Flux flow oscillators for sub-mm wave integrated receivers

Abstract

The results of a detailed study of the microwave linewidth of Nb-Al0,-Nb flux flow oscillators (FFO) are presented. The dependence of the FFO linewidth on the junctions parameters has been measured by using an improved technique based on harmonic mixing in the frequency range 250 -600 GHz. Experimental data are compared with theoretical estimates to evaluate the influence of the possible mechanisms responsible for the broadening of the FFO linewidth. The origins of the increased linewidth at the transition from the resonant to the "pure" fluxflow regime are discussed. The results of the linewidth measurements for the FFO locked to an external synthesizer via a wideband feedback loop are presented. The possibility of phase locking of the Josephson oscillator has been demonstrated experimentally. A FFO linewidth as low as 3.3 kHz (determined by the resolution bandwidth of the spectrum analyzer) has been measured at 310 GHz; it is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction.

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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.…”

“…This fact is very important for linewidth calculations. It gives us the experimental evidence that the wide-band fluctuations are present in the magnetic field, because according to [16] at small R d the FFO linewidth is mainly determined by fluctuations of the magnetic field. Since an influence of the wide-band fluctuations in the bias resistors feeding the control line (CL) is negligibly small [16], it means that there is a channel for transfer of the wide-band fluctuations from the bias current I B to the FFO via the magnetic field.…”

“…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.…”

“…Recent results on phase locking of an FFO to an external reference oscillator have been used to develop the practical concept of the integrated receiver with phase-locked loop [9,16,19] presented schematically in Fig 4. Following this concept a 350 GHz receiver chip containing a quasioptical SIS mixer, a phase-locked flux flow oscillator, a harmonic SIS mixer and an SIS multiplier as a source for the harmonic mixer (optional) has been designed, fabricated and successfully tested.…”

“…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.…”