| Summary: | We compare the design of two 0.85 THz SIS mixers fed with a radial probe antenna aligned to the E-Plane of
the input full-height rectangular waveguide connected to a drilled smooth-walled horn. Both designs employ
the same 0.5 µm2 hybrid Nb/AlN/NbN tunnel junction technology, sandwiched between a NbTiN ground and
aluminium wiring layer fabricated on top of a 40 µm quartz substrate. The two designs is differed by how we tune
out the unwanted junction capacitance for broadband performance. The first design uses the commonly-used
twin-junction tuning scheme; whilst the second design utilises an end-loaded scheme. We successfully achieve
close to 2× the double sideband quantum noise performance for both schemes, but the twin-junction design is
less sensitive to fabrication accuracy of planar circuit components utilised. However, the end-loaded design offers
a much better IF bandwidth performance, almost twice wider than the twin-junction design. The need for an
ultra-wide IF bandwidth mixer is becoming more pressing and important for the future and up-coming upgrades
of various millimetre (mm) and sub-mm astronomical instruments, hence we conclude that the end-loaded design
is a better solution for the THz heterodyne mixing applications.
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