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IEEE Transactions on Microwave Theory and Techniques

Performance of a two-junction array SIS-mixer operating around 345 GHz

Cornelia E. Honingh, Gert J. de Lange, M. M. T. M. Dierichs, H. Schaeffer, Th. de Graauw, T. M. Klapwijk

Published Apr 1, 1993• 14 citations
Publisher

Abstract

The authors have made a detailed study of the gain and noise of a SIS (superconductor-insulator-superconductor) heterodyne receiver at 345 GHz. An array of two Nb-Al/sub 2/O/sub 3/-Nb SIS junctions in series are used as the mixing element. The array is operated in a waveguide mount with a backshort and an E-plane tuner. The best receiver noise temperature achieved is 140 K DSB (double sideband). The embedding impedances were determined by fitting theory to the measured pumped curves. High-quality fits were obtained, providing the first detailed test of the Tucker-theory at frequencies above 300 GHz. The impedances found by this method are in very good agreement with impedances measured in a scale model at 3.3 GHz. From these embedding impedances, the gain and noise of the mixer were calculated over a full bias range using the Tucker theory in the three-port low-IF approximation. The measured dependence of mixer gain and noise on bias voltage, pump power and embedding impedance is in good agreement with theory. However the absolute values show discrepancies that appear to be independent of the bias parameters of the mixer.>

DOI
10.1109/22.231655

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IEEE Transactions on Magnetics•Mar 1, 1991•21 citations
The authors have made a detailed study of the gain and noise of a SIS (superconductor-insulator-superconductor) heterodyne receiver at 345 GHz. An array of two Nb-Al/sub 2/O/sub 3/-Nb SIS junctions in series are used as the mixing element. The array is operated in a waveguide mount with a backshort and an E-plane tuner. The best receiver noise temperature achieved is 140 K DSB (double sideband). The embedding impedances were determined by fitting theory to the measured pumped curves. High-quality fits were obtained, providing the first detailed test of the Tucker-theory at frequencies above 300 GHz. The impedances found by this method are in very good agreement with impedances measured in a scale model at 3.3 GHz. From these embedding impedances, the gain and noise of the mixer were calculated over a full bias range using the Tucker theory in the three-port low-IF approximation. The measured dependence of mixer gain and noise on bias voltage, pump power and embedding impedance is in good agreement with theory. However the absolute values show discrepancies that appear to be independent of the bias parameters of the mixer.>
The noise and gain of a heterodyne waveguide mixer using Nb/Al/sub 2/O/sub 3//Nb superconducting tunnel junctions were measured in the 400-500-GHz frequency range. Three different arrays of two junctions in series are analyzed. The minimum receiver noise temperature is 120 K DSB at 480 GHz, measured with an array having integrated tuning stubs. The authors compare data of the pumped I-V curves with the Werthammer-Tucker theory and demonstrate an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs. a mixer noise temperature of 90+or-30 K and a DSB mixer gain of -12.5+or-0.6 dB were measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows good qualitative agreement, indicating the applicability of this theory to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage is 220 K at 495 GHz. This frequency is 85% of the reduced gap frequency, indicating that Nb superconductor insulator-superconductor (SIS) mixers can be used up at least the gap frequency of 680 GHz.>
The measured gain and noise of a SIS waveguide mixer at 345 GHz have been compared with theoretical values, calculated from the quantum mixer theory using a three port model. As a mixing element, we use a series array of two Nb-Al2O3-Nb SIS junctions. The area of each junction is 0.8 sq microns and the normal state resistance is 52 omega. The embedding impedance of the mixer has been determined from the pumped DC-IV curves of the junction and is compared to results from scale model measurements (105 x). Good agreement was obtained. The measured mixer gain, however, is a factor of 0.45 plus or minus 0.5 lower than the theoretical predicted gain. The measured mixer noise temperature is a factor of 4-5 higher than the calculated one. These discrepancies are independent on pump power and are valid for a broad range of tuning conditions.
The noise and gain of a heterodyne waveguide mixer employing Nb/A120JNb superconducting tunnel junc- tions is measured in the 400-500 GHz frequency range. Three different arrays of two junctions in series are analyzed. An array with integrated tuning stubs has lowest receiver noise temperatures of T, = 120 K DSB at 480 GHz, which is to our knowledge the lowest value reported in this frequency range. We compare data of the pumped I-V curves with the Wert- hamer-Tucker theory and demonstrate for the first time an excellent agreement at frequencies up to 500 GHz. For an array without integrated tuning stubs, a mixer noise tempe- rature T- = 90 f 30 K and a DSB mixer gain G-= -12.5 f 0.6 dB is measured. A comparison of the measured gain versus bias voltage with the quantum theory of mixing shows a good qualitative agreement, indicating the applicability of this theory, to Nb tunnel junctions up to 500 GHz. The noise temperature of an array with a lower gap voltage, is 220 K at 495 GHz. This frequency is 85 % of the reduced gap frequen- cy, indicating that Nb SIS mixers can be used up to at least the gap frequency of 680 GHz. I. INTRODUCTION
The development of ultra-low-noise receivers at frequencies of >200 GHz is primarily driven by astronomical applications. The most sensitive millimeter-wave receivers that have presently been constructed make use of the strong mixing afforded by superconductor-insulator-superconductor (SIS) tunnel junctions. In a collaborative project between UKC and RAL, a 230 GHz laboratory receiver has been built using a lead alloy SIS junction as the mixing element, mounted in 4:1 reduced-height waveguide, terminated at one end with a circular cross-section, adjustable backshort. Best results indicate a double-sideband receiver noise temperature of T/sub R/(DSB) approximately=120 K, which represents state-of-the-art performance at this frequency. Initial difficulties in achieving good mixing performance prompted an investigation into the importance of the 'quality' of mixer block backshort tuning elements. An analysis of the mixer equivalent circuit is presented which allows for loss in a non-ideal mixer block, and 3-port mixer model calculations are discussed which incorporate this analysis into the Tucker quantum theory of mixing.
A heterodyne receiver using an SIS (superconductor-insulator-superconductor) waveguide mixer with two mechanical tuners has been built and characterized over the frequency range of 460 GHz to 630 GHz. This receiver is designed for radioastronomy applications. The mixer uses high-current-density, 0.25- mu m/sup 2/, Nb/AlO/sub x//Nb SIS tunnel junctions with integrated superconductive RF circuits to tune the junction capacitance. A DSB (double sideband) receiver noise temperature as low as 200+or-17 K has been obtained at 540 GHz. In addition, negative differential resistance has been observed in the DC I-V (current-voltage) curve at 487-491 GHz.>
The possibility of overcoming the main drawbacks of the traditional series arrays by parallel connection of the SIS (superconductor-insulator-superconductor) junctions for DC and IF are demonstrated experimentally. The proposed mixing elements avoid the influence of the junction nonuniformity in the array, provide the optimal matching on both RF input and IF output, and tune out junction capacitance. Experimentally tested arrays contained up to eight high-quality Nb-AlO/sub x/-Nb junctions with R/sub n/=20-70 Omega at junction area 8/14 mu m/sup 2/. The best values of T/sub r/ (SSB) were measured to as low as 5 K on the mixer cold input flange in the 45-55-GHz bandwidth at an ambient temperature of 4.2 K. A mixer gain of up to 10 dB was obtained. The gain values were limited by bias stability at very high or even negative array output impedance. An experimental receiver noise temperature (SSB) of less than 20 K has been achieved.