Resonance-free low-pass filters for the AC Josephson voltage standard

We have designed and characterized superconducting integrated circuits for the ac Josephson voltage standard that demonstrate significantly improved performance. The typical circuit consists of an array of superconductor-normal metal-superconductor Josephson junctions, which are placed in a transmission line and biased with a broad-band (dc-to-15 GHz) pulse-drive waveform. Additional low-speed (dc and audio frequency) bias and output leads contain on-chip inductors that act as low-pass filters. The operating margins of the array were improved by adding resistive shunts across these inductors to damp their intrinsic resonances. These resonances had previously degraded the integrity of the broad-band signal driving the array. We present simulations and measurements of these improved circuits that demonstrate no resonances in the range of 0.1-20 GHz. Moreover, the operating margins of the ac Josephson voltage standard were improved.