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Applied Physics Letters

Impedance conversion using quantum limit nonreciprocity for superconductor-insulator-superconductor mixer compensation

Stephen R. Whiteley

Published May 1, 1982• 2 citations
Publisher

Abstract

It is shown how a superconductor-insulator-superconductor (SIS) mixer when inductively terminated at the if port has inductive reactance at the signal port. This reactance may be used to compensate for the geometric capacitance of a conventionally operated SIS mixer over twice the bandwidth available through signal port resonance techniques.

DOI
10.1063/1.93249

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It is shown how a superconductor-insulator-superconductor (SIS) mixer when inductively terminated at the if port has inductive reactance at the signal port. This reactance may be used to compensate for the geometric capacitance of a conventionally operated SIS mixer over twice the bandwidth available through signal port resonance techniques.
Superconductor-insulator-superconductor (SIS) junction geometrical capacitance with out of phase current I/sub kk/ impedance component forms sufficient junction reactance X/sub SIS/=( omega C+B/sub Q/)/sup -1/. It is suggested that the way to resonate out both omega C and B/sub Q/ is by using an additional identical SIS junction connected to the first through a long line impedance inverter and RF+DC biased symmetrically to the first. Pumped I-V curves without quantum reactance and frequency impedance patterns of the system are calculated. Calculations demonstrated the presence of high and even negative induced dynamic resistance regions at high-order quasiparticle steps for the case of SIS junction reactance complete compensation. The suggested method may be used in SIS mixers and detectors for a better RF matching.
Virtually all analyses of the superconductor-insulator-superconductor (SIS) quasiparticle mixer have employed the quantum theory of mixing in its three-frequency approximate form. This approximation is valid only in the limit of very large junction capacitance. For finite capacitance, these analyses may be seriously in error. To remedy this, a computer program has been developed which uses the quantum theory of mixing in its most general form, treating the large-signal properties of the mixer in the time domain. The terminating impedances at the harmonics of the local oscillator frequency and their sidebands can be arbitrary. Using this analysis, the effect of finite junction capacitance on one SIS mixer's performance is described. This gives an insight into the range of validity of the three-frequency model.
We have fabricated a superconductor-insulator-superconductor (SIS) mixer using a couple of junctions connected in parallel through a stripline inductor. The junctions have significantly larger area, i.e. larger capacitance, and smaller normal resistance than conventional ones. In order to obtain a good impedance matching between the source and the junctions, an impedance transformer made of a superconducting stripline was integrated with the junctions. The capacitance of the junctions was tuned out by the inductor to obtain a broadband operation without mechanical tuning elements. It was shown that the double sideband noise temperature of the receiver employing this type of mixer device was less than 40 K over the bandwidth of 90-115 GHz. The lowest receiver noise temperature of /spl sim/20 K, which is only 4 times as large as the quantum limited photon noise of hv/k/sub B/, was obtained around 105 GHz.>
We propose an optimization method for a superconductor-insulator-superconductor (SIS) mixer-based amplifier circuit that employs SIS up- and down-converters. Such a novel amplifier configuration could enable superior low-noise performance for future broadband amplifier systems, such as those for large arrays of superconducting sensors or qubits. Optimization was achieved by identifying a solution in which the mutually dependent impedances of the SIS mixers converge under given conditions. A circuit design using this method predicted an amplifier gain of approximately 20 dB and a noise temperature of approximately 2 K at 4 K operation when practical current-voltage characteristics of Nb SIS junctions are used in the simulation. To verify the validity of this method, an amplifier circuit was prepared using waveguide Nb/AlOx/Nb mixer modules. By varying the phase delay between the two SIS mixers, changes in gain were observed, resulting from different convergence conditions of their mutually dependent impedances, which closely matched the predictions made using this method. These results are promising for the development of an integrated circuit including SIS amplifiers and superconducting detector arrays, with bandwidths of 10 GHz or above. This may also enable novel nonreciprocal microwave circuits such as gyrators, isolators, and circulators.
A conventional superconductor-insulator-superconductor (SIS) mixer, that is pumped by a single-frequency local oscillator (LO) functions as a phase-insensitive linear amplifier and may achieve a noise level near the quantum limit of one-half photon added noise per unit bandwidth. A phase-sensitive linear amplifier may have less noise than this quantum limit. To overcome the quantum noise limit with an SIS mixer, a two-LO technique which makes the mixer's gain dependent upon the phase of the incoming signal was used. The phase sensitive gain of this two-LO mixer is experimentally demonstrated. The gain variation from maximum to minimum is more than 20 dB. Theoretical predictions of the noise of the two-LO mixer are presented.
A detailed analysis of the effects of the quantum susceptance on the performance of superconductor-insulator-superconductor (SIS) mixers is performed. It is found that the principal effects of the quantum susceptance are to change the DC bias at which optimum coupling of the signal to the mixer occurs, and to change the output admittance at the IF frequency, thus changing the available gain. It is shown that at the signal port the principal effect is to change the bias voltage at which optimum coupling to the mixer occurs. A more dramatic effect occurs at the IF port, where the quantum susceptance leads to negative values of the output conductance, which causes the available gain to be infinite.