Frequency Conversion Using Weak Multiply Connected Josephson Junctions

Abstract

A Josephson tunneling device essentially consisting of a lead-tin solder bead encircling a 5-mil niobium wire has been used to produce some frequency conversion effects. Thin spots in the oxide coating of the niobium wire give rise to two or more parallel weak-contact regions between the solder and the niobium. A typical voltage vs junction current plot displays an antisymmetric tunneling curve with zero voltage across the junction until a certain critical current Ijc is supplied. A field current IH flowing through the niobium wire produces a magnetic flux which links the multiply connected region(s) formed by the penetration depths of the solder and the niobium and the distance(s) between the parallel weak contacts. The critical current Ijc is a periodic function of the magnetic flux threading the multiply connected region(s), the modulation period being proportional to one flux quantum. When the device is biased with a junction current Ij in the interval Ijc≤Ij≤3Ijc, a modulated dc voltage appears across the junction as IH is varied. The period of the oscillations is independent of the junction bias Ij. The number of cycles ΔN produced for a given absolute change | ΔIH | defines the junction constant Kj of the device. Kj depends upon the geometry of the device and the penetration depths of the superconductors used in its construction. If a time varying IH is applied to the niobium wire an output frequency f0 can be obtained, given by the relation f0=Kj | dIH/dt |. For a triangular IH the device acts as a frequency multiplier giving f0=4KjIHm fin, where IHm is the current amplitude of the triangular waveform and fin its frequency. A sinusoidal IH gives rise to a frequency-modulated output: f0=2πKjIHm fin | cos2πfint |. Other input waveforms yield other frequency-modulated outputs.