First Results of the Sideband-Separating Mixer for ALMA Band 9 Upgrade

Last year, the design and implementation details of a new modular sideband-separating mixer block, intended as an upgrade for the current single-ended ALMA Band 9 mixers, were presented at this conference. In high-frequency observation bands like ALMA Band 9 (600-720 GHz), which is strongly influenced by atmospheric noise, employment of sideband separating mixers can reduce, by roughly a factor of two, the integration time needed to reach a certain signal-to-noise ratio for spectral line observations. Alternatively, in the same integration time, a sufficiently larger selection of sources can be accessed. Two prototype mixer blocks were produced on a micro milling machine, and equipped with production Band 9 SIS mixer devices that have independently been tested in double-sideband mode. Here, we present the results of the first measurements, notably, the noise temperature, image rejection, LO pumping balance and IF response. We also present in detail a procedure of the image rejection ratio measurement, which is fast and can be used for single sideband mixers, so that a second IF chain is not required.