A High-Performance 650-GHz Sideband-Separating Mixer—Design and Results

We presented the design and experimental results of a new sideband-separating (2SB) mixer for 600-720 GHz (ALMA Band 9). Although designed following the classical quadrature hybrid architecture, the emphasis of the optimization was this time not placed on ultimate phase and amplitude balance in the hybrid, but on the reduction and active suppression of reflections and standing waves. It turns out that, once the phase and the amplitude are sufficiently balanced, the image rejection ratio (IRR) starts being dominated by unbalanced parasitic reflections in the signal path, and further perfection of the hybrid's balance is not the way toward higher IRR. Four operational waveguide blocks were produced. IRRs above 15 dB over the entire band were obtained, depending critically on mixer matching. The noise temperature was not more than 20-30 K above the value expected from the individual mixer devices. With the RF circuits optimized thus, the performance becomes limited by the IF system, where similar interference and standing wave problems seem to occur. These are exacerbated when, for compactness (i.e., in arrays), cryogenic isolators between the mixers and amplifiers are omitted. We show that, because of the high performance of the mixers, an IRR of 10 dB can still be obtained without isolators. When, on the other hand, highly matched amplifiers are used, we expect that the full image rejection performance can be achieved even in array receivers. These 2SB mixers are intended for the future 2SB receivers on the APEX (Chile) and LLAMA (Argentina) observatories and for deployment in any other observatory that would benefit from sideband separation in the 600-720-GHz band.