The Atacama Pathfinder EXperiment (APEX) is a 12 m antenna now operating at the Llano Chajnantor on the Atacama
Desert in Northern Chile at 5100 m altitude. APEX will be equipped with single-pixel heterodyne receivers covering
211 - 1500 GHz frequency range. We present a sideband separation (2SB) mixer using superconducting-insulator-superconductor
(SIS) junction for the APEX band 2, 275-370 GHz. The 2SB mixer is based on a previous development
of a double sideband (DSB) mixer, which is currently installed at the APEX telescope. This DSB receiver has a noise
temperature of about 40-50 K across the band, and, as installed on one of the best site on the Earth, yields total DSB
system noise temperatures of about 100 K for excellent weather.
The 2SB mixer layout uses a modular approach with two identical DSB mixers, independently tested, having similar
characteristics, and combined with an intermediate waveguide block, containing a 3 dB-90° branch-line coupler for the
RF signal and a 3 dB-180° divider for the LO signal. The LO signals are injected into the mixer using a novel
waveguide directional coupler based on 2 quartz chips containing E- probes, allowing to couple -15 dB of the LO to the
RF path. At the conference, we will present the first measurements of this 2SB mixer, together with the current
performance the DSB receiver at the APEX telescope.