TITLE:
Microwave Design and Performance of PTB 10 V Circuits for the Programmable Josephson Voltage Standard
AUTHORS:
Franz Müller, Thomas J. Scheller, Jinni Lee, Ralf Behr, Luis Palafox, Marco Schubert, Johannes Kohlmann
KEYWORDS:
Josephson Voltage Standard, Programmable Josephson Voltage Standard (PJVS), SNS Josephson Junction, NbxSi1-x Josephson Junction
JOURNAL NAME:
World Journal of Condensed Matter Physics,
Vol.4 No.3,
August
15,
2014
ABSTRACT: At Physikalisch-Technische Bundesanstalt
(PTB), superconducting 10 V circuits for the programmable Josephson voltage standard
(PJVS) are routinely manufactured on the basis of NbxSi1-x barrier junctions. This paper describes in detail the basic design principles for
an operating frequency of 70 GHz. It starts with single junctions, discusses
their insertion into microstriplines and closes with the whole microwave circuit
containing 69,632 NbxSi1-x barrier junctions arranged over
128 microstriplines connected in parallel. The microwave attenuation of this junction
type is a key parameter for the 10 V design and we report its experimental determination.
Special attention has been devoted to subarrays with just a few Josephson junctions
in one of the outermost striplines. The arrangement of these subarrays determines
the optimum performance of the complete 10 V series array. The high performance
of programmable 10 V circuits fabricated at PTB is characterized by measured operating
margins in all subarrays of more than 1 mA centered at the same dc bias current.
The observed modulation of the current margins, when changing the frequency around
70 GHz, is explained by microwave reflections caused by the rf waveguide inside
the cryoprobe. We determine the current margins in dependence of the output power
of a microwave synthesizer and show that 60 mW is sufficient to achieve current
margins larger than 1 mA.