Instrumentation & Measurement Magazine 24-3 - 18

Digital Sampling Techniques for
AC Josephson Standards at
NRC-Canada
Waldemar G. Kuerten Ihlenfeld

he paper describes research and development (R&D)
work being conducted at the NRC in the field of ac
Metrology to establish a Josephson based system
to provide traceability to the SI to measurements of low frequency ac quantities via digital sampling techniques. Latest
achievements suggest that measurement uncertainties bearing
parts in 107 or much less for frequencies up to 1 kHz are attainable. The ongoing R&D efforts also tackle challenges related
to extending the application of NRC Josephson systems to
frequencies towards the MHz range using special digital sampling techniques aided by custom designed instrumentation.

Josephson Voltage Standards
The quantum Josephson Effect was discovered fifty-eight
years ago [1] and represented a breakthrough in physics, electronics and ultimately instrumentation, since it can be viewed
as a perfect frequency to voltage converter. Interestingly, it became available for ac metrology only rather recently [2], [3]
after diligent R&D efforts from national metrology institutes
(NMIs) of developed countries. Today, Josephson standards
embody the SI unit volt and allow a very accurate characterization of time-varying (ac) signals, a realm formerly dominated
by thermal converters. This capability is of great scientific and
economic importance for any industrial country.
There are two types of Josephson voltage standards (JVS):
the programmable Josephson voltage standard (PJVS) and
the Josephson arbitrary waveform standard (JAWS). The first
works like a conventional digital-to-analog (DAC) converter,
generating quantum accurate step-approximated voltage
waveforms. Each synthesized voltage plateau is an exact multiple of hf/(2e), where h stands for the Planck constant, e the
elemental charge and f the microwave frequency irradiating
the Josephson junctions, all stacked in thousands in a chip
operating at very low temperatures (at ~ 4 K). With the 2019 redefinition of the SI base units, the Josephson constant KJ known
as 2e/h has the exact value of 483,597 GHz/V. The JAWS, also
known as the pulse-driven Josephson arbitrary waveform
synthesizer [4], operates like a sigma-delta DAC, where a
multitude of quantum accurate pulses are fast generated and
18

filtered to produce a continuous waveform of known spectral
content. While a PJVS was designed to cover low frequencies
from dc to nearly about 5 kHz and up to 10 V, a JAWS (Fig. 1)
covers a wider frequency range up to hundreds of kHz [4] but
still has lower output voltage capability.

AC Voltage Measurements with JVS
JVS are exquisitely delicate and sensitive devices displaying
feeble current generation capacity. Therefore, voltage measurements must be made under low load conditions (i.e., high
impedance) to avoid drawing excessive current that could
drive the junctions out of their quantum or superconducting
state, and in consequence, impair or invalidate altogether any
measurement attempt.
JVS' signals are measured either directly or differentially,
as depicted in Fig. 2. For best results, both the JVS and device
under test (DUT) represented by source VAC must have the
same amplitude and signal frequency (derived from an accurate time-base, as from a Cesium clock for the JVS). A single

Fig. 1. The NRC Josephson arbitrary waveform standard/synthesizer (JAWS)
was fabricated by the National Institute for Science and Technology (NIST,
USA). The Dewar in the center contains the chip with the Josephson junctions
under cryogenic temperatures.

IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE

May 2021

Instrumentation & Measurement Magazine 24-3

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