Upgrading the SMA 600 GHz Receivers

We present the results of an ongoing effort to improve the sensitivity of the Submillimeter Array (SMA) 660 GHz receivers, including upgrades to the local oscillator (LO) units, improved telescope-receiver coupling optics, and upgraded 660 GHz mixers and receiver inserts. New tunerless varistor multiplier chains have resulted in increased local oscillator (LO) power across the frequency band (> 10 !W) and have enabled the upgrade of receiver coupling optics inserts, including the use of a fine (400 lines-per-inch) metal mesh as an RF/LO diplexer. This has resulted in a decrease in overall system complexity and an improvement of about 0.5 dB in signal beam transmission. This work has focused on improving the performance of the double sideband (DSB) SIS receivers designed for operation in the 630-700 GHz frequency range. In collaboration with University of Koln (KOSMA), a new batch of 660 GHz Nb/Al-AlOx/Nb SIS devices with end-loaded stub integrated tuning structures was designed and fabricated. We present an overview of this work and a summary of performance achieved both in a laboratory liquid helium cooled cryostat and in the SMA cryostat environment. First-light SMA observations with an upgraded 600 GHz receiver insert are shown.