Superfine resonant structure on IV-curves of long Josephson junction and its influence on flux flow oscillator linewidth

The Josephson Flux Flow Oscillator (FFO) has proven to be a perfect on-chip local oscillator for integrated submm receivers; a noise temperature (DSB) below 100 K has been achieved at 500 GHz. Recently a FFO linewidth as low as 1 Hz has been measured in the frequency range 270 - 440 GHz. A new technique for both linewidth measurements and phase locking of the FFO is developed; this method employs an offchip harmonic multiplier. By measuring the frequency of the FFO radiation emission, its IV-curve (IVC) can be reconstructed with an accuracy better than 1 nV. A superfine resonant structure with a voltage spacing of about 20 nV and extremely low differential resistance has been observed in the FFO IVCs. This resonant structure modifies the performance of the FFO compared to the one expected from the "averaged IVC". The influence of this resonant structure on phase locking is discussed. Also results of FFO phase noise measurements are presented.