Radiation linewidth of flux-flow oscillators

Abstract

The results of linewidth measurements on flux-flow oscillators (FFOs) of a new design with improved parameters are presented. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated both with the bias current and the control-line current (applied magnetic field) are taken. A profile of the FFO radiation line is measured in different regimes of FFO operation and compared to the theoretical models. A Lorentzian shape of the FFO line is observed both at Fiske steps (FSs) in the resonant regime and on the flux-flow step (FFS) at high voltages. A phenomenological model of the FFO linewidth taking into account all known noise sources (both internal and external) is used to explain the FFO linewidth dependence on the experimental parameters. Finally, we discuss the feasibility of using an electronic phase-locking loop (PLL) over the entire FFO operational frequency band.

Form and width of the spectral line of a Josephson flux-flow oscillator

“…Besides, magnetic field fluctuations and parametric effects that may lead to additional broadening of the linewidth, were not considered in those papers. Recently, importance of accounting magnetic field fluctuations has been discussed in [13].…”

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“…It means that the low frequency external fluctuations can be almost fully suppressed by appropriate filtering and shielding in combination with frequency stabilization system. The data presented in [15] demonstrate that the profile of the FFO emission line is Lorentzian even at the "plateau", which is observed in the dependence of the FFO linewidth on differential resistance [10,16] at R d < 0.01 Ω. This fact is very important for linewidth calculations.…”

“…The profile of the FFO line recorded when biased at the steep Fiske step (FS), where the differential resistance is extremely small, can be different [10] from the one measured on the smooth FFS. Nevertheless, very recently it was shown [15] that the FFO spectrum measured at the Fiske step is also very well described by a Lorentzian. It means that the low frequency external fluctuations can be almost fully suppressed by appropriate filtering and shielding in combination with frequency stabilization system.…”

“…This has never been taken into account in the theoretical considerations. If we include the wide-band fluctuations via the CL in the numerical model [16] by an empirical coefficient K (as a multiplier to the measured value of R d CL ), this allows us to fit the experimental linewidth data to the calculated both on the Fiske steps and on the flux flow step [15]. This empiric approach seems to be very similar to that derived recently by A. Pankratov [17] on the base of the same assumption concerning the influence of wide-band fluctuations via magnetic filed.…”

“…Detailed analyses of the measured dependence of the radiation linewidth [10,15,16] on the FFO parameters provides us with information and knowledge for further optimization of the FFO design. In order to reduce both the bias and control line differential resistances a new design of the FFO electrodes has been made.…”