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Conference on Precision Electromagnetic Measurements

Optimized 1 V and 10 V Josephson series arrays

F. Muller, R. Pöpel, J. Kohlmann, J. Niemeyer, W. Meier, Th. Weimann, L. Grimm, F.-W. Dunschede, P. Gutm

Published Dec 23, 2002• 2 citations
Publisher

Abstract

By means of a new fabrication method and detailed design investigations we have been able to optimize the array circuits for the Josephson voltage standard. Even in very long arrays (4800 junctions in one stripline) a yield of 1 mV per junction could be achieved. The microwave power at 70 GHz required for the stable operation of 1 V and 10 V circuits could be reduced down to 1 mW and 10 mW (antenna input), respectively.

DOI
10.1109/cpem.1996.546659

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By means of a new fabrication method and detailed design investigations we have been able to optimize the array circuits for the Josephson voltage standard. Even in very long arrays (4800 junctions in one stripline) a yield of 1 mV per junction could be achieved. The microwave power at 70 GHz required for the stable operation of 1 V and 10 V circuits could be reduced down to 1 mW and 10 mW (antenna input), respectively.
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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.
A new type of microwave circuit for the generation of frequency-scaled voltages is investigated. The circuit is designed as a superconducting microstrip line resonator and series arrays of Josephson tunnel junctions located within the rf current antinodes. The circuits are designed to obtain highly accurate voltages at low rf power level and low frequencies of about 10 GHz. For a resonator circuit with 432 junctions driven at about 10 GHz we have observed steps up to 0.25 V, the lifetime of which is about 20 minutes.>
Programmable Josephson voltage standard (PJVS) circuits were developed that operate at 16 GHz to 20 GHz with operating margins larger than 1 mA. Two circuit designs were demonstrated, each having a total of ~ 300,000 junctions, which were divided into either 16 or 32 sub-arrays. Triple-stacked junctions were used in order to fit the ~300,000 junctions on each circuit. The amorphous NbxSi1-x-barrier junction technology provided a high degree of uniformity of the barrier and junction electrical properties, which was necessary to achieve the 1 mA operating margin. Although the margins on both the 16-array and 32-array circuits were much greater than 1 mA, the circuit yield for the 16-array design was lower because the longer arrays are more sensitive to defects. The use of lumped-element microwave splitters and tapered arrays significantly reduced the microwave input power and increased the operating margins of these designs. In addition, the broadband microwave response of the designs allowed the PJVS output voltage to be continuously adjusted by using the microwave frequency while remaining on margins.
Series arrays of Nb–PdAu–Nb Josephson junctions were fabricated with characteristics ideally suited for application in programmable voltage standards and D/A converters with fundamental accuracy. Large arrays of junctions with applied microwave power showed constant voltage steps with current amplitudes as large as 7 mA. A novel coplanar waveguide design enabled uniform microwave power coupling to a five-segment array of 8192 junctions, so each segment had constant voltage steps over the same bias range. The 8192-junction device generated 1.1 mA steps at 186 mV with 11 GHz power and a maximum constant voltage step of 260 mV at 15.34 GHz.
We have investigated a new type of microwave circuit for the generation of frequency-scaled Josephson voltages useful for the representation of highly accurate reference voltages. The circuit consists of a superconducting microstrip resonator and series arrays of Josephson junctions integrated within its rf current antinodes. The circuit is designed to operate at low frequencies of about 10 GHz to simplify the microwave techniques. At a resonator circuit with four Josephson junctions driven at 10.9 GHz we have obtained stable overlapping Shapiro steps up to 4.6 mV at a step width of about 70 µA. A more advanced double-resonator circuit with altogether 216 junctions allows to generate scaled reference voltages up to 0.16 V at steps of about 25 µA in width and at a microwave frequency of 11.1 GHz.