Design and Performance of Waveguide Mixers with All NbN tunnel junctions on MgO substrates

In this paper we present the development of low-noise waveguide mixers with NbN/AlN/NbN tunnel junctions at frequency approaching 1THz. The mixer was designed to be compatible with MgO substrate. Mixers of such a design demonstrated much improved receiver sensitivity. The improvement is a result of reduction of signal dissipation in waveguide and leakage into IF port by adopting a full-height waveguide and an effective RF choke filter respectively. Two types of tuning circuit, namely parallel-connected twin-junction (PCTJ) and half-wavelength self-resonance distributed junction (DJ), are designed and evaluated. Both theoretical and measurement results show that the PCTJ design is superior in terms of gain and sensitivity due to smaller loss in the tuning circuit.