Abstract:
The invention of the superconductor-insulator-superconductor (SIS) tunnel
junction has produced major progress in the development of low-noise detectors. The
physical phenomenon that provides the basis of mixing is photon-assisted tunneling of
quasiparticles across the SIS tunnel barrier. SIS mixers are used extensively in
submillimetre-wave astronomy and many terahertz applications. It is vitally important
in critical measurements to understand precisely how saturating SIS mixers behave.
The ability to examine the large-signal transfer characteristics, both in-band and outof-
band, of new designs, before they are fabricated, is highly beneficial.
The work presented in this thesis is about the investigation of SIS mixer
saturation using the MultiTone mixer model developed by Withington, Kittara and
Yassin (2003). This model is based on the full quantum mechanical description of the
superconducting tunnel junction, and does not assume that the terminal voltages at the
signal, image, and intermediate frequencies are vanishingly small. Simulation results
of saturation properties for SIS mixers operating in the single side band and double
sideband configurations with various circuit parameters are given. Emphasis is placed
on the effect of junction nonlinearities, the bias point, the intermediate frequency
bandwidth and the out-of-band signals to the mixer saturation. The simulations have
shown that single-junction SIS mixers illuminated by the room temperature radiation
in the common hot and cold load noise calibration have a 2% reduction in gain.