Development of the SIS Heterodyne Mixer

45-m telescope
 For spectral-line observations, it is important to operate the heterodyne mixer receiver in the single-sideband (SSB) mode, while rejecting power from the image sideband, because atmospheric noise coming from the image sideband increases the system noise temperature significantly. In the case of mixer receivers operated in the double-sideband (DSB) mode, it is difficult to perform an accurate intensity calibration using the “chopper wheel” method, because it is difficult to achieve precise measurement of the image rejection ratio (IRR). Most telescopes, therefore, adopt one of the following techniques to operate the mixer receivers in the SSB mode: (1) a mechanically tunable interferometer as an image-rejection SSB filter, (2) a mixer block equipped with backshort tuners, and (3) a sideband-separating (2SB) method by quasi-optics usinga wire grid, two focusing mirrors, and two mixer horns. Each technique requires quasi-optics and/or tuning mechanics, which makes the gain of the receivers unstable, and the loss in the optics increases the total noise temperature of the receiver systems. Moreover, each SSB method, except for 2SB, can access only one sideband simultaneously, i.e., the upper sideband (USB) or the lower sideband (LSB), which is a well-known feature of the heterodyne mixer receivers in the SSB mode.

 In order to improve such disadvantages of the heterodyne mixer receivers, we have been developing a 2SB SIS mixer at 200 GHz based on waveguide split block. The RF/LO coupler is the same as the scaled model described by Asayama et al. (2004). The measured SSB receiver noise temperatures in the intermediate frequency (IF) range of 4-8 GHz (fc = 6 GHz ) are less than 100 K in the local oscilator (LO) frequency range of 225-231 GHz, and minimum value of around 70 K is achieved at 225 GHz. The IRRs are more than 10 dB in the frequency range of 220-230 GHz. We have installed the sideband-separating SIS mixer into 60 cm radio telescope at Nobeyama Radio Observatory and successfully simultaneous observed 12CO (J=2-1) spectrum at 230 GHz in USB and 13CO (J=2-1) spectrum at 220 GHz in LSB.


Reference
Asayama, S. et al., "An Integrated Sideband-Separating SIS Mixer based on Waveguide Split Block for 100 GHz Band with 4.0-8.0 GHz IF", Int. J. Infrared Millimeter Waves, 25, 107 (2004)
Nakajima, T., "The Simultaneous Observation of Molecure Lines with 200 GHz SIS Receiver on the 60 cm Radio Telescope", master thesis (Osaka Pref. Univ. 2005)