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In order to better understand the variation mechanism of ozone abundance in the middle atmosphere, the simultaneous monitoring of ozone and other minor molecular species, which are related to ozone depletion, is the most fundamental and critical method. From the 1980s until now, at Nagoya University, a superconductor-insulator-superconductor (SIS) mixer in the 100-200 GHz bands for radio astronomy has been developed and applied to the observations of atmospheric molecules. However, only one or two molecular emission lines are monitored due to the narrow frequency range of the current spectroradiometer. For the simultaneous observation of multiple molecular lines using a spectroradiometer, the development of a highly sensitive receiver with a wide frequency band is critical. In this study, a waveguide-type multiplexer was developed for the expansion of the observation frequency range of a millimeter-wave spectroradiometer, to conduct the simultaneous monitoring of multiple atmospheric molecular lines. With reference to the literature, this paper details the first development of a multiplexer in the 200 GHz band using the waveguide technique for atmospheric observation. In addition, the performance of the multi-band receiving system using the multiplexer at cryogenic temperatures with an SIS mixer was evaluated, as discussed in this paper.

The multiplexer contains a cascaded four-stage sideband-separating filter circuit, which comprises two quadrature hybrid couplers and two BPFs. Eight hybrid couplers were used in the multiplexer, and the waveguide circuits were based on the same design. The frequency characteristics of different numbers of branched lines (n = 2-10) in the hybrid couplers were simulated using HFSS software, and nine branches were employed. Four BPFs were designed, and the pass frequency bands of each stage were as follows: 243-251 GHz for the first stage, 227-235 GHz for the second stage, 197-205 GHz for the third stage, and 181-189 GHz for the fourth stage. This validated the use of the waveguide circuit for a multi-band receiving system based on the frequency multiplexer technique. The designed waveguide multiplexer circuit was fabricated on an aluminum alloy (A6061) split-block component composed of two pieces by milling. The assembled unit had dimensions of 38 mm x 70 mm x 20 mm, and the five waveguide interfaces were UG387 flanges. The machining error was measured using an optical measuring microscope in three dimensions. The errors with respect to the slots of the hybrid couplers and cavity length of the BPFs were within ±4 um.

The characteristics and performances of the multiplexer in a 4-K dewar, under the same conditions employed for atmospheric observation, were measured in the laboratory. Given that the total receiver noise was dependent on the input signal loss of the multiplexer at the front of the SIS mixer, the Trx were measured and compared with respect to the inclusion and exclusion of the multiplexer in the receiver system. As a result, the more significant increase in Trx using the multiplexer, when compared with the SIS mixer, can be attributed to the transmission loss of the waveguide circuit in the multiplexer. The IRR of the receiving system with the multiplexer was measured. CW test signals were generated for the measurement of the gains of a heterodyne receiver at the USB and LSB. The IRRs were measured at the center of each IF band and the values were more than 25 dB. Moreover, the IRRs were significantly higher than those of the previous 2SB mixers, which were 10-20 dB. This indicates that the high and stable IRR performance can be attributed to the waveguide-type multiplexer for the separation of the sideband signals. Finally, a simulation experiment to observe target atmospheric molecular lines using the multiplexer was performed in the laboratory to prove the usefulness of the proposed receiver concept. A simulated signal generation circuit for oscillation and emission of target molecular lines was constructed, and the spectra in each output port of the receiver were measured, and the target spectral lines are clearly found in each sideband. Thus, it was successfully demonstrated that this new receiver concept using the multiplexer works as a multi-band atmospheric spectroradiometer.