Self-pumping effects and radiation linewidth of Josephson flux-flow oscillators

Abstract

Flux-flow oscillators ͑FFO's͒ are being developed for integration with a SIS mixer for use in submillimeter wave receivers. The present work contains a detailed experimental study of the dc, microwave, and noise properties of Nb-AlO x -Nb FFO's. A model based on the Josephson self-pumping effect is proposed for an explanation of the experimental current-voltage characteristics. A reliable technique based on harmonic mixing is used to determine the spectral linewidth of the radiation emitted by the integrated FFO's up to 600 GHz. Comprehensive measurements of the dependence of the linewidth on the dynamic resistance and the applied magnetic field have been performed. In the resonant regime a linewidth as small as 200 kHz is obtained at 450 GHz. The experimental data are compared with recent theoretical predictions. ͓S0163-1829͑97͒02233-9͔

Influence of surface losses and the self-pumping effect on current-voltage characteristics of a long Josephson junction

“…The damping increases significantly ͑up to three times͒ for voltages above v b = 1.9 as predicted in Ref. 7. In difference with Ref.…”

“…Below v b , the IVC consists of nearly vertical, equally spaced voltage spikes, the so-called Fiske steps, which are due to electromagnetic resonances ͑Fiske resonances͒ in the JTJ. Above v b , the damping parameter is drastically increased due to the so-called self-pumping effect, 7 which is explained as a resonant tunneling of quasiparticles similar to the well-known photon-assisted tunneling ͑PAT͒. As it was shown in Ref.…”

“…4, ␣ is considered in a large scale, so if one cuts the curves up to voltages 2.4, and consider in more detail the currents below 0.04, one will see the same qualitative behavior of ␣ as in Ref. 7, with the only exception, we do not observe the small step at v g /5, since we consider pumping the FFO by its own radiated signal, which is rather small for small currents/voltages, while in Ref. 7 this pumping is considered to be with a constant amplitude irrespectively to the junction's biasing point.…”

“…The dynamical and fluctuational properties of the FFO have been intensively studied both experimentally and theoretically. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In contrast to many other types of oscillators, the FFO fluctuations are mainly caused by internal wideband sources, such as thermal and shot noise, that result in a spectral line of emission with nearly perfect Lorentzian shape. The power in the so-called "wings" decreases much slower with frequency offset from the carrier than the Gaussian shaped spectral line obtained when external noise sources are dominant.…”

Towards a phase-locked superconducting integrated receiver: prospects and limitations

“…The resolution, which is determined by both the instantaneous linewidth of the local oscillator and its long-time stability, should be much less than 1 ppm of the center frequency. Previous measurements of the linewidth of the FFO have demonstrated that the intrinsic linewidth increases considerably at voltages above a boundary voltage of the order of 1/3 of the gap voltage as a result of the abrupt increase of the internal damping due to Josephson self-coupling [10]. Note that the observed FFO linewidth is almost one order of magnitude wider [4,10] than predicted by the theory for a lumped Josephson tunnel junction [11,12].…”