Instrumentation & Measurement Magazine 23-2 - 8

mini ion pump for pumping of this cryostat is used.
The array is irradiated
by a compact 68-78 GHz
synthesizer.
The measured relative
standard type A uncertainties between output voltages
of N4-21 and the primary
state voltage standard of
Russia is only a few parts in
108. Such low uncertainties
are sufficient for the application of voltage standard
N4-21 in the highest-level
metrology. The implementation of the link between
JJs and Zener diodes largely
eliminates the uncertainty
contributions associated
with Zeners' nonlinear drift,
environmental effects, and
the impact due to shipping.
Fig. 2. The current-voltage characteristics of the first JJ array of 620 junctions at 79.2 K: (a) without and (b) with the
external electromagnetic wave irradiation at a frequency of 77.465 GHz. Enlarged portions of the I-V curves demonstrate
the amplitude and the steepness of the step with 2 nV resolution at voltage of about 0.1 V.

high-temperature superconductor bicrystal JJs with dry nitrogen temperature of about 77 K and Zener diodes is shown in
Fig. 3 [39].
It has output voltages from −10 to 10 V in steps of 0.1 V and
the nominal voltages ±1.025 V. The voltage standard N4-21 includes four units: cryocooler unit; measuring unit; control and
power supply unit; and laptop. The required output voltage of
the N4-21 is formed by the precision dividers, which are automatically calibrated against VJ ≈ 25 mV. The turnkey standard
N4-21 is powered by 220 V or 110 V ac voltages. The HTS JJs array is located in a cryostat and cooled by a small cryocooler. A

Fig. 3. Photo of the cryocooler-based table top Josephson voltage standard
N4-21: 1- Cryocooler unit, 2- Measuring unit, 3- Control and power supply unit
and 4- Laptop [39] (IEEE ©2019, used with permission).
8	

Future Applications
of Bicrystal Junctions

As it was mentioned, the
PJVS uses LTS niobium
junction arrays with three constant voltage steps, n = 0, ±1
[17]. The array for an m-bit PJVS was divided into m sections
containing the JJs sequence equal to 1, 2, 4, 8, ..., 2m−1. The total number of junctions in the array was N=2m-1. By applying
bias currents to the appropriate set of sections, arbitrary voltages with amplitudes not exceeding in steps of Nf/KJ were
obtained.
In contrast to the niobium arrays, the HTS arrays demonstrate only two quantum steps, n = ±1. The utilization of two
instead of three quantum steps requires a new, slightly different series array circuit design [38], [40]. In this case the N=2m
junctions in the array are divided into m + 1 segments (bits).
The number of junctions in the first m segments, including the
most significant bit (MSB) of the array, corresponds to the binary code (1, 2, 4, 8...). The last (m+1) segment consists of only
one junction. Zero voltage can be achieved if the MSB is biased
oppositely to all of the remaining segments. By changing the
code of the first bits, one can get any desired voltage in steps of
2Nf/KJ. The sign of the output voltage is defined by the MSB,
and the additional bit has the opposite sign. It is important to
note that, in this case, the maximal step amplitude can be equal
to ΔI1max = 1.16Ic, in contrast to the case with three stable states
in which ΔI1max = 0.54Ic.

The Promise of High-Tc Josephson
Junctions
In high-Tc compounds, the superconducting order parameter
which describes the fundamental properties of the quantum

IEEE Instrumentation & Measurement Magazine	

April 2020



Instrumentation & Measurement Magazine 23-2

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