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Physica C Superconductivity

Linewidth and phase locking of Josephson flux-flow oscillators

J. Mygind, V. P. Koshelets, S. V. Shitov, Lyudmila V. Filippenko, V L Vaks, A. M. Baryshev

Published May 1, 2000• 1 citation
Publisher
DOI
10.1016/s0921-4534(99)00691-7

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The combination of narrow linewidth and wide band tunability makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated sub-mm wave receivers for, e.g., spectral radio astronomy. The feasibility of phase locking the FFO to an external reference oscillator is demonstrated experimentally. A FFO linewidth as low as 1 Hz (determined by the resolution bandwidth of the spectrum analyzer) has been measured in the frequency range 270–440 GHz relative to a reference oscillator. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The results of residual FFO phase noise measurements are also presented. Finally, we propose a single-chip fully superconductive receiver with two superconductor–insulator–superconductor mixers and an integrated phase-locked loop.
Oscillators based on unidirectional viscous flow of magnetic flux quanta in long Josephson tunnel junctions with high damping have been experimentally investigated by harmonic mixing and frequency multiplication at frequencies up to 450 GHz. An integral spectral linewidth of two flux-flow oscillators (FFO) as low as 750 kHz was found at 280 GHz. Presently no theory exists for the linewidth of the FFO. Above a few MHz the experimental linewidth scales with the square of the dynamic resistance of the dc I-V curve as found for short Josephson junctions and oscillators based on resonant motion of fluxons in under-damped long Josephson tunnel junctions. Based on calculations assuming wideband thermal noise, we find that the effective temperature of the FFO appears to be much higher than its physical temperature. Different models are discussed in order to explain this discrepancy. Finally harmonic phase locking of a FFO at 355 GHz is demonstrated.
External phase locking of a Josephson flux flow oscillator (FFO) to a 10 MHz reference oscillator is demonstrated experimentally in the frequency range 270-440 GHz. A linewidth as low as 1 Hz (as determined by the resolution bandwidth of the spectrum analyser) has been measured. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The combination of narrow linewidth, wide band tunability and low noise is important for spectral radio astronomy applications.