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Superconductor Science and Technology

Synthesis of sinusoidal signals with a Josephson arbitrary waveform synthesizer

Helko E. van den Brom, Ernest Houtzager, Oleg A. Chevtchenko, G. Wende, Marco Schubert, Torsten May, Hans‐Georg Meyer, Oliver Kieler, J. Kohlmann

Published Mar 27, 2007• 11 citations
Publisher

Abstract

A Josephson arbitrary waveform synthesizer (JAWS) has been used for the synthesis of bipolar waveform voltage signals. The two major changes with respect to earlier work are the use of Josephson arrays consisting of more junctions and the use of a new type of pattern generator. With a multi-branch Josephson array from IPHT, the synthesized sinusoidal voltages have amplitudes up to 9.5 mV zero to peak and frequencies in the range from 100 Hz to 150 kHz. For instance, for a 9.5 mV sine wave of 122 kHz, the spectrum of the synthesized signal shows that higher harmonics are at least 86 dB below the fundamental. Comparable suppression was achieved with a single-branch Josephson array from PTB generating a 17 mV sine wave of 180 Hz. Additionally, with a PTB array, sinusoidal signals with amplitudes up to 57 mV zero to peak are generated with higher harmonics typically 60 dB to 70 dB below the fundamental at frequencies below a few hundred hertz.

DOI
10.1088/0953-2048/20/5/001

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A Josephson arbitrary waveform synthesizer (JAWS) has been used for the synthesis of bipolar waveform voltage signals. The two major changes with respect to earlier work are the use of Josephson arrays consisting of more junctions and the use of a new type of pattern generator. With a multi-branch Josephson array from IPHT, the synthesized sinusoidal voltages have amplitudes up to 9.5 mV zero to peak and frequencies in the range from 100 Hz to 150 kHz. For instance, for a 9.5 mV sine wave of 122 kHz, the spectrum of the synthesized signal shows that higher harmonics are at least 86 dB below the fundamental. Comparable suppression was achieved with a single-branch Josephson array from PTB generating a 17 mV sine wave of 180 Hz. Additionally, with a PTB array, sinusoidal signals with amplitudes up to 57 mV zero to peak are generated with higher harmonics typically 60 dB to 70 dB below the fundamental at frequencies below a few hundred hertz.
The Josephson arbitrary waveform synthesizer (JAWS) allows AC voltages to be generated with fundamental accuracy. Lumped arrays of superconductor/normal conductor/superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 mum2to 4 mum2in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra.
The establishment at PTB of an AC Josephson voltage standard (Josephson Arbitrary Waveform Synthesizer- JAWS) based on pulse-driven Josephson arrays is focused on achieving an output voltage of at least 1 Vrms which is required for many metrology applications. In this paper, we approached this goal by increasing the number of active junctions in two ways. Firstly, we fabricated arrays containing triple-stacked SNS-type Josephson junctions with NbxSi1-xas barrier material. We obtained current operation margins of about 0.2 mA with arrays of up to 9000 Josephson junctions at an rms voltage of 355 mV. Secondly, we used a new 8-channel ternary pulse pattern generator (PPG) to bias up to 8 arrays connected in series. An output voltage of VRMS= 1006 mV(VPP= 2.845 V) was achieved by using 8 arrays (arranged on 4 separate chips) with 63000 junctions in total. Higher harmonics are suppressed by at least -116 dBc. The fabrication process and the experimental setup will be described, as well as experimental results that are leading towards our 1 V goal.
Superconductor-normal metal-superconductor(SNS) Josephson junction series arrays are developed and investigated for applications in the Josephson arbitrary waveform synthesizer (JAWS). The series arrays contain up to about 10 000 junctions arranged in a meanderlike design. AC voltages and arbitrary waveforms are synthesized by operation of the Josephson junctions by short current pulses. Higher harmonics are at least 110 dB below the fundamental. To verify the generated waveforms, a spectrum analyzer that has initially been calibrated using a binary-divided programmable Josephson voltage standard is used.
Sub-μm\mu{\rm m}μm SNS Josephson Junction Arrays for the Josephson Arbitrary Waveform Synthesizer
The experimental set-up of a Josephson arbitrary waveform synthesizer (JAWS) has been further optimized to generate AC voltages with fundamental accuracy. Josephson junction series arrays for the JAWS, consisting of sub-mum SNS type junctions (superconductor-normal conductor-superdonductor), are fabricated using a technology which combines electron-beam lithography and chemical-mechanical polishing. Bipolar pulses are realized by the superposition of unipolar pulses from a pulse pattern generator and a continuous sinusoidal microwave provided by a signal generator. A second-order delta-sigma modulator is optimized to generate a high-quality pulse code.
The performance of the NIST Josephson arbitrary waveform synthesizer has been improved such that it generates a root-mean-square (rms) output voltage of 1 V with an operating current range greater than 2 mA. Our previous 1 V JAWS circuit achieved this same maximum voltage over a current range of 0.4 mA by operating every Josephson junction in its second quantum state. The newest circuit synthesizes 1 V waveforms with the junctions operating in the first quantum state. The voltage per array is doubled because the number of junctions in each array was doubled through the use of improved microwave circuit designs that increased the bias uniformity to the junctions. We describe the circuit improvements and device operation, and we demonstrate the system capabilities by showing measured spectra of a 1 Hz sine wave and a dual-tone waveform. With only two arrays of the new circuit, we also synthesized a 128 mV sine wave without a compensation bias signal, which is one of the bias signals required for achieving 1 V. This is the same rms output voltage achieved with the previous circuit using four arrays.
Superconductor/normal conductor/superconductor (SNS) Josephson junction series arrays are the subject of research for the realization of a Josephson arbitrary waveform synthesizer (JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephson junctions are investigated for this application at the Physikalisch-Technische Bundesanstalt (PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in the center line of a coplanar waveguide transmission line. To increase the ac output voltage, the Josephson junction series array is designed as a meander-like structure. A broadband response of the arrays is required for the complex pulse operation of the JAWS. The arrays are investigated by dc measurements and for test purposes under continuous microwave irradiation at different frequencies. Flat Shapiro steps were measured for meander-like series arrays with 21 parallel lines containing up to 10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of 214 mV Peak was synthesized under pulse mode operation.