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Superconductor Science and Technology

Phase-locked Josephson flux flow local oscillator for sub-mm integrated receivers

J. Mygind, C. Mahaini, Pavel N. Dmitriev, An. B. Ermakov, V. P. Koshelets, S. V. Shitov, Alexander S. Sobolev, Mikhail Yu. Torgashin, Victor V. Khodos, V L Vaks, Paul R. Wesselius

Published Nov 21, 2002• 1 citation
Publisher

Abstract

The Josephson flux flow oscillator (FFO) has proven to be one of the best on-chip local oscillators for heterodyne detection in integrated sub-mm receivers based on SIS mixers. Nb–AlO x –Nb FFOs have been successfully tested from about 120 to 700 GHz (gap frequency of Nb) providing enough power to pump an SIS mixer (about 1 μW at 450 GHz). Both the frequency and the power of the FFO can be dc-tuned. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated with both the bias current and the control-line current (applied magnetic field) have been performed. The FFO line is Lorentzian both in the resonant regime, on Fiske steps (FSs), and on the flux flow step (FFS). This indicates that internal wide-band noise is dominant. A phenomenological noise model can account for the FFO linewidth dependence on experimental parameters. The narrow free-running FFO linewidth achieved, in combination with the construction of a wide-band phase-locked loop (PLL) system, enable us to phase lock the FFO in the frequency range 490–712 GHz where continuous frequency tuning is possible.

DOI
10.1088/0953-2048/15/12/314

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IEEE Transactions on Applied Superconductivity•Jun 1, 1997•8 citations
The Josephson flux flow oscillator (FFO) has proven to be one of the best on-chip local oscillators for heterodyne detection in integrated sub-mm receivers based on SIS mixers. Nb–AlO x –Nb FFOs have been successfully tested from about 120 to 700 GHz (gap frequency of Nb) providing enough power to pump an SIS mixer (about 1 μW at 450 GHz). Both the frequency and the power of the FFO can be dc-tuned. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated with both the bias current and the control-line current (applied magnetic field) have been performed. The FFO line is Lorentzian both in the resonant regime, on Fiske steps (FSs), and on the flux flow step (FFS). This indicates that internal wide-band noise is dominant. A phenomenological noise model can account for the FFO linewidth dependence on experimental parameters. The narrow free-running FFO linewidth achieved, in combination with the construction of a wide-band phase-locked loop (PLL) system, enable us to phase lock the FFO in the frequency range 490–712 GHz where continuous frequency tuning is possible.
A Josephson flux flow oscillator (FFO) is the most developed superconducting local oscillator for integration with an SIS mixer in a single-chip submm-wave receiver. Recently, using a new FFO design, a free-running linewidth /spl les/10 MHz has been measured in the frequency range up to 712 GHz, limited only by the gap frequency of Nb. This enabled us to phase lock the FFO in the frequency range 500-712 GHz where continuous frequency tuning is possible; resulting in an absolute FFO phase noise as low as -80 dBc at 707 GHz. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated SIS harmonic mixer. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field has been studied in order to further optimize the FFO design. A new approach with a self-shielded FFO has been developed and experimentally tested.
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.
A Superconducting Integrated Receiver (SIR) comprises in a single chip a planar antenna combined with a superconductor-insulator-superconductor (SIS) mixer, a superconducting Flux Flow Oscillator (FFO) acting as a Local Oscillator (LO) and a second SIS harmonic mixer (HM) for the FFO phase locking. In this report an overview of the FFO development and optimization is presented. In order to overcome temperature constraints and extend operation frequency of the fully Nb SIR we have developed and studied Nb-AlN-NbN circuits with a gap voltage Vg up to 3.7 mV and extremely low leak currents (Rj/Rn > 30). Continuous tuning of the frequency due to bending and overlapping of the Fiske steps and a possibility to phase lock the Nb-AlN-NbN FFO at any frequency in the range 350-750 GHz has been experimentally demonstrated. After optimization of the FFO design the free-running linewidth between 7 and 1.5 MHz has been measured in the frequency range 350 - 750 GHz, which allows to phase-lock from 50 to 95 % of the emitted FFO power. New designs of the FFO intended for further improvement of its parameters are under development, but even at the present state the Nb-AlN- NbN FFOs are mature enough for practical applications. These achievements enabled development of a 500 - 650 GHz integrated receiver for the Terahertz Limb Sounder (TELIS) project intended for atmosphere study and scheduled to fly on a balloon in 2008.
The results of a detailed study of the microwave linewidth of Nb-AlO/sub x/-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" flux-flow 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.
Comprehensive measurements of the flux flow oscillator (FFO) radiation linewidth are performed using an integrated harmonic SIS mixer; the FFO linewidth and spectral line profile are compared to a theory. An essential dependence of the FFO linewidth on frequency is found; a possible explanation is proposed. The results of the numerical solution of the perturbed sine–Gordon equation qualitatively confirm this assumption. To optimize the FFO design, the influence of the FFO parameters on the radiation linewidth is studied. A novel FFO design at a moderate current density has resulted in a free-running FFO linewidth of about 10 MHz in the flux flow regime up to 712 GHz, limited only by the gap frequency of Nb. This relatively narrow free-running linewidth (along with implementation of a wide-band phase locking loop system) allows continuous phase locking of the FFO in the wide frequency range of 500–710 GHz. These results are the basis for the development of a 550–650 GHz integrated receiver for the terahertz limb sounder (TELIS) intended for atmosphere study and scheduled to fly on a balloon in 2005. We report here also on the design of the second generation of the phase-locked superconducting integrated receiver chip for TELIS.
Presently Flux Flow Oscillators (FFOs) are being developed for integration with superconductor-insulator-superconductor (SIS) mixers intended for submillimeter wave integrated receivers. Here we report on a detailed study of the dc and microwave properties of Nb-AlO/sub x/-Nb FFOs. A model based on the Josephson self-pumping effect is proposed for explanation of the experimentally measured dc I-V curves (IVC). Extensive measurements of the spectral linewidth of the radiation emitted from a FFO have been performed up to 600 GHz using a new reliable technique based on harmonic mixing. The results are compared to recent theoretical predictions.