Kecheng Xiao

A Low-Noise 230 GHz SIS Receiver for Radio Astronomy Employing Untuned Nb/AlOx/Nb Tunnel Junctions

A heterodyne receiver based on a ∼1/3 reduced height rectangular waveguide SIS mixer with two mechanical tuners has been built for astronomical observations of molecular transitions in the 230 GHz frequency band. The mixer used an untuned array (ωRnCj≈3, Rn≈70 Ω) of four Nb/AIOx/Nb tunnel junctions in series as a nonlinear mixing element. A reasonable balance between the input and output coupling efficiencies has been obtained by choosing the junction number N=4. The receiver exhibits DSB (Double Side Band) noise temperature around 50 K over a frequency range of more than 10 GHz centered at 230 GHz. The lowest system noise temperature of 38 K has been recorded at 232.5 GHz. Mainly by adjusting the subwaveguide backshort, the SSB (Single Side Band) operation with image rejection of ≥ 15 dB is obtained with the noise temperature as low as 50 K. In addition, the noise contribution from each receiver component has been studied further. The minimum SIS mixer noise temperature is estimated as 15 K, pretty close to the quantum limit ħv/k∼11 K at 230 GHz. It is believed that the receiver noise temperatures presented are the lowest yet reported for a 230 GHz receiver using untuned junctions.

A 230 GHz low noise subharmonically pumped SIS mixer

A 230 GHz Low Noise Subharmonically Pumped (SHP) SIS mixer has been developed and tested at 115 GHz LO. The mixer has a two backshorts waveguide structure and employs an array of four 1.7 /spl mu/m/sup 2/ Nb/AlO/sub x//Nb SIS junctions in series with /spl omega/RnC/spl sim/3 at 230 GHz. Harmonic quantum mixing occurred over the whole experimental frequency range of 208-234 GHz (LO:114-117 GHz), with the corresponding DSB noise temperatures of lower than 150 K and a lowest value of 75 K at /spl sim/230 GHz. The present noise results are believed to be the lowest yet reported for a SHP mixer.

Ground-based millimeterwave instrument for measurement of stratospheric ClO using a superconductive (SIS) receiver

Abstract In 1997, Nagoya University and the National Institute for Environmental Studies (NIES) started a joint research project to develop a high-sensitivity ground-based millimeter-wave receiver capable of detecting thermal emission from the rotational lines of chlorine monoxide (ClO) and other constituents in the stratosphere. We have already constructed ozone sensors at 110 GHz employing an SIS receiver. In this project, we shall develop a 200–300 GHz SIS receiver system. The receiver noise temperature at 230 GHz is expected to be better than 50 K in single sideband. In 1999, we will install this instrument at Las Campanas Observatory at an altitude of ∼2400 m (29°S, 71°W) in Chile. This project is a first dedicated effort to measure O 3 , ClO and other constituents in South America where the polar vortex in the Antarctic sometimes tends to move away from the South Pole, and this study should be able to provide valuable information on their behavior, giving us a better understanding of the mechanism of O 3 depletion in the mid-latitude region of southern hemisphere.

An experimental study of harmonic SIS mixing at 205-235 GHz

A 230-GHz subharmonically pumped waveguide mixer employing superconducting tunnel junctions has been developed. We present, in this paper, an experimental study of harmonic superconductor-insulator-superconductor (SIS) mixing at 230 GHz, focusing mainly on its noise behavior. The mixer has a double-tuned waveguide structure and employs an array of four 1.7-/spl mu/m/sup 2/ Nb-AlOx-Nb SIS junctions in series, with /spl omega/R/sub n/C/sub j//spl sim/3 at 230 GHz. Harmonic quantum mixing occurred over an experimental frequency range of 205-235 GHz (local oscillator: 112.5-117.5 GHz), exhibiting corresponding double sideband noise temperatures of lower than 150 K, with a lowest value of 75 K at /spl sim/230 GHz. The measured mixer noise is believed to be the lowest yet reported for a mixer using subharmonic-pump configuration at this frequency. A phenomenon that we attribute to the third harmonic SIS mixing has also been observed.

A Novel LO Power Injection Method for SIS Mixers

We propose a novel LO power injection method developed for SIS mixers in this paper. Based on the feature of extremely small LO power requirements of SIS quantum mixing, the new method fulfills SIS pumping through a DC/IF route based built-in LO path, which is composed of an additional LO waveguide and the existing microstrip choke filter on the junction substrate. With the new method, traditional external LO diplexers(e.g., crossguide-couplers or beamsplitters) become unnecessary, resulting in a lower loss, compact, and stable receiver system. Experiments at 110- and 230 GHz bands have shown that the present method is efficient in coupling sufficient pumping power to SIS junctions from general LO sources, and the receiver sensitivities have a further improvement of about 10 K. We expect this method is also able to be applied into submillimeter wave band for SIS mixers.