We have built a waveguide SIS mixer with Nb-based parallel-connected twin j
unctions at 800 GHz which is above the gap frequency of Nb (690 GHz). Heter
odyne mixing was performed at 810 GHz and a receiver noise temperature of 5
80 K (DSB), which includes noise contributions due to a vacuum window and a
beam splitter, was achieved using Nb/AlOx/Nb junctions with Nb striplines,
The performance of this mixer is quite good and it is shown that efficient
mixing in Nb-based SIS junctions is still possible above the gap frequency
, SIS mixers with Al striplines have also been fabricated and good DC I-V c
haracteristics have been observed. Heterodyne measurements of these mixers
are now under preparation.