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arXiv:1112.2803 · Journal of Applied Physics

The effect of bias feed profile on the linewidth of noisy Josephson flux flow oscillator

Ekaterina A. Matrozova, Andrey L. Pankratov, L. S. Revin

Published Sep 1, 2012• 12 citations
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Abstract

For creation of a noisy non-stationary spectrometer operating in the sub-THz frequency range, the emitting power and spectral characteristics of a flux-flow oscillator based on a long Josephson junction are considered. The effect of bias current profiles on the spectral linewidth of a long overlap Josephson junction is investigated and compared with the inline junction geometry. It has been demonstrated that the spectral linewidth can be increased by a factor of three with a moderate reduction of the emitted power.

DOI
10.1063/1.4748151

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Linewidth and DC properties of the flux-flow oscillator with mixed inline-overlap bias

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A Flux-Flow Oscillator (FFO) is a long Josephson junction in which a unidirectional flow of magnetic flux quanta is maintained by an applied DC magnetic field and a bias current. In the overlap geometry the FFO voltage and thus its oscillation frequency is controlled independently by the (overlap) bias current and the so-called "control line" (inline) current, which creates the magnetic field at the ends of the FFO. We have studied both DC properties and the linewidth of the emitted radiation for an FFO, which can be biased in different ways; as a four current terminal object. The bias current of the mixed inline-overlap type is applied through the terminals commonly used for the "control line" current. In this configuration the bias current also contributes to the magnetic field at one of the two FFO ends. This changes the steepness of the junction's I-V characteristics and therefore the bias current dynamic resistance. The experimental FFO linewidth was found to be independent of the bias configuration and determined by the dynamic resistance of the junction with pure overlap bias current. A new method has been developed to evaluate the difference in the DC magnetic field at the ends of the junction. It may be attributed to control line redistribution caused by the impedance matching RF circuit connected to the FFO at the end where the fluxon chain annihilates and radiation is emitted. We found that the opposite end, where the fluxons enter the junction, appears to be about three times more sensitive to variations of the magnetic field than the radiating end.

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Theoretical and experimental studies have been made of the effects of junction geometry on the performance of the flux-flow type Josephson oscillator. It is shown theoretically that the height and the slope of the current steps appearing in dc I-V curves of the junction, which determine the available power and the radiation linewidth of the oscillator, respectively, are dependent strongly upon self fields in the junction. In order to study the effects of junction geometries on the self fields, computer simulation and experiments have been carries out for an in-line junction, an overlap junction and an overlap junction with a small projection on one side. The best performance is obtained for the overlap junction with a moderate projection on side as expected theoretically.

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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.

Physical review. B, Condensed matter•Mar 1, 1995•43 citations
For creation of a noisy non-stationary spectrometer operating in the sub-THz frequency range, the emitting power and spectral characteristics of a flux-flow oscillator based on a long Josephson junction are considered. The effect of bias current profiles on the spectral linewidth of a long overlap Josephson junction is investigated and compared with the inline junction geometry. It has been demonstrated that the spectral linewidth can be increased by a factor of three with a moderate reduction of the emitted power.
A Flux-Flow Oscillator (FFO) is a long Josephson junction in which a unidirectional flow of magnetic flux quanta is maintained by an applied DC magnetic field and a bias current. In the overlap geometry the FFO voltage and thus its oscillation frequency is controlled independently by the (overlap) bias current and the so-called "control line" (inline) current, which creates the magnetic field at the ends of the FFO. We have studied both DC properties and the linewidth of the emitted radiation for an FFO, which can be biased in different ways; as a four current terminal object. The bias current of the mixed inline-overlap type is applied through the terminals commonly used for the "control line" current. In this configuration the bias current also contributes to the magnetic field at one of the two FFO ends. This changes the steepness of the junction's I-V characteristics and therefore the bias current dynamic resistance. The experimental FFO linewidth was found to be independent of the bias configuration and determined by the dynamic resistance of the junction with pure overlap bias current. A new method has been developed to evaluate the difference in the DC magnetic field at the ends of the junction. It may be attributed to control line redistribution caused by the impedance matching RF circuit connected to the FFO at the end where the fluxon chain annihilates and radiation is emitted. We found that the opposite end, where the fluxons enter the junction, appears to be about three times more sensitive to variations of the magnetic field than the radiating end.
Presently the Josephson Flux Flow Oscillator (FFO) is the best on-chip integrated local oscillator in all-superconducting sub-millimeter wavelength receivers based on SIS mixers. Potential applications are spectral investigations in astronomy, atmospheric pollution, biophysics, etc. Theoretically, the linewidth of the emitted radiation from the FFO - as for all other Josephson oscillators - should be given by the well-known lumped junction expression containing only internal noise and quantities measured from the DC I-V characteristic. The FFO, however, differs from the other members of the Josephson oscillator family in needing also an external magnetic field bias. Several theoretical models have been proposed adding also external noise. Experimentally the linewidth may be fitted by a simple extension of the lumped junction expression including also the measured dynamic resistance of the magnetic bias current. We suggest a simple solution to this based on the assumption that the DC bias current e.g. due to geometrical asymmetries creates an additional contribution to the magnetic field in the junction.
Theoretical and experimental studies have been made of the effects of junction geometry on the performance of the flux-flow type Josephson oscillator. It is shown theoretically that the height and the slope of the current steps appearing in dc I-V curves of the junction, which determine the available power and the radiation linewidth of the oscillator, respectively, are dependent strongly upon self fields in the junction. In order to study the effects of junction geometries on the self fields, computer simulation and experiments have been carries out for an in-line junction, an overlap junction and an overlap junction with a small projection on one side. The best performance is obtained for the overlap junction with a moderate projection on side as expected theoretically.
The Flux Flow Oscillator (FFO) is a long Josephson junction in which a DC bias current and a DC magnetic field maintain a unidirectional viscous flow of magnetic flux quanta. The theoretical linewidth of the electromagnetic radiation generated at the end boundary is due to internal current fluctuations as given by the usual expression for the lumped junction. Experimentally, however, the linewidth deviates significantly both in magnitude and functional dependence. We suggest a simple solution based on the assumption that the bias current creates an additional magnetic field in the junction. This is supported by linewidth measurements on FFO's subjected to different bias configurations.
A Flux-flow oscillator based on a long Josephson junction is a local oscillator in fully superconducting integrated receivers (SIRs). The low frequency voltage fluctuations define the free-running FFO radiation linewidth, an important parameter for employing the SIR as a submm spectrometer. The DC voltage of the oscillator is independently controlled by both the dissipative DC bias current I B and the loss-free external magnetic field at the two FFO ends. It is mainly the natural wideband current noise of the Josephson barrier, which is converted into the fluctuations of voltage by the differential resistance Rd on the current I B . We have developed and experimentally demonstrated the method for suppression of the voltage fluctuations by convey of the bias current noise into the anticorrelated noise of the magnetic field. The experiments were made with Nb/AlN/Nb FFOs, having a specially designed shunting structure at the end, where flux quanta enter the junction.
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.