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We present design and evaluations of a submillimeter double-ridged waveguide ortho-mode transducer (OMT) for ALMA Band 8 (385-500 GHz) cartridge receiver. The measured transmission loss of the OMT at 4 K was 0.4-0.5 dB according to noise measurements with an SIS mixer. The polarization isolation was measured to be larger than 29 dB from quasioptical measurements. The OMT consists of a Bfifot junction and a double-ridged guide. A robust design with allowable mechanical errors of 20 μm has been demonstrated.

We have developed a cartridge-type receiver covering the frequency band of 385-500 GHz as a qualification model of Atacama Large Millimeter/submillimeter Array (ALMA) band 8. It receives two orthogonal polarizations and down-converts the sideband-separated signals to intermediate frequencies (IF) of between 4 and 8 GHz. The cartridge-type receiver consists of cold optics, two feed horns, a wire grid, mirrors, two sideband-separating SIS mixers, cryogenic multipliers of a local oscillator (LO), cryogenic and warm IF amplifiers, a cartridge body, and sensors/wirings. These components were individually tested, and then the cartridge was integrated and tested as a complete assembly. We have also developed equipment for efficiency tests of both the components and the integrated receiver. The single sideband (SSB) noise temperature of this receiver is 100 K at the band center and 300 K at the band edges. The beam pattern and cross-polarization pattern are consistent with a physical optical calculation. The amplitude stability is around $3 \times 10^{-4}$ in 1 s. The phase stability is less than 2.0D on a time scale of 0.1 s to 10 min. These results are promising for a receiver in the ALMA.

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In this paper, we report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500 GHz) receivers. Nb-based SIS junctions of a current density of 10 kA cm 2 and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA receiver specification, which requires wide frequency coverage as well as low noise temperature. Parallel-connected twin junctions (PCTJ) are designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe's feed point, thereby making it much easier to match the low-impedance PCTJ over a wide frequency band. In addition, a superconducting magnet fixed onto the compact mixer block to provide efficient magnetic field coupling is designed. The SIS mixer demonstrates a minimum double-sideband receiver noise temperature of 108 K at the band center and temperatures of less than 167 K over the whole band (for an intermediate-frequency range of 4-8 GHz).Index Terms-Atacama large millimeter/submillimeter array, noise temperature, SIS mixer, wideband performance.

Band10 of the Atacama Large Millimeter Array (ALMA) is a planned heterodyne receiver covering the frequency range 790 GHz-950 GHz. The waveguide mixers are superconductor insulator superconductor (SIS) Nb Al AlO Nb junctions integrated with NbTiN SiO 2 Al tuning circuits. Our junction definition process has been greatly simplified by introducing an inductively-coupled-plasma (ICP) etcher to the fabrication. The etching process, which employs a low-pressure CF 4 O 2 plasma, allows to remove the entire Nb Al AlO Nb tri-layer in a single step. SIS mixers with current densities up to 10 kA cm 2 and quality factor above 15 have been fabricated with good reproducibility. Minimum receiver noise temperature, corrected for optical losses, is 210 K at 860 GHz. With the goal to further improve the performance, optimization of circuit design and fabrication technology are discussed.Index Terms-ICP etching, SIS mixer, submillimeter wave receiver.

We have developed a 400-500 GHz low-noise balanced SIS (Superconductor Insulator Superconductor) mixer, which is based on a waveguide RF quadrature hybrid coupler. The RF quadrature hybrid was designed and fabricated as a broadband hybrid with good performance at 4 K. The fabricated RF quadrature hybrid was measured at room temperature with a submillimeter vector network analyzer to check amplitude and phase imbalance between two output ports. Then the balanced mixer was assembled with the RF hybrid, two DSB mixers, and a 180 • IF hybrid. Several important parameters such as noise temperature, LO power reduction, and IF spectra were measured. The LO power reduction is defined as how much LO power the balanced mixer saves compared with a typical single-ended mixer. The measured noise temperature of the balanced mixer was ∼ 55 K at the band center which corresponds to ∼ 3 times the quantum noise limit (hf/k) in DSB, and ∼ 120 K at the band edges. The noise performance over LO frequency was almost the same as that of the worse DSB mixer used in the balanced mixer. In addition the LO power required for the balanced mixer is ∼ 11 dB less than that of the single-ended mixers.