Induced Current Effects in Josephson Voltage Standard Circuits
Anna E. Fox, Grace E. Butler, Miranda L. Thompson, Paul D. Dresselhaus, Samuel P. Benz
A study has been performed to understand the effects of magnetic field induced current in Josephson voltage standard circuits. Inductive filtering is an essential component in Josephson voltage standard devices for isolation of the microwave and DC bias signals within the circuit, however currents induced by bias currents flowing in the inductor coil cause undesirable effects in the neighboring junctions. These undesirable effects are amplified as circuits grow in density, with limited die space causing inductors to be placed in close proximity to devices. This study characterizes critical current suppression of a junction array as induced currents are introduced into the circuit under various conditions. Results show that critical current suppression is both a function of inductor magnetic field but also of the coupling to the nearby junction structure . Design variations explore solutions for re-routing or reducing induced currents . A solution providing a shield around the inductors proved to be best for protecting the junction circuit from induced currents. Results from actual Josephson voltage standard devices with shielded low pass filters confirm the technique.