Design of an on-Chip Sideband Separating (2SB) SIS Balanced Mixer for 400 to 500 GHz on a Silicon Membrane

Superconductor-Insulator-Superconductor (SIS) tunnel junctions are currently used as heterodyne mixers with quantum limited sensitivity for receivers in astronomy. Well-engineered technology offers the opportunity to replace the single-ended double-sideband (DSB) mixers by balanced or sideband separating (2SB) mixers. 2SB mixers are used to prevent the increase of system noise due atmospheric noise in the image sideband. They are usually made in waveguide technology, using an assembly of separate pre-tested units. The large size of these mixers makes it difficult to build multi-pixel-array receivers where the footprint of each pixel must be reduced (10 mm $\times$ 10 mm e.g. for CHAI receiver). We are developing the RF part of an on-chip 2SB balanced SIS mixer between 400 and 500 GHz, in the same technology as the existing balanced mixer. The total size of the complete RF part is 2.3 mm $\times$ 1.7 mm, including 3 hybrids, 4 SIS junctions, a LO-power divider and a RF load. The mixer reduces the LO AM noise due to the balanced mixers. For testing the RF performance of the chip we designed a prototype block with 4 separate SIS mixer IF-outputs. In addition THz spectrometer measurements are planned show the sideband suppression with detection of rotational lines of CH${}_3$CN.