kHz, ±0.29-mV sinusoidal voltage waveform is successfully synthesized.
We describe the design of an oversampling single-flux-counting analog-to-digital converter with the estimated bandwidth of a few tens of MHz, based on Rapid Single-Flux-Quantum (RSFQ) devices. It consists of an input analog circuit, high-speed quantizer with an original analog/digital negative feedback loop, decimation (comb) filter, and supporting subsystems including clock distribution circuits, quantization signal conditioner, and output drivers. We have applied a new approach, the 'symmetrizing' negative feedback, to free the quantizer from the intrinsic hysteresis without dynamic range sacrifice. The 16-bit version of the system comprises about 1,500 Josephson junctions and consumes 1.2 mW of DC power, when implemented using Nb-trilayer technology with 1 kA/cm/sup 2/, 3.5 /spl mu/m Josephson junctions. All subsystems of the converter have been successfully tested and testing of the system as a whole is in progress.>
A new A/D converter which includes a comparator, and a reversible binary counter with DC outputs has been designed, fabricated, and tested. The comparator generates two trains of the single-flux-quantum (SFQ) pulses in response to increasing or decreasing input signal. The pulses are transferred through SFQ transmission lines to the adding and diminishing inputs of a reversible counter. The reversible counter has been realized by supplementing the usual counter with the SFQ transmission lines, splitters; and confluence elements for sending diminished pulses directly to each bit. Nondestructive read-out of the counter contents is carried out by SFQ/DC converters connected to each counter bit. The integrated circuit is fabricated using 5- mu m Nb-AlO/sub x/-Nb Josephson junction technology with a critical current density about 500 A/cm/sup 2/. External Mo shunts of the junctions provide the value of beta /sub c/<or=1 and I/sub c/R approximately=300 mu V. The A/D conversion is studied for low-frequency signals with an external clock. High-frequency performance of the reversible counter has been tested.
Recently we designed and successfully tested a prototype of a superconducting digital-to-analog converter (DAC) based on processing single flux quantum (SFQ) pulses. Two unique features make the device very attractive for metrology applications: (i) The generation of output voltage is based on the fundamentally accurate Josephson frequency-to-voltage relationship. (ii) The device does not require any external signals with frequencies higher than 100 MHz. It is expected that the DAC will be used as a key component of novel standards and calibrators of ac voltage with frequency up to 1 MHz. The implemented prototype has low magnitude of output voltage (about 10 mV) which is far lower than the 1 to 10 V target magnitude.
Precise voltage generation is a unique feature of single-flux-quantum (SFQ) circuits, in addition to their high-speed digital signal processing with low power consumption. We investigated SFQ pulse-frequency modulation D/A converters for metrological applications. In our SFQ-based D/A converters, the maximum output voltage is determined by the maximum SFQ pulse-frequency at the pulse number multiplier, and by the voltage multiplication factor at the voltage multiplier. In this study, we present our new design for a double-flux-quantum amplifier (DFQA) that works as a quantum voltage multiplier. In the new parameter set, we tuned the damping parameters of the Josephson junctions to realize proper propagation of SFQ pulses. A 1000-fold DFQA designed with the new parameter set was fabricated using a 25-μ A/μ m2Nb/AlOx/Nb integration technology. A 1000-fold voltage multiplication was confirmed for the input voltage up to 43 μV, with a corresponding SFQ repetition frequency of 21 GHz. That is, the output voltage reached 43 mV.
Digital to analog converters based on the Josephson effect are promising for voltage standards, because they produce voltage steps with ultimate precision and stability. In this paper, we describe a project to develop a Josephson D/A converter designed for synthesizing a sinusoidal waveform with metrological accuracy. The D/A converter is based on RSFQ (Rapid Single Flux Quantum) logic circuits, and consists of a frequency multiplier (FM), a pulse distributor (PD), and a number of voltage multipliers (VMs). Each VM circuit, corresponding to the n-th bit digital code, multiplies the number of SFQ pulses by a factor of 2/sup n/. By gating the input SFQ pulses from the FM to the VMs using the PD circuits, a programmable output voltage is obtained. Possible sources of uncertainties in the measurement of the rms value of the synthesized sine wave are discussed.