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Conference on Precision Electromagnetic Measurements

Josephson Based Spectrum Analyzer

L. Palafox, Jonas Herick, Ralf Behr, Oliver Kieler, M. Schubert

Published Jul 1, 2018• 2 citations
Publisher

Abstract

Differential measurements relative to a Josephson waveform provide very accurate measurements of ac waveforms. Pulse driven arrays, used in Josephson Arbitrary Waveform Synthesizers, can generate sine waves with very high spectral purity. These two Josephson based measurement tools can be combined to study undesired distortion components present in the frequency spectrum of high quality waveform generators. For a proof of principle experiment, we cancelled the fundamental tone of a waveform and measured the third harmonic that was generated.

DOI
10.1109/cpem.2018.8500926

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Differential measurements relative to a Josephson waveform provide very accurate measurements of ac waveforms. Pulse driven arrays, used in Josephson Arbitrary Waveform Synthesizers, can generate sine waves with very high spectral purity. These two Josephson based measurement tools can be combined to study undesired distortion components present in the frequency spectrum of high quality waveform generators. For a proof of principle experiment, we cancelled the fundamental tone of a waveform and measured the third harmonic that was generated.
We present the combination of two Josephson waveform synthesizers to generate waveforms with low harmonic distortion [below$-$110 dBc (carrier)] and amplitudes above the 1-V level. Fast and flexible code programming enables pulse-driven Josephson arrays to synthesize arbitrary waveforms. We have used the pulse-driven system to cancel the harmonic content from the binary array that can reach 1-V amplitudes. The quantum nature of the Josephson synthesizer ensures very stable amplitudes (better than6×10−86 \times 10^{-8}6×10−8) and no frequency drift. This performance, together with the ability to synthesize arbitrary waveforms, can make this synthesizer a relevant tool for characterizing precision electronics and ac voltage metrology.
The past few decades have witnessed great progress being made from the first voltage standards based on arrays of Josephson junctions. Continuous improvements in the fabrication process have increased the quality and “user-friendliness” of these superconducting circuits. Combined with the improvements in measuring electronics, the range of applications of Josephson-based measurements with unmatched precision has extended from DC into AC voltage metrology for signal frequencies up to 500 kHz. In this poster we will present several Josephson-based measurement systems developed for metrology applications. Two versions of quantum AC standards will be used as Josephson references: Programmable Josephson Voltage Standards (PJVS), which are based on serial arrays with up to 300,000 Josephson junctions, and pulse-driven arrays with more than 50,000 junctions, typically referred to as Josephson Arbitrary Waveform Synthesizers (JAWS). PJVS can reach amplitudes of up to 80 V, while pulse-driven arrays can reach up to 1 V or even 4 V. The advantage of pulse-driven Josephson voltage standards is that they are ideally suited as quantum-based voltage synthesizers for arbitrary waveforms or sine waves with high spectral purity. Therefore, they can be used directly to characterize or calibrate ADCs. In addition, JAWS are particularly useful in Josephson impedance bridges, which perform automated impedance calibrations at the 10-9 uncertainty level over a wide frequency range from 50 Hz to 80 kHz in a single bridge. We will further show how PJVS are used in commercial quantum voltmeters for DC and AC voltages and are typically used as quantum voltage references in measurement systems. Using differential sampling based on PJVS, uncertainties at the level of a few parts in 108 for frequencies up to the kHz range can be achieved. Such quantum-based AC voltage calibrations are at least 30 times faster than conventional AC-DC transfer and offer even better uncertainties. This idea has been quickly adopted by many National Metrology Institutes to establish such systems for AC voltage calibrations. As an example, the BIPM will now start worldwide onsite comparisons to ensure the uniformity of the new method which we introduced in 2006. But developments continue. Recently, a sub-sampling technique was used to extend the frequency range of the AC quantum voltmeter to up to 100 kHz with uncertainties below 5 parts in 106. In addition, a novel method, also for differential sampling, enables the measurement of arbitrary waveforms and the calibration of specific tones. Our poster will provide an overview of these Josephson-based metrological applications and present the latest developments from this fast-developing area.
The equivalence of two sinusoidal waveforms synthesized by pulse-driven Josephson arrays using two Josephson arbitrary waveform synthesizers (JAWS) has been investigated by a direct, high-precision comparison. The arrays are based on double-stacked SNS-type Josephson junctions (superconductor-normal conductor-superconductor) with NbxSi1-xas barrier material. Two arrays, consisting of 4000 junctions each, are arranged on one silicon chip and are connected in opposite voltage direction directly at the chip at 4.2 K. Careful phase tuning between the two JAWS systems allowed the minimization of the residual ac voltage, which was characterized by a fast digitizer and a lock-in amplifier. For an output voltage of 8.8 mV, a difference voltage of 0.14 nV with 0.25 nV uncertainty was measured, i.e., (1.6 ±2.9) × 10-8in relative units. This result demonstrates the excellent agreement of both voltages and therefore the quantization of the JAWS itself.
We have developed a Josephson digital-to-analog converter, otherwise know as a Josephson arbitrary waveform synthesizer, with 300 mV peak output voltage. This is the first system of its kind with demonstrated quantum accuracy. We show precision synthesized waveforms from dc to 100 kHz with measured distortion and harmonic content below 115 dBc (dB below the carrier or fundamental). The heart of the system is a superconducting microwave integrated circuit with two Josephson junction arrays biased in parallel for the microwave drive and connected in series to double the voltage for the audio frequency output waveforms. New superconducting integrated circuits with twice as many stacked junctions and improved microwave circuits have allowed us to more than double the output voltage of our recent system. We also demonstrate that quantum-based synthesized arbitrary waveforms (not just sine waves) can be used for precision measurements of a high-performance commercial analog-to-digital converter.
We demonstrate a new bias technique that uses low-pass and high-pass filters to separate the current paths of the sampling and signal frequencies in a Josephson waveform synthesizer. This technique enables the output voltage of the array to be directly grounded by removing the low-frequency common mode signal that previously prevented direct measurement of the array voltage with low-impedance instruments. We directly measure the harmonic spectra of 1 kHz and 50 kHz synthesized sine waves. We also use a thermal transfer standard to compare the rms voltages of synthesized sine waves at frequencies from 1 kHz to 50 kHz. Finally, we describe a new circuit that should enable a significant increase in output voltage by allowing several distributed arrays to be biased in parallel at high frequency, while combining their low frequency output voltages in series.