A Superconducting Integrated Receiver (SIR) comprises in a single chip a planar antenna combined with a superconductor-insulator-superconductor (SIS) mixer, a superconducting Flux Flow Oscillator (FFO) acting as a Local Oscillator (LO) and a second SIS harmonic mixer (HM) for the FFO phase locking. In this report an overview of the FFO development and optimization is presented. In order to overcome temperature constraints and extend operation frequency of the fully Nb SIR we have developed and studied Nb-AIN-NbN circuits with a gap voltage Vg up to 3.7 mV and extremely low leak currents (Rj/Rn > 30). Continuous tuning of the frequency due to bending and overlapping of the Fiske steps and a possibility to phase lock the Nb-AIN-NbN FFO at any frequency in the range 350-750 GHz has been experimentally demonstrated. After optimization of the FFO design the free-running linewidth between 7 and 1.5 MHz has been measured in the frequency range 350 - 750 GHz, which allows to phase-lock from 50 to 95 % of the emitted FFO power. New designs of the FFO intended for further improvement of its parameters are under development, but even at the present state the Nb-AIN-NbN FFOs are mature enough for practical applications. These achievements enabled development of a 500 - 650 GHz integrated receiver for the Terahertz Limb Sounder (TELIS) project intended for atmosphere study and scheduled to fly on a balloon in 2008.
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