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IEICE Electronics Express

100GHz operation of a 4-bit single-flux-quantum pulse-frequency modulator designed for bipolar D/A conversion

Yoshinao Mizugaki, Naonori Sega, Hiroshi Shimada

Published May 15, 2022• 1 citation
Publisher

Abstract

We designed and operated a 4-bit single-flux-quantum pulse-frequency modulator for bipolar D/A conversion. It was based on our previously-proposed “sum of selected bit sequence” configuration. Two functions, a synchronization scheme with a reference signal source and a complement number system for realization of bipolar output voltage, were implemented. A test circuit excluding the output analog stage (dual double-flux-quantum amplifier) was fabricated using a 10 kA/cm2 Nb/AlOx/Nb integration process. The synchronization function was evaluated in terms of the average voltages, which agreed well with the expected values. The bipolar D/A conversion was successfully emulated by using the positive and negative output signals. The maximum over-biased input voltage was determined to be 230 µV, of which the corresponding SFQ repetition frequency was 111 GHz.

DOI
10.1587/elex.19.20220194

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We designed and operated a 4-bit single-flux-quantum pulse-frequency modulator for bipolar D/A conversion. It was based on our previously-proposed “sum of selected bit sequence” configuration. Two functions, a synchronization scheme with a reference signal source and a complement number system for realization of bipolar output voltage, were implemented. A test circuit excluding the output analog stage (dual double-flux-quantum amplifier) was fabricated using a 10 kA/cm2 Nb/AlOx/Nb integration process. The synchronization function was evaluated in terms of the average voltages, which agreed well with the expected values. The bipolar D/A conversion was successfully emulated by using the positive and negative output signals. The maximum over-biased input voltage was determined to be 230 µV, of which the corresponding SFQ repetition frequency was 111 GHz.
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.
Abstract We have demonstrated digital-to-analog (D/A) operations using single-flux-quantum (SFQ) pulse-frequency modulation (PFM) D/A converters for future AC voltage standards. In this paper, for the improvement of SFQ-PFM D/A converters, we investigated a double-flux-quantum amplifier (DFQA) and a magnetically-coupling SFQ driver/receiver circuit (MC-SFQ-DR) fabricated using a 10 kA cm −2 Nb/AlO x /Nb integration process. The critical current density J c of 10 kA cm −2 was four times larger than that of the integration process we had used. A DFQA and an MC-SFQ-DR included unshunted Josephson junctions, and therefore, it was unclear if the high- J c process improved their performances. We measured test chips cooled in a liquid helium bath. The maximum input voltages for a +20 fold and a −20 fold DFQA were 147 and 126 μ V, for which the corresponding Josephson frequencies were 70.9 and 61.1 GHz. It was confirmed that the operation frequencies of the DFQAs and MC-SFQ-DR were improved approximately two-fold.
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.
Single-flux-quantum (SFQ) circuits can generate highly accurate average voltage. We have developed SFQ digital-to-analog converters (DACs) for application of new ac voltage standards. In this paper, we present a bipolar voltage amplifier (BVA) based on a double flux quantum amplifier (DFQA) for generation of intrinsic bipolar voltage in an SFQ-DAC. The BVA realizes the identical multiplication factors for the positive and negative output, whereas the conventional DFQA has different multiplication factors in the bipolar output. The BVA can be used for the output stage of the SFQ- DAC. We have designed a 19-fold BVA and confirmed that the 19-fold BVA operated for input frequency of up to 31.2 GHz in positive voltage output and 33.6 GHz in negative voltage output.
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.
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.