Victor K. Kornev

Bi-SQUID: a novel linearization method for dc SQUID voltage response

We show that the voltage response of a dc SQUID can be substantially linearized by the introduction of a nonlinear inductance. The inductance tuning allows us to achieve response linearity close to 120 dB. Such a nonlinear inductance can be easily formed using Josephson junction inductance. The additional junction and main inductance form a single-junction SQUID and hence the device can be called a bi-SQUID. To obtain high dynamic range commensurate to the high response linearity, one can use a serial array of nonlinear inductance dc SQUIDs. Experimental studies of a single bi-SQUID and serial arrays of bi-SQUIDs are reported and discussed.

Sensitivity of the balanced Josephson-junction comparator

The sensitivity of a balanced comparator composed of two overdamped Josephson junctions fed by single flux pulses is calculated, taking into account both thermal and quantum fluctuations. The results of the analysis are compared with those of recent experiments. Ultimate resolution of the balanced comparator with feasible junction parameters is estimated to be as high as approximately 50 pA/Hz/sup 1/2/ at 4 K and approximately 10 pA/Hz/sup 1/2/ at T to 0 K. Performance limits of the device are compared with those of its single-junction counterpart.

Active Electrically Small Antenna Based on Superconducting Quantum Array

We introduce Superconductive Quantum Arrays and propose to use these structures as active Electrically Small Antennas (ESA). Several prototypes of the active ESA were fabricated using Nb process with a critical current density of 4.5 kA/cm2 and experimentally evaluated. The magnetic field to voltage transfer function linearity up to 70 dB was measured, and transfer factor dV/dB up to 6.5 mV/μT was observed for the ESA prototype containing 560 cells.

Linear Bi-SQUID Arrays for Electrically Small Antennas

Recently we proposed so-called bi-SQUID based on a 3-junction SQUID circuit capable of providing highly linear voltage response. In this report, we present the experimental evaluation of series arrays of 20 and 128 bi-SQUIDs fabricated with a 4.5 kA/cm2 Nb process as well as a prototype of an active electrically small antenna based on series array of 12 bi-SQUIDs. Both the origins of imperfections of the observed response linearity and the possible ways of the linearity improvement are discussed.

Design and Experimental Evaluation of SQIF Arrays With Linear Voltage Response

Differential circuits consisting of two series arrays of 10-junction parallel SQIFs were developed, designed and fabricated with 4.5 kA/cm2 Nb HYPRES process. The differential voltage response evolution with applied magnetic field providing opposite frustration of the serial arrays was analysed in detail. Linear differential response with amplitude as high as 22 mV was observed for the serial arrays of 108 parallel SQIFs. It was shown that the response linearity is kept within some range of the applied frustrating magnetic field.

Performance Advantages and Design Issues of SQIFs for Microwave Applications

We consider applications of SQIFs as amplifiers for gigahertz frequency range. SQIF-like structures are able to provide much higher dynamic range and linearity than a dc SQUID. We also analyse design limitations imposed by finite coupling inductances and stray capacitances. Possible ways of resolving design issues are discussed.

High Linearity SQIF-Like Josephson-Junction Structures

Recently we reported design approaches for the synthesis of multi-SQUID structures capable of providing high linearity voltage response. These structures were developed to form periodic voltage responses. This paper presents possible design solutions for multi-element structures providing SQIF-like high linearity voltage response. The approach is based on the use of a differential scheme of two parallel SQIFs with arrays oppositely frustrated by applied magnetic field deltaB. The differential scheme enables a high efficiency synthesis of highly linear SQIF response by subtraction of deviations from linear law.

Development of SQIF-Based Output Broad Band Amplifier

High-performance single flux quantum (SFQ) pulse amplifier (driver) based on superconducting quantum interference filter (SQIF) or near regular array of SQUIDs has been developed, fabricated, and tested. The driver part coupling method and circuit optimizations are discussed. The first test results of the driver prototype are reported. The experimental results confirm performance advantages of this driver design approach.

Superconducting Quantum Arrays

Superconducting Quantum Arrays (SQAs) based on integration of quantum cells each consisting of two Josephson-junction parallel arrays [Differential Quantum Cell (DQC)] are analyzed for applications in broadband radio frequency systems. These SQAs are capable of providing highly linear magnetic signal to voltage transfer with high dynamic range. Both detail study of the quantum cells with realistic parameters and analysis of their characteristics, including voltage response linearity, are presented and discussed. A prototype of an active electrically small antenna is implemented based on an SQA containing 560 DQCs. We demonstrated the SQA voltage response swing as high as ~ 100 mV at a transfer factor of ~ 6.5 mV/μT.

Output Power and Loading of Superconducting Quantum Array

Using numerical simulation, we investigate the output power and loading tradeoffs for Superconducting Quantum Arrays (SQAs) suggested for the implementation of broadband radio-frequency systems, including active electrically small antennas capable of providing highly linear transfer characteristics with a high dynamic range. Standard matching loads used for linear microwave networks, including passive antennas, are not applicable for the active SQA-based circuits generally due to the strong shunting effect restricting the attainable linearity. At the same time, the output power of an active device can be increased by a power supply even for the impedance-mismatched circuits. Both the optimum achievable balance of the shunt effects and getting the required output power are analyzed and discussed in detail.