Jaani Nissilä

The Josephson two-terminal-pair impedance bridge

We present the connection of two programmable Josephson arrays generating synchronous waveforms to measure impedance ratios?the Josephson two-terminal-pair impedance bridge. This approach is more flexible than conventional bridges at the same level of uncertainty. The Josephson bridge can measure over a wider frequency range, over a wider range of impedance ratios than conventional two-terminal-pair bridges. Furthermore, the phase angle between the two impedances can take any value. As a first application, we present measurements of a 1?:?1 resistance ratio at the 10?k? level in the frequency range between 25?Hz and 10?kHz. The uncertainties are better than a few parts in 108 and hence comparable to those of conventional impedance bridges. Quantization at up to 10?kHz was confirmed by varying the bias current of the Josephson arrays, resulting in constant resistance ratios within the measurement resolution.

AC voltage standard based on a programmable SIS array

An AC voltage standard is being developed based on phase sensitive detection of the amplitude of the fundamental frequency component of the output of a programmable Josephson voltage array. The setup is described and requirements for relative uncertainties less than 10/sup -7/ at 1 kHz and 1 V are discussed. According to preliminary experiments, the constructed current bias is able to drive the array from -1 to +1 V within less than 100 ns.

Programmable Josephson Arrays for Impedance Measurements

Arbitrary impedance ratios can be determined with high accuracy by means of a programmable Josephson system. For a 1 : 1 resistance ratio at 10-kΩ level, we demonstrate that the novel system allows measurements over a wide frequency range from 25 Hz to 6 kHz. Uncertainties are in the range of a few parts in 108 and thus comparable to those of conventional impedance bridges. Two methods for four-terminal-pair impedance measurements have been investigated, i.e., the potential comparison circuit and the coaxial setup. Both methods are capable of measuring from dc to 6 kHz with uncertainties to a few parts in 108. The potential comparison circuit has an upper bound at 6 kHz due to the use of the sampling method. The four-terminal-pair coaxial setup has the potential to decrease the relative uncertainty down to 10-9 once systematic errors are analyzed and canceled.

A precise two-channel digitally synthesized AC voltage source for impedance metrology

A two-channel AC voltage source based on digital synthesis is reported. We present measurement results of some of the key properties at 1 kHz with applications like digital impedance bridges in mind. Measurements show that amplitude ratio of the channels has a stability (Allan standard deviation) of one part in 108 for a 30 min measurement. The phase difference between the channels is also stable within 0.1 μrad.

Josephson impedance bridges as universal impedance comparators

Josephson impedance bridges have been reported to provide uncertainties close to that from the best conventional bridges when comparing two 10-kΩ resistors. The accuracy of a few parts in 108 has been extended to the comparison of two 100-pF capacitance standards, significantly improving the uncertainty for these measurements at power line frequencies. Preliminary measurements of Josephson quadrature bridges are also presented with uncertainties at the level of 1.6 × 10−6 (k = 1).

Realization of a square-wave voltage with externally-shunted SIS Josephson junction arrays for a quantum AC voltage standard

A quantum-based ac voltage standard operating from about 1 Hz up to 10 kHz at 1 V is being developed. The standard is based on relating the output of a stable sine generator to a square wave obtained by biasing a nonhysteretic Josephson junction array alternately at steps n = -2 and n = +2. We have constructed a bias current source and a buffer amplifier connected to the array which enable fast switching between steps. In this paper, we describe the system used for generation of the square wave and present experimental results which show that the accuracy of the fundamental frequency component amplitude of the square wave is sufficient for realization of a standard with accuracy better than 1 /spl times/ 10/sup -6/.

Strong Attenuation of the Transients' Effect in Square Waves Synthesized With a Programmable Josephson Voltage Standard

This paper addresses the effect of transients on the operation of a 1-V programmable Josephson voltage standard for frequencies ranging between 125 Hz and 4 kHz. A detrimental effect of the transients occurring during the transition between different voltage steps is to make the ac Josephson voltage dependent on the bias current in the junctions. In other words, the current margins where the array behaves as a quantum standard are reduced to zero: the voltage steps have a slope. However, by using square waveforms, the effect of the transients can be reduced to a level where this slope is no longer measurable. This observation will probably lead to a simplification in the development of a new type of high-precision Josephson-based waveform synthesizer.

Stimulated power generation in ES-SIS junction arrays

We present experimental results on new programmable Josephson voltage standards based on frequency dependently damped superconductor-insulator-superconductor (SIS) junctions. We introduce a design able to generate two independent floating voltages up to 1.5 V using a microwave frequency of 70 GHz. The array is developed for dc and ac voltage and impedance metrology. The design is optimized in terms of speed, power consumption, and stability. Stimulated power generation is shown to affect the performance of long SIS arrays. The collective dynamics of Josephson junctions is analyzed and a boundary condition for the design of long arrays is derived.

Fast Josephson Arrays for Voltage and Impedance Metrology

We introduce an optimized Josephson double array generating two independent voltages up to 1.5V using only one microwave input. Such an array, combined with the quantum Hall resistance, can provide the basic traceability for an electricity metrology laboratory from DC voltage to capacitance. We describe the array and present preliminary experimental results from DC measurements. Extending the output voltage up to 10V is discussed

A quantum voltmeter for precision AC measurements

An AC quantum voltmeter has been developed for precision AC voltage measurements into the kHz range. A special sub-sampling technique is used such that no information of the AC voltage under test is lost. A comparison of the AC quantum voltmeter and an ultra-stable precision waveform generator has been performed. The Allan deviation analysis demonstrates a voltage output stability of 2 × 10-8 in 1 minute at the level of 2.5 VRMS. The measurements are validated with an AC-DC transfer measurement using a planar multijunction thermal converter.