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Conference on Precision Electromagnetic Measurements

Superconductor digital-to-analog converter based on processing of SFQ pulses

Vasili K. Semenov, P.N. Shevchenko, Yu. A. Polyakov

Published Nov 27, 2002• 9 citations
Publisher

Abstract

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.

DOI
10.1109/cpem.1998.700136

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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.
The design and operation of a 9 bit superconductive single‐flux‐quantum (SFQ) digital‐to‐analogue converter (DAC) are presented. The DAC comprises a 9 bit variable SFQ pulse number multiplier and a 100‐fold voltage multiplier. An SFQ pulse‐frequency modulation technique is employed for coding the analogue output voltage. The DAC is fabricated using an Nb Josephson integration technology. Its output voltage waveform, the amplitude of which is up to 2.54 m V pp , is controlled by digital input signals from a room‐temperature data generator.
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.
Abstract One of the unique applications of single-flux-quantum (SFQ) circuitry is a digital-to-analog converter (DAC) of which the output voltage is defined by the Josephson effect. Recently, we have designed and operated rapid single-flux-quantum (RSFQ) DACs with frequency modulation of SFQ pulse sequences. In this paper, we report our design and test results of a 4-bit RSFQ-DAC based on the configuration of the “sum of selected bits sequence (Σ-SBS)”. By adjusting the timing margins, in addition to the fabrication using the Nb integration process with the critical current density as high as 10 kA/cm2, the maximum output voltage up to 260 μV was demonstrated.
Superconducting Josephson devices have been developed for various electronics fields such as highly sensitive magnetometers, voltage standards, and high-speed digital signal processing. In this paper, the author describes two kinds of superconducting digital-to-analog （D/A） converters that generate quantum voltages. One is a balanced-ternary D/A converter comprising asymmetric superconducting quantum interference devices, in which the generation of either a positive or a negative zero-crossing Shapiro step is controlled by an external magnetic flux. The other is a single-flux-quantum （SFQ） D/A converter, in which SFQ pulse-frequency modulation is employed. They generate quantum voltage defined by the reference microwave frequency and the AC Josephson effect.
We present a single-flux-quantum (SFQ)-based digital-to-analog converter (DAC) generating bipolar output voltages, in which the key component is a polarity-switchable double-flux-quantum amplifier (PS-DFQA). The DAC comprised a dc/SFQ converter, an 8-bit variable pulse-number multiplier (PNM), and a ±8-fold PS-DFQA integrated on a single chip. SFQ pulse-frequency modulation was employed to realize variable output voltage amplitude, for which the multiplication factor of the variable-PNM was controlled by a commercial data generator situated at room temperature. The variable-PNM realized 8-bit resolution with a multiplication factor between 0 and 255. Bias currents fed to the ±8-fold PS-DFQA were polarity switched in synchronization with the digital code for the variable-PNM. The whole circuits including I/O elements were designed using SFQ cell libraries, and fabricated using a niobium integration process. Sinusoidal bipolar voltage waveform of 0.38 mVpp was demonstrated using a reference signal source of 43.94 MHz.
We have developed and demonstrated a digital to analog converter DAC which uses an SFQ counter to precisely divide an input reference oscillator to produce a set of binary frequencies/voltages. The binary input gates the output SFQ pulses of the counter flip flops to a passive summing network, producing an analog output current. The DAC is asynchronous (no clock to reset latching circuits), low power, and requires only N equal matched resistors. We have built and tested a 4-bit Nb DAC with data clocking rates up to 1 GHz. Using the DAC we have generated arbitrary wave forms including ramps and sine waves. By measuring the harmonic content of sine wave outputs, we deduce values for the DAC linearity.