Flux flow oscillators for sub-mm wave integrated receivers

The results of a detailed study of the microwave linewidth of Nb-AlOx-Nb fl ux flow oscillators (FFO) are presented. The dependence of the FFO linewidt h on the junctions parameters has been measured by using an improved techni que based on harmonic mixing in the frequency range 250 - 600 GHz, Experime ntal data are compared with theoretical estimates to evaluate the influence of the possible mechanisms responsible for the broadening of the FFO linew idth, The origins of the increased linewidth at the transition from the res onant to the "pure" flux-flow regime are discussed. The results of the line width measurements for the FFO locked to an external synthesizer via a wide band feedback loop are presented. The possibility of phase locking of the J osephson oscillator has been demonstrated experimentally. A FFO linewidth a s low as 3.3 kHz (determined by the resolution bandwidth of the spectrum an alyzer) has been measured at 310 GHz; it is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction.