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We present design and testing of a pseudo-random number generator (PRNG) and a variable pulse number multiplier (VPNM) which are digital circuit subsystems in an integrated quantum voltage noise source for Jonson noise thermometry. Well-defined, calculable pseudo-random patterns of single flux quantum pulses are synthesized with the PRNG and multiplied digitally with the VPNM. The circuit implementation on rapid single flux quantum technology required practical circuit scales and bias currents, 279 junctions and 33 mA for the PRNG, and 1677 junctions and 218 mA for the VPNM. We confirmed the circuit operation with sufficiently wide margins, 80–120%, with respect to the designed bias currents. [ABSTRACT FROM AUTHOR]

Abstract: For establishing a new generation of ac voltage standards, digital-to-analog converters (DACs) based on rapid single flux quantum (RSFQ) technology, which enable synthesis of arbitrary waveforms with fundamental accuracy, are developed. An RSFQ-DAC consists of three main subsystems: a pulse-number multiplier (PNM), a pulse distributor (PD) and voltage multipliers (VMs). Conventional RSFQ-DACs generate arbitrary voltages by switching a binary array of the VM cells. In this paper, we propose a Variable-PNM for a new RSFQ-DAC based on frequency modulation. The output voltage is determined by the multiplication factor m of the Variable-PNM that consists of a ring oscillator, an n-bit counter and a decoder. We designed a 2-bit Variable-PNM using the CONNECT cell library. The circuit was fabricated using the ISTEC Nb standard process (STP2). We have confirmed the correct functionalities for 2-, 4-, 6- and 8-fold multiplication in functional testing at low speed. [Copyright &y& Elsevier]

We have developed a sub-μm Nb/Al-AlNx /Nb Josephson junction and integrated circuit fabrication process using deep-UV lithography and inductively coupled plasma etch tools. The baseline process consists of 11 masking steps including ground plane, PdAu resistor, Nb/A1-AlNx/Nb trilayer, and two additional Nb wiring layers. The AINx tunnel barriers are grown with plasma nitridation. These junctions exhibit low subgap leakage even at current densities exceeding 100 kA/cm2. The critical current spread of a series array of 50-kA/cm2, 0.6 μm diameter junctions is under 3%. For very high current density applications, these junctions are a good candidate to replace Nb/Al-A1Ox /Nb junctions particularly in future generations of very high speed, rapid single flux quantum logic circuits. In this paper we discuss our baseline fabrication process and device characterization including junction capacitance extraction from direct measurements of the Josephson plasma frequency. [ABSTRACT FROM AUTHOR]

A 10-kA/cm2 Nb/A1Ox,/Nb junction technology for rapid single flux quantum (RSFQ) integrated circuits has been developed. Modifying our standard 1.6-kA/cm2 cell library, we have implemented elementary cells used to build RSFQ digital-to-analog converters for ac voltage standard applications. The 10-kA/cm2 library cells have been fabricated by utilizing existing process tools. Correct operations of the circuits have been confirmed at low speed. The maximum operating frequencies evaluated by an average-voltage measurement technique were consistent with the scaling rules of RSFQ circuits. [ABSTRACT FROM AUTHOR]

Abstract: A digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) circuits is a potential device for establishing a new generation of ac voltage standards. We are developing RSFQ D/A converters that can generate an ac waveform with an accurately defined rms value. Our goal in the near future is the synthesis of an ac waveform with a 100-mV peak voltage that will allow accurate characterization of the thermal voltage converters currently used for setting ac–dc transfer standards. We have designed, fabricated and successfully tested key subsystems of the RSFQ D/A converter including a 10-stage pulse-number multiplier, a 1-bit slice of a pulse distributor and a 9.5-bit voltage multiplier. [Copyright &y& Elsevier]

One of the remaining challenges in the application of superconducting electronics is the interfacing between superconducting and semiconducting environments. The voltage and speed mismatch between RSFQ pulses and semiconducting read-out electronics makes it necessary to amplify as well as stretch the RSFQ pulses. Moreover, circuits based on HTS (High Temperature Superconductor) technology are very attractive since they can operate under considerably relaxed cooling effort, which is one of the main problems with LTS (Low Temperature Superconductor) circuits. Within the European project SuperADC, a HTS second order sigma delta modulator and a pulse stretcher, used as an interface between the modulator and the first semi-conducting amplifier stage, have been designed at Twente University and will be presented here. [ABSTRACT FROM AUTHOR]

We present the development of a rapid single flux quantum (RSFQ) standard cell library for middle-scale applications. The library includes elementary cells frequently used to build large circuit blocks. The standard cells were designed for a simple process with 8 mask levels. The circuits were optimized and laid out to minimize effects of high parasitic inductance associated with a single ground plane layer. Circuit parameters on the wafer were determined by electrical measurements on parametric test circuits, which gave feedback on the design and process. All cells were individually fabricated and tested, and the design and layout were confirmed. [Copyright &y& Elsevier]

We investigated the effects of magnetic fields induced by bias currents on rapid single flux quantum (RSFQ) circuits. We measured the mutual inductance between a signal line and a bias line in the circuit. The mutual inductance values obtained were in the range of 5–50 fH. We simulated operation of a 2-stage shift register under the magnetic fields induced by an additional control current. The calculated operating regions were significantly reduced by the magnetic fields induced by the control currents on the order of 10 mA. The experimental results obtained for the shift register agreed with the simulation. These results suggest that it is important to take the effects of current-induced magnetic fields into account in the design of large-scale RSFQ circuits. [Copyright &y& Elsevier]

Rapid single flux quantum (RSFQ) digital-to-analog (D/A) converters that synthesize arbitrary waveforms with metrological accuracy are under development. In the D/A converter, a magnetically coupled voltage multiplier (VM) is utilized as a precise frequency-to-voltage converter that generates a voltage proportional to the input frequency. To increase the output voltage of the VM, parameters that determine the cycle time of the VM cell were optimized. Experimental results obtained with 16-stage VMs showed that the maximum operating frequency of the new VM was 50 GHz, which is twice that of the previous type. [Copyright &y& Elsevier]

After more than a decade of research in single flux quantum (SFQ) circuits, a high level of understanding the requirements for designing them with high-Tc superconductors (HTS) has been gained. Recently, the fabrication processes are being developed to be more and more in accordance with practical requirements for the design of real digital circuits. The contribution describes the complex relationship of desired high operation temperature and required low bit-error probability on a quantitative level. Suitable guidelines for circuit dimensioning are given. The discussion is oriented at one of the contemporary technological approaches. It is concluded that, despite some previous estimations, HTS SFQ circuits can work stable theoretically also at a relatively high operation temperature. [Copyright &y& Elsevier]

The rapid single flux quantum (RSFQ) mixed-signal technology offers a way to enhance performance and reduce complexity and cost of SQUID systems. Ultra-sensitive analog SQUID front ends can be combined with low-power and ultra-fast RSFQ digital circuits on a single chip to form mixed-signal circuits combining the sensitivity and noise level of an “analog SQUID” and the increased slew-rate, dynamic range, linearity, and simple multi-channel readout electronics of digital RSFQ circuits. We have developed several RSFQ-based circuits to perform sensing, digitization, and timing of various weak signals including outputs of detector arrays. The key enabling factor for the implementation of all these SQUID-RSFQ circuits is the availability of a high-yield, low-noise fabrication process. [Copyright &y& Elsevier]