Tac's Page

The 45-m radio telescope in the Nobeyama Radio Observatory (NRO) is one of the largest millimeter-wave telescopes in the world. Many observational studies have been conducted using this telescope, including investigations of star formation, galaxy evolution, and interstellar chemistry. The NRO 45-m telescope has been employed in a wide range of research fields and it is clear that it is an important instrument at present. Several receivers have been developed and operated at the NRO in the millimeter band, especially at λ~3 mm, which is the highest frequency range in the NRO 45-m telescope.

Over this 10 years, several new receivers were developed with novel waveguide-type sideband-separating (2SB) mixer technology and they entered open-use operation in the 45-m telescope, and the SIS mixer device in the 100-GHz band was a parallel-connected twin-junction (PCTJ) type device. The PCTJ circuit comprises two identical SIS junctions and a stripline inductor to tune out the capacitance of the junctions. However, we found that the mixer with the PCTJ device in the 100-GHz band on the 45-m telescope exhibited severe gain saturation when connected to a load of 273 K during chopper-wheel calibration.

In this study, we developed a series-connected array of SIS junction device in the 100-GHz band DSB mixer and it was installed in the multi-beam receiver FOREST on the NRO 45-m telescope. An array junction mixer (N > 3) was designed in order to reduce the gain compression under hot load temperatures compared with the existing PCTJ type mixer.

The impedance transformer from the feed point to the junction array employed a CLCPW, which comprised two sections with high (~90 Ω) and low (~10 Ω) characteristic impedance transmission lines. The structure of this tuning line is simple and easy to fabricate, and the RF impedance matched in a wide frequency range. The RF/LO coupling efficiency was more than 92% and the expected receiver noise temperature was approximately two times the quantum limit, at least from 75 GHz to 125 GHz.

We fabricated series-connected array devices where N = 3-6 and measured their performance in the laboratory. As a result, we successfully developed suitable array junction devices with ~15-30 K of Trx(DSB) in the range of 80-120 GHz. The minimum value was fairly similar to the performance of the PCTJ device, but the RF frequency range was significantly wider than that for the existing device. We considered the effective length extension at the edge of planar circuit, but the impact of this effect on the noise performance was not clearly confirmed. Our results indicated that this effect possibly had a small impact on the device in the millimeter wavelength range, or it may have been too small to measure with our measurement system.

The gain compression values were measured as 21.2±1.2% and 10.6±0.7% for the N = 3 and N = 5 mixers, respectively, at a load of 273 K in the DSB mode. Thus, the equivalent gain compression with the N = 5 device using a 2SB mixer was expected to be approximately 6%, which is sufficient to meet the requirements for open-use with FOREST on the 45-m telescope.

Finally, the newly developed array junction mixer was installed in the FOREST receiver and we successfully obtained the 12CO (J = 1-0) molecular line toward IRC+10216 with the NRO 45-m telescope.