... For example, only about 50 % of the FFO power can be phase-locked by the present PLL system at the free-running FFO linewidth of 7 MHz. ... D EPENDENCE OF THE Abstract -- A comprehensive experimental study of the Flux Flow Oscillator (FFO) linewidth on its parameters has been carried out in order to obtain free-running FFO linewidth well below 10 MHz and ensure phase-locked operation of a Superconducting Integrated Receiver (SIR). ... Presented at ISEC-2005 2 FFO length for tested FFOs. ...