Oral Presentation

Demonstration of Digital Sideband Separation with a Wideband Receiver at 210–360 GHz for the FINER Project

Author(s): Masato Kato (NagoyaU), Akio Taniguchi (KIT), Masato Hagimoto, Yoichi Tamura, Kianhong Lee (NagoyaU), Haoran Kang, Takafumi Kojima, Shun Ishi, Ryohei Kawabe (NAOJ), Takeshi Sakai (UEC), Taku Nakajima (Suwa University of Science), and the FINER team

Presenter: Masato Kato (Nagoya University)

Far-infrared nebular emission lines (e.g., [O III] 88 µm, [C II] 158 µm) are powerful probes of galaxy formation at the epoch of reionization. Recent JWST and Euclid explorations have revealed many high-redshift galaxy candidates, but spectroscopic observations with ALMA have been limited to the southern hemisphere, leaving those in the northern hemisphere largely unexplored. Far Infrared Nebular Emission Receiver (FINER, Tamura et al. 2024) for the Large Millimeter Telescope (LMT) in Mexico is a 120–360 GHz SIS receiver system comprising ALMA’s Band 4+5 and 6+7, and using 10.24 GHz wideband digital spectrometers (DRS4, Hagimoto et al. 2024). Leveraging the LMT’s large collecting area and good atmospheric conditions, LMT-FINER will offer an efficient redshift search capability comparable to ALMA. For single-dish spectroscopy in sub/millimeter bands, where atmospheric transmission rapidly varies with frequency, achieving a high sideband rejection ratio (SRR ~ 25 dB) is a major challenge, as it is necessary to remove atmospheric noise that leaks from the image sideband. Amplitude and phase imbalances in the typical hybrid analog circuits limit SRR to ~ 10–15 dB. Thus, digital sideband separation (DSBS, Morgan & Fisher 2010) that digitally compensates for the imbalances is a key technology to improve SRR. However, its practical application of DSBS over a wide bandwidth and its long-term stability have remained to be fully demonstrated.
Here, we report the DSBS performance evaluation of DRS4 combined with FINER’s Band 6+7 receiver. DSBS digitally recombines USB and LSB signals to cancel leakage, determining the complex gains per frequency channel before an observation is essential. By using a signal generator to create reference monochromatic signals and measuring their leakage, we estimate gains. We achieve SRR > 20 dB over 35 GHz (210–227 and 233–250 GHz with a 230 GHz LO), which corresponds to a 5 to 20 dB improvement over performance without DSBS. We also confirm the estimated gains remain stable for 8 hours, suggesting the recalibration per night. These findings demonstrate that DSBS provides practical SRR and stability required for redshift surveys with FINER’s Band 6+7 receiver.