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PTB 10 Volt Chip Successfully Tested on the International Level

30.10.1995

Owing to the considerable improvement of the operational conditions in the new clean-room centre of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, the Laboratory for Thin-layer Structures has succeeded in optimizing the circuits for Josephson voltage standards to such a degree that stable d.c. voltages greater than 10 Volt are supplied.

In the most advantageous case, the microwave power necessary for operation at 1 Volt and at 10 Volt is approx. 1 mW and 10 mW, respectively, at the circuit's antenna input and could thus be reduced by about a factor 6 compared with previous versions. The optimum value of 1 mV per Josephson tunnel element was reached even for strings with up to 6000 elements. In practice, the new design allows excellent stability to be achieved with only two strings of 800 elements each for 1 Volt and with four strings with 3000 elements each for 10 Volt.

The number of operative circuits obtained could be clearly increased for 10 volt in particular. One of the best 10 volt circuits has in the interim been successfully tested by the Burau International des Poids et Mesures (BIPM) in Paris. Measurable deviations from the expected voltage value did not occur, the stability of the circuit proved to be as excellent as that of a 1 volt circuit, and the width of the 10 Volt step was greater than 20 µA.

The improved technology applied in designing the circuit arrangement allowed problems connected with the cooling of frozen flux quantums to be surmounted. Owing to the switching to high-melting, stable materials and passivation of sensitive circuit elements by elements less susceptible to faults, it was possible to substantially improve the quality of the tunnel elements.

With these results, the international standard has probably been surpassed without the scope for further optimization being fully used.