We discuss the design and optimisation of two full height waveguide SIS mixers for astronomical applications both covering the 275-370 GHz frequency band: a Double Side Band (DSB) and a Single Side Band (SSB) mixer. The expected SSB receiver noise temperature referred to the mixer input are in the range 23-35 K for the DSB and 25-38 K for SSB mixer. A 30 % operating bandwidth can be achieved in the DSB case by using an "end-loaded" tuning stub to tune out the junction capacitance of 75 fF (junction size 1 µm2) followed by two quarter-wave transformer sections. A similar operating bandwidth is obtained in the SSB mixer by using a parallel tuning inductor with a radial microstrip stub. Single junctions are mounted on a 80 mm thick quartz which stretches only part way across the waveguide. In the SSB mixer the image rejection is obtained using a mechanically rugged noncontacting backshort with a circular cross-section which can be moved inside a circular waveguide. Both mixers have a 4 GHz IF passband and a central IF frequency of 6 GHz.
A stability criterion for intrinsically DSB and SSB mixers under typical operating conditions has been derived. We have shown that when an inductive series matching structure is used to compensate the junction capacitance, the SSB mixer cannot be operated over a wide frequency range in a stable way. An inductive parallel matching structure allows us to fulfil the necessary conditions of stability. Receiver performance has been optimised for both mixers in order to guarantee a low mixer noise temperature while maintaining adequate gain and stable operations over the whole frequency band of interest.
View a pdf version of ALMA Memo #351.
Last modified: March 05, 2001
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