We present a microwave circuit for Josephson voltage standards. Here, the Josephson junctions are integrated in a microwave transmission line designed as coplanar strips (CPS). The new layout offers the possibility of achieving a higher scale of integration and to considerably simplify the fabrication technology. The characteristic impedance of the CPS is about 50 Ω, and this should be of interest for programmable Josephson voltage standard circuits with SNS or SINIS junctions. To demonstrate the function of the microwave circuit design, conventional 10 V Josephson voltage standard circuits with 17 000 Nb/AlOx/Nb junctions were prepared and tested. Stable Shapiro steps at the 10 V level were generated. Furthermore, arrays of 1400 SINIS junctions in this microwave layout exhibited first-order Shapiro steps.

For a more wide-spread use of Josephson voltage standards, cryogen-free operation, by means of an appropriate closed-cycle refrigerator, is highly desirable. In this work we present a low-noise pulse tube cooler (PTC) that is capable to cool the voltage standard circuits to temperatures below 4 K.As a low loss dielectric waveguide with a very small thermal conductivity a Teflon strip was used for the microwave transmission. The matched transition from the WR12 rectangular waveguide to the Teflon strip and vice versa was made using special exponential tapers.Programmable 1 Volt 14 bit SINIS and 10 Volt SIS arrays assembled in the PTC operated well, i.e., demonstrated a similar performance to that measured in liquid helium. The Josephson junction arrays were integrated in coplanar strips instead of the commonly used microstriplines for the microwave transmission lines. The 1 Volt SINIS array contained 8,192 Josephson junctions (JJ's). It generated an inherently stable Josephson voltage step of 1.19 V at a driving microwave frequency of 70 GHz. The step width was 150 A and the critical current of the junctions was 550 A. Similar results were obtained under liquid helium cooling conditions. In another experiment a 10 Volt voltage standard chip with 19,700 SIS JJ's was installed. It was possible to generate stable Shapiro steps at the 10 V level. Index Terms-Cryocooler, Josephson junctions, microwave dielectric waveguide, voltage standard.

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Single nonhysteretic superconductor-metal-superconductor Josephson junctions consisting of Nb-Ti-Nb were prepared using the well-known anodization process for the isolation of the junctions. The 4×4 μm2 junctions show overdamped Josephson current-voltage characteristics at 4.2 K. The characteristic voltage IcRn is about 8.0 μV at a critical current density jc of about 29 kA/cm2. Under the influence of 10.5 GHz microwave irradiation the constant-voltage steps of n=0 and ±1 are observed with current amplitudes of about 3 mA.

Central to quantum-based arbitrary waveform synthesizers is a Josephson junction array driven by short pulses. Its linear dimensions should be small compared to the line wavelength in order to generate an ac voltage with quantum accuracy. Therefore, we use coplanar strip transmission lines to integrate a large number of cross-type SNS Josephson junctions into multiple parallel arrays. First measurements show that a circuit with 1,760 junctions exhibits Shapiro steps over a large frequency range. By using pulse driving the chip generates unipolar voltages up to 6.4 mV at a clock frequency of 3.5 GHz, the maximum frequency of the available pulse pattern generator.

Superconductor -normal metal -superconductor (SNS) junctions a r e well-suited for large-scale integrated superconducting circuits. Potential applications a r e programmable voltage standards, digital-to-analog converters a n d large-scale RSFQ circuits. T h e application of SNS junctions provides some advantages, e.g. a simplified fabrication technology a n d reduced parasitic inductances. But it requires a technological process with dimensions down to the submicrometer region to achieve sufficiently high characteristic voltages IcRN.In our process the normal metal interlayer is titanium. Therefore the sidewall insulation of the junctions can be made by anodization, as it is known from the Nb-AI technology. NbTi-Nb Josephson junctions a n d junction a r r a y s were fabricated down t o the submicrometer range with I,R, values up to 87 microvolt depending on the titanium thickness. Their measured critical currents have a small spread of about 10 percent.Shapiro steps a r e observed under the influence of microwave irradiation. For the application of such junctions in a programmable Josephson voltage standard a new type of microwave circuit using coplanar strips is proposed a n d successfully tested in 10 V SIS circuits.

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