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Sideband Separating Mixer for atmospheric window 790-950 GHz

Andrey V. Khudchenko

Published Jun 6, 2019• 0 citations
Publisher

Abstract

We have developed a Sideband Separating (2SB) Mixer based on superconductor-insulator-superconductor (SIS) junctions for frequency range 790-950 GHz. In the first test this mixer demonstrated the noise temperature from 450 to 900 K and the image rejection ratio above 15dB in 95% of the band. The SIS mixers used here have DSB noise temperature form 220 to 400 K. These results show that RF waveguide hybrid structure for such a high frequencies can be manufactured with losses level below 0.8 dB. It makes attractive possible ALMA upgrade of Band 10 receivers to 2SB option, because the sensitivity for spectral lines observations can be improved by about 30% by using 2SB mixers instead of DSB ones.

DOI
10.5281/zenodo.3240358

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•Sep 1, 2009•1 citation
We have developed a Sideband Separating (2SB) Mixer based on superconductor-insulator-superconductor (SIS) junctions for frequency range 790-950 GHz. In the first test this mixer demonstrated the noise temperature from 450 to 900 K and the image rejection ratio above 15dB in 95% of the band. The SIS mixers used here have DSB noise temperature form 220 to 400 K. These results show that RF waveguide hybrid structure for such a high frequencies can be manufactured with losses level below 0.8 dB. It makes attractive possible ALMA upgrade of Band 10 receivers to 2SB option, because the sensitivity for spectral lines observations can be improved by about 30% by using 2SB mixers instead of DSB ones.
We present the design and results of characterization of a new sideband separating (2SB) mixer for 800-950 GHz, based on superconductor-insulator-superconductor (SIS) junctions. This is the first waveguide 2SB SIS mixer demonstrated at such a high frequency. The design is following the classical quadrature hybrid architecture, meanwhile additional attention was put on the reduction of reflections in the RF structure in order to minimize the RF imbalance, to achieve a high image rejection ratio (IRR). The RF waveguide block was manufactured by micromilling and populated by single-ended SIS mixers developed earlier for upgrade of the CHAMP+ high-band array on the APEX telescope. These SIS mixers have double-sideband (DSB) noise temperatures from 210 to 400 K. The assembled 2SB mixer yields a SSB noise temperature from 450 to 900 K, with an IRR above 15 dB in 95% of the band. Comparing the DSB and the SSB sensitivities, we find that the waveguide losses are as low as expected and do not exceed 0.6 dB. The presented mixer is a prototype for use in a 2SB dual polarization receiver planned for deployment on the APEX telescope.
The Atacama Large Millimeter Array (ALMA) is the largest radio astronomical enterprise ever proposed. When completed, each of its 64 constituting radio-telescopes will be able to hold 10 heterodyne receivers covering the spectroscopic windows allowed by the atmospheric transmission at the construction site, the altiplanos of the northern Chilean Andes. In contrast to the sideband-separating (2SB) receivers being developed at low frequencies, double-side-band (DSB) receivers are being developed for the highest two spectroscopic windows (bands 9 and 10). Despite of the well known advantages of 2SB mixers over their DSB counterparts, they have not been implemented at the highest-frequency bands as the involved dimensions for some of the radio frequency components are prohibitory small. However, the current state-of-the-art micromachining technology has proved that the structures necessary for this development are attainable. Here we report the design, modeling, realization, and characterization of a 2SB mixer for band 9 of ALMA (600 to 720 GHz). At the heart of the mixer, two superconductor-insulator-superconductor (SIS) junctions are used as mixing elements. The constructed instrument presents an excellent performance as shown by two important figures of merit: noise temperature of the system and side band ratio, both of them within ALMA specifications.
A modular sideband-separating (2SB) receiver for 600-720 GHz has been built and tested. The used modular design allows to characterize all the parts separately, including testing of the superconductor-insulator-superconductor (SIS) junctions individually in a double-sideband mode before building them in to the 2SB assembly. The developed 2SB mixer has a single sideband noise temperature below 380 K in the entire operating band and reaches a level of 200 K in the best point. At the same time, the image rejection ratio (IRR) was demonstrated to be better than 11.5 dB over the entire band. However, we have found a discrepancy between the observed and expected performance. To investigate this problem, we have developed and applied methods to characterize RF and intermediate frequency imbalances of a fully assembled 2SB mixer using the SIS junction properties. As result, the IRR of our mixer was found to be limited by the RF imbalance, which is caused by complex standing waves created by reflections from the SIS junctions, the RF hybrid and the RF absorption load.
Abstract We have designed and mass-produced low-noise sideband separating superconductor–insulator–superconductor (SIS) mixers for the Atacama Large Millimeter/submillimeter Array (ALMA) band 4 over the frequency range of 125–163 GHz. An integrated design was adopted for the band 4 sideband separating (2SB) mixer block because of the advantages it offers in terms of compactness, reduced testing time and lower cost. The mixer chip was designed to be robust for handling errors to avoid performance degradation caused by generation of the higher order mode in the mixer chip slot. Detailed analyses revealed its robustness and ability to ensure mass production of the 2SB mixers. Using the robust mixer design in addition to well-established waveguide technologies, all of the 2SB mixers met ALMA specifications for noise temperature and image rejection ratio.
The Band3 ALMA front end receiver incorporates sideband-separating (2SB) superconductor-insulator-superconductor (SIS) RF hybrid assemblies to achieve low noise temperatures. The 2SB RF hybrid assembly consists of a feedhorn to direct the RF signals, RF and IF quadrature hybrids for separating the USB and LSB signals, and two SIS mixers to down convert the 100 GHz RF signals to 4-8 GHz IF signals. These assemblies require careful matching of the phase and amplitude of the signals in the USB and LSB pathways to minimize receiver noise. The individual components undergo careful characterization and tests to build suitable 2SB RF hybrids assemblies.
This licentiate is focused on the design and development of Superconductor-Insulator-Superconductor (SIS) junction mixer for ALMA Band 5, one of the ten frequency bands of the Atacama Large Millimeter Array (ALMA) project. ALMA Band 5 is a dual polarisation sideband separation heterodyne receiver covering radio frequencies (RF) from 163 GHz to 211 GHz, with 4-8 GHz intermediate frequency (IF). Amongst the other ALMA bands, Band 5 is the lowest frequency band, which uses all-cold optics. That puts strong constrains on the dimensions of all the receiver components. For the SIS mixer, we employ a MMIC-like approach where most of the DSB mixer components, including LO injection circuitry with novel directional coupler, are integrated on the same crystal quartz substrate. The mixer uses a lumped filter technique to extract the IF frequencies, which is non-traditional for SIS mixers. A custom made superconducting Lange coupler with suspended air bridges to connect the fingers of Lange coupler is used. Experimental measurements of the double sideband mixer show state of the art performance with the receiver noise temperature below 30 Kelvin across the entire band. The noise temperature for sideband separation mixer is below 60 Kelvin with sideband rejection better than 12 dB over most of the band.