Linewidth and phase locking of Josephson flux-flow oscillators

We report on measurements of the linewidth of the emitted radiation from Jo sephson Flux Row Oscillators (FFOs). Frequency and phase locking to an exte rnal 10 MHz reference oscillator an demonstrated experimentally in the freq uency range 270-440 GHz. A linewidth as low as 1 Hz (as determined by the r esolution bandwidth of the spectrum analyzer) has been measured. This linew idth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The damping mechanisms are discussed and related to the self-excitation Of quasiparticles. Narrow linewidth, wide-ba nd tunability and low noise are important for radio astronomy and air- and space-borne spectroscopy for atmospheric research and environmental monitor ing. (C) 2000 Elsevier Science B.V. All rights reserved.