A Sideband-Separating Mixer Upgrade for ALMA Band 9

The ALMA band 9 (600-720 GHz) receiver cartridge, as currently being produced, features two single-ended (dual sideband) SIS mixers in orthogonal polarisations. In the case of spectral line observations in the presence of atmospheric backgound, the integration time to reach a certain desired signal to noise level can be reduced by about a factor of two by rejecting the unused sideband. A proof-of-concept sideband separating mixer has been successfully designed and produced over the past few years, the results of which have been presented earlier at this conference. At the time that the band 9 cartridge got its final form, however, this mixer was not yet ready for series production. Here, we present a design study that investigates the feasibility of upgrading the current ALMA band 9 cartridge to full dual-polarisation sideband separating capability, with minimal impact on the overall structure of the cartridge. The goal is to re-use as many of the parts and techniques of the existing cartridge as possible to provide a minimal-cost upgrade path, to be available in a couple of years.