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IEEE Transactions on Magnetics

Accurate noise measurements of superconducting quasiparticle array mixers

William R. McGrath, Antti V. Räisänen, P. L. Richards, R. Harris, Frances L. Lloyd

Published Mar 1, 1985• 37 citations
Publisher

Abstract

We have constructed a 30-40 GHz test apparatus which allows us to measure the noise temperatures of SIS mixers with an accuracy of better than ±1K. This is a factor of six improvement over earlier measurements. The most accurate measurement made thus far of a mixer which uses a single Pb-alloy junction yielded Tm= 9.2 ± 0.9 K; and mixer gain: Gm= 0.240 ± 0.005. In addition, SIS mixers employing arrays of N = 1, 5, 10, 25, and 50 tunnel junctions in series have been tested. The input power required to saturate the array mixers was found to increase as N2and the gain and noise temperature of the array mixers were independent of N.

DOI
10.1109/tmag.1985.1063708

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We have constructed a 30-40 GHz test apparatus which allows us to measure the noise temperatures of SIS mixers with an accuracy of better than ±1K. This is a factor of six improvement over earlier measurements. The most accurate measurement made thus far of a mixer which uses a single Pb-alloy junction yielded Tm= 9.2 ± 0.9 K; and mixer gain: Gm= 0.240 ± 0.005. In addition, SIS mixers employing arrays of N = 1, 5, 10, 25, and 50 tunnel junctions in series have been tested. The input power required to saturate the array mixers was found to increase as N2and the gain and noise temperature of the array mixers were independent of N.
We have made a systematic experimental study of the performance of millimeter-wave quasiparticle heterodyne mixers which use arrays of SIS tunnel junctions. Sets of arrays with N =1, 5, 10, 25, and 50 junctions in series were fabricated by photolithography. All of the arrays in a given set were made on a single silicon wafer so that their response time parameter omega/sub S/R/sub N/C would be the same. Junction areas were scaled so that the total impedance was the same for each array in a set. Sets of arrays from four wafers with values of omega/sub S/R/sub N/C ranging from 2.6 to 13 were evaluated in mixers at 33 and 36 GHz. These measurements showed that the signal power required to saturate the mixers varies as N/sup 2/ and that the conversion efficiency is nearly independent of N for all values of omega/sub S/R/sub N/C. The mixer noise temperature is independent of N for large values of omega/sub S/R/sub N/C. Therefore, the dynamic range of an SIS quasiparticle mixer can increase in proportion to N/sup 2/. For small values of as omega/sub S/R/sub N/C, however, the mixer noise increases systematically with N. This correlation suggests that the junction capacitance affects the coupling between junctions that can contribute to the noise.
The SIS quasiparticle tunnel junction has been demonstrated to be the most sensitive mixing element for millimeter wave receivers [1,2]. Conversion gain greater than 1 and noise temperature approaching the quantum limit [3,4] has been observed. The required local oscillator power, which can be as small as 10−8W, leads to a low saturation threshold for these mixers [5,6]. The practical benefits, including higher saturation power, of using series arrays are sufficiently important that a substantial number of experiments using arrays have been reported [2,6,7,8,9]. It has not been clear, however, whether there has been any sacrifice in mixer conversion efficiency (gain) or noise. We present a systematic experimental investigation of mixing performance in series arrays of N junctions, where N = 1, 5, 10, 25 and 50.
Experiments on the performance of superconductor-insulator-superconductor (SIS) tunnel junctions as heterodyne mixers at 40 GHz are reported. The techniques developed for the fabrication of multi-junction SIS series arrays differ from, and are considerably simpler than, other methods described in the literature. Results are reported on mixer noise temperature, conversion loss, saturation, local oscillator power levels and mixer dynamic range. Factors affecting the mixer's conversion performance are identified and comparisons are made with other reported results. It is concluded that mixer performance in the present experiments is limited by imperfect impedance matching at the signal frequency, due to relatively large values of parasitic capacitance in the junctions. Future work is aimed at both reducing the junction capacitance and redesigning the mixer mount to give improved r.f. matching.
Accurate measurements of the noise and gain of superconducting-insulating-superconducting (SIS) mixers using small-area (1 mu m/sup 2/) Ta/Ta/sub 2/O/sub 5//Pb/sub 0.9/Bi/sub 0.1/ tunnel junctions were made. The authors measured an added mixer noise of 0.61+or-0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta. Detailed comparison between theoretical predictions of the quantum theory of mixing has been carried out, and noise and gain have been experimentally measured. The shapes of the I-V curves pumped at the upper and lower sideband frequencies were used to deduce values of the embedding admittances at these frequencies. Using these admittances, the mixer noise and gain predicted by quantum theory are in excellent agreement with experiment.