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IEEE Transactions on Applied Superconductivity

Josephson Arbitrary Waveform Synthesizer With Two Layers of Wilkinson Dividers and an FIR Filter

Nathan E. Flowers-Jacobs, Steven B. Waltman, Anna E. Fox, Paul D. Dresselhaus, Samuel P. Benz

Published Jun 23, 2016• 32 citations
Publisher

Abstract

The output voltage of Josephson arbitrary waveform synthesizers (JAWS) is limited by the number of Josephson junctions (JJs) that can be driven by a single pulse-generator channel. Here, we double the number of JJs driven by one generator channel to 51 200 JJs by distributing the pulse bias between four JJ arrays by use of two layers of Wilkinson dividers. We use this single bias to generate a voltage at 1 kHz with an rms magnitude of 1 V. This voltage is quantum-accurate over an operating current range of 1.4 mA. For comparison, the operating current range of a recent design that uses a single layer of Wilkinson dividers is twice as large, but requires two pulse-bias channels to generate 1 V. We also show that we can recover this performance by incorporating in the pulse generator a finite-impulse response (FIR) filter that acts as an equalizer. The FIR filter creates a custom transfer function that compensates for the bandwidth-limited transfer function of the Wilkinson dividers. Optimizing the FIR filter parameters increases the operating current range from 1.4 to 2.7 mA. This ability to drive additional JJ arrays with a single pulse-generator channel will enable future JAWS chips and systems to achieve significantly larger output voltages. This will increase the voltage range for JAWS calibrations of ac thermal converters and improve precision voltage measurements that require quantum-accurate, stable, and distortion-free waveforms with a large signal-to-noise ratio.

DOI
10.1109/tasc.2016.2582800

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IEEE Transactions on Applied Superconductivity•Jul 11, 2014•40 citations
The output voltage of Josephson arbitrary waveform synthesizers (JAWS) is limited by the number of Josephson junctions (JJs) that can be driven by a single pulse-generator channel. Here, we double the number of JJs driven by one generator channel to 51 200 JJs by distributing the pulse bias between four JJ arrays by use of two layers of Wilkinson dividers. We use this single bias to generate a voltage at 1 kHz with an rms magnitude of 1 V. This voltage is quantum-accurate over an operating current range of 1.4 mA. For comparison, the operating current range of a recent design that uses a single layer of Wilkinson dividers is twice as large, but requires two pulse-bias channels to generate 1 V. We also show that we can recover this performance by incorporating in the pulse generator a finite-impulse response (FIR) filter that acts as an equalizer. The FIR filter creates a custom transfer function that compensates for the bandwidth-limited transfer function of the Wilkinson dividers. Optimizing the FIR filter parameters increases the operating current range from 1.4 to 2.7 mA. This ability to drive additional JJ arrays with a single pulse-generator channel will enable future JAWS chips and systems to achieve significantly larger output voltages. This will increase the voltage range for JAWS calibrations of ac thermal converters and improve precision voltage measurements that require quantum-accurate, stable, and distortion-free waveforms with a large signal-to-noise ratio.
The root-mean-square (rms) output voltage of the NIST Josephson arbitrary waveform synthesizer (JAWS) has been doubled from 1 V to a record 2 V by combining two new 1 V chips on a cryocooler. This higher voltage will improve calibrations of ac thermal voltage converters and precision voltage measurements that require state-of-the-art quantum accuracy, stability, and signal-to-noise ratio. We achieved this increase in output voltage by using four on-chip Wilkinson dividers and eight inner-outer dc blocks, which enable biasing of eight Josephson junction (JJ) arrays with high-speed inputs from only four high-speed pulse generator channels. This approach halves the number of pulse generator channels required in future JAWS systems. We also implemented on-chip superconducting interconnects between JJ arrays, which reduces systematic errors and enables a new modular chip package. Finally, we demonstrate a new technique for measuring and visualizing the operating current range that reduces the measurement time by almost two orders of magnitude and reveals the relationship between distortion in the output spectrum and output pulse sequence errors.
This paper describes recent developments for increasing the output voltage of the Josephson Arbitrary Waveform Synthesizer (JAWS) at PTB. For this purpose, we developed modified broadband Wilkinson power dividers, which enable the operation of two or four Josephson junction series arrays per RF channel of the pulse pattern generator and thus the generation of increased output voltages. In detail, we designed and investigated two different Wilkinson power dividers: the one-stage three-section Wilkinson power divider with extended bandwidth and the two-stage single-section Wilkinson divider with more outputs to combine four parallel series arrays. They were both integrated with triple-stacked Josephson junction series arrays. Additionally, a new modulator pulse amplifier was introduced into the JAWS set-up to provide sufficient power for all the junction arrays. Our measurement results show that spectrally pure sinusoidal waveforms were successfully generated with both types of power dividers. With the one-stage three-section Wilkinson power divider, we obtained RMS voltages of nearly 53 mV at a clock-frequency of 15 GHz combined with a test array of 3000 Josephson junctions. As for the two-stage single-section Wilkinson power divider combined with a test array of 6000 Josephson junctions, we synthesized RMS output voltages of about 105 mV. These test results show a promising prospect for quantum voltage standard applications.
We report on the design, fabrication, and measurement of a very high frequency band Josephson arbitrary waveform synthesizer (VHF-JAWS) at frequencies from 1 kHz to 50.05 MHz. The VHF-JAWS chip is composed of a series array of 12 810 Josephson junctions (JJs) embedded in a superconducting coplanar waveguide. Each JJ responds to a pattern of current pulses by creating a corresponding pattern of voltage pulses, each with a time-integrated area related to fundamental constants ash/2e{{h/2e}}h/2e. The pulse patterns are chosen to produce quantum-based single-tone voltage waveforms with an open-circuit voltage of 50 mV rms (–19.03 dBm output power into 50Ω\OmegaΩload impedances) at frequencies up to 50.05 MHz, which is more than twice the voltage that has been generated by previous RF-JAWS designs at 1 GHz. The VHF-JAWS is “quantum-locked,” that is, it generates one quantized output voltage pulse per input current pulse per JJ while varying the dc current through the JJ array by at least 0.4 mA and the amplitude of the bias pulses by at least 10 %. We use the large bias pulse quantum-locking range to investigate one source of error in detail: the direct feedthrough of the current bias pulses into the DUT at VHF frequencies, which adds an unwanted, in-band component to the measured voltage. We reduce this error by high-pass filtering the current bias pulses and measure the error as a function of input pulse amplitude using two techniques: 1) by measuring small changes over the quantum-locking range and 2) by passively attenuating the input pulse amplitude so that the nonlinear JJs no longer generate voltage pulses while the error is only linearly scaled.
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.
This paper describes the status of developing Josephson arbitrary waveform synthesizer (JAWS) chips at NIM (National Institute of Metrology, China). To obtain high junction integration density and fewer data input channels, the chip employs an on-chip Wilkinson power divider and inside/outside dc blocks, enabling both arrays to be driven by a single pulse-generator channel. In addition, the tapered coplanar waveguide structure is used to ensure the microwave uniformity of the long-junction array. Each array consisted of 4000 double-stack Nb/NbxSi1−x/Nb junctions, and 16,000 junctions are integrated in the chip in total. The JAWS chip demonstrates good performance, capable of synthesizing a 300 mV root mean square (rms) voltage with exceptionally low harmonic distortion. Dc and ac voltage-current characteristics measurements indicate that the junctions are with a critical current of 2.5 mA, and a normal-state resistance of 4.5 mΩ per junction. Contact aligners are manually operated to fabricate the chips, and process errors in the fabrication are estimated in this paper.
The Josephson arbitrary waveform synthesizer (JAWS) is a series array of thousands of superconducting Josephson junctions that are biased by current pulses such that the array produces voltage waveforms with quantum accuracy. Intrinsically accurate voltage waveforms synthesized with the quantized pulses from Josephson junctions were first demonstrated in 1996. Ten years later, a commercial ac standard was calibrated at an output root-mean-square (RMS) amplitude of 100 mV with the first practical superconducting digital-to-analog converter. Since then, many different bias techniques, pulse-drive electronics, and device technology have been developed and improved in order to achieve a maximum of 138 mV output RMS voltage per Josephson array. In this paper, we report new bias electronics and demonstrate two new pulse-bias techniques. The first technique has demonstrated 250 mV output RMS voltage per 6400-junction array and may enable a practical 1 V system with only four arrays. The second bias technique reduces inductance-related error signals at the signal frequency and should reduce systematic errors for waveforms with frequencies greater than 1 MHz.