H.‐J. Köhler

Josephson voltage standard at low drive frequencies

The authors report on the design and operation of a superconducting microstrip resonator containing four series-connected highly hysteretic Josephson junctions in the top layer. This arrangement generates constant-voltage steps at X-band drive frequencies useful for a Josephson voltage standard. At 12 GHz, steps up to 5 mV have been obtained with step amplitudes of about 100 mu A. The resonator principle can be used to save microwave power and to simplify the microwave circuit. To obtain sufficiently large and stable steps the critical current density and the junction length must be matched carefully with the order of maximum step used for the operation of the Josephson voltage standard. The X-band microwave drive frequencies offer substantial advantages in terms of costs and handling of microwave equipment.<<ETX>>

Josephson voltage standard microwave circuits operating at 10 GHz

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.<<ETX>>

Josephson voltage standard microwave circuits fabricated in a modified all-niobium technology for operation at low drive frequencies

A resonant type of cryoelectronic microwave circuit for the generation of frequency-scaled Josephson voltages is presented. The circuits consist of superconducting microstripline resonators with series arrays of Josephson tunnel junctions located on the microwave current antinodes. This microwave design was chosen to obtain highly accurate Josephson voltages at low microwave power levels and low drive frequencies. The resonator circuits have been fabricated in both niobium-lead and all-niobium technology. At present, we have observed long-term stable Josephson voltages up to 0.75 V for niobium-lead circuits with 1320 junctions and 0.66 V for all-niobium circuits with 1456 junctions, respectively.

Theoretical investigations on series connections of Josephson junctions

Abstract Based on the Stewart-McCumber model we investigate the current-voltage characteristics of series connections of non-identical Josephson junctions irradiated by a microwave. By means of the first harmonic approximation we derive relations for the mean points and the heights of the steps. The attached stability problem is discussed within the theory of the Mathieu differential equations. As a result of our numerical simulations we get a requirement for the maximum permissible deviations of the junction parameters.