The noise and gain of a heterodyne waveguide mixer employing Nb/Al2O3/
Nb superconducting tunnel junctions with an on-chip integrated tuning
element are measured and analyzed at 680-750 GHz and at 840 GHz. The l
owest receiver noise temperatures are 400 K (double side band) at 720
GHz and 1500 K (3000 K including the beam splitter loss) at 840 GHz. W
e compare data of the pumped I-V curves with the quantum theory of mix
ing and demonstrate good agreement at frequencies well above the gap f
requency.