Showing papers on "Noise (radio) published in 2008"

Characteristics of ambient seismic noise as a source for surface wave tomography

Abstract: [1] Interstation cross correlations of ambient seismic noise from 1 year of continuous data at periods between 6 and 50 s are used to study the origin of the ambient noise using stations located in Europe, southern Africa, Asia, and three regions within North America. The signal-to-noise ratios (SNR) of Rayleigh waves for positive and negative correlation time lags at periods of 8, 14, 25 and 50 s are used to determine the azimuthal distribution of strong ambient noise sources. Ambient noise in both the primary (10–20 s) and secondary microseism bands (5–10 s) comes dominantly from the directions of relatively nearby coastlines with stronger noise occurring in the Northern Hemisphere in northern winter and in the Southern Hemisphere in southern winter, consistent with the hypothesis that oceanic microseisms are generating this noise. The observed differences in the directivity of noise in the primary and secondary microseism bands are the consequence of propagation and attenuation, rather than the location of generation. At intermediate and long periods (>20 s), there is much less seasonal variation in both signal strength and directivity. We argue that our results are explained most simply by near-coastal sources rather than deep ocean sources at all periods. Although the dominant ambient noise sources are distributed inhomogeneously in azimuth, strong ambient noise emerges from most directions when using recordings that are 1 year in duration. Simulations illustrate that this is what ensures the accuracy of the empirical Green's functions and ambient noise tomography.

A semiempirical model for two-level system noise in superconducting microresonators

Abstract: We present measurements of the low-temperature excess frequency noise of four niobium superconducting coplanar waveguide microresonators, with center strip widths sr ranging from 3 to 20 µm. For a fixed internal power, we find that the frequency noise decreases rapidly with increasing center strip width, scaling as 1/s r 1.6 . We show that this geometrical scaling is readily explained by a simple semiempirical model which assumes a surface distribution of independent two-level system fluctuators. These results allow the resonator geometry to be optimized for minimum noise.

Equalization-enhanced phase noise for coherent-detection systems using electronic digital signal processing

Abstract: In coherent optical systems employing electronic digital signal processing, the fiber chromatic dispersion can be gracefully compensated in electronic domain without resorting to optical techniques. Unlike optical dispersion compensator, the electronic equalizer enhances the impairments from the laser phase noise. This equalization-enhanced phase noise (EEPN) imposes a tighter constraint on the receive laser phase noise for transmission systems with high symbol rate and large electronically-compensated chromatic dispersion.

300 Ghz transmission system

Abstract: A 300 GHz transmission system, designed for terahertz communication channel modelling and propagation studies, is introduced. It consists of an autarkic transmitter and detector units based on Schottky diode mixer technology. The system performance is characterised with regard to link budget and noise. For demonstration, analogue video signals have been transmitted over distances of up to 22 m.

Spectral analysis of seismic noise induced by rivers: A new tool to monitor spatiotemporal changes in stream hydrodynamics

Abstract: Analysis of continuous seismic data recorded by a dense passive seismological network (Hi-CLIMB) installed across the Himalayas reveals strong spatial and temporal variations in the ambient seismic energy produced at high frequencies (>1 Hz). From June to September 2003, the high-frequency seismic noise is observed to increase up to 20 dB (relative to (m/s)(2)/Hz) for all the stations located along a steep 30-km-long narrow and deeply incised channel of the Trisuli River, a major trans-Himalayan river. The early summer increase in high-frequency energy is modulated by a 24-h periodicity where the minimum of seismic noise level is reached around noon and the maximum is reached late in the evening. A detailed study of seismic noise amplitude reveals a clear correlation with both regional meteorological and hydrological data along the Trisuli River. Seasonal increase in ambient noise coincides with the strong monsoon rainfall and a period of rapid melting of snow and ice in the high elevations. The observed 24-h cyclicity is consistent with the daily fluctuation of the precipitation and river discharge in the region. River-induced seismic noise is partly generated by stream turbulence, but this mechanism fails to explain the observed clockwise hysteresis of seismic noise amplitude versus water level. This pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bed load transport. This points out the potential of using background seismic noise to quantify in continuous river bed load and monitor its spatial variations, which remain difficult with classical approaches.

A near-IR variability study of the Galactic black hole: a red noise source with no detected periodicity

Abstract: We present the results of near-infrared (2 and 3 microns) monitoring of Sgr A*-IR with 1 min time sampling using the natural and laser guide star adaptive optics (LGS AO) system at the Keck II telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2005 July and 2007 August. Sgr A*-IR is detected at all times and is continuously variable, with a median observed 2 micron flux density of 0.192 mJy, corresponding to 16.3 magnitude at K'. These observations allow us to investigate Nyquist sampled periods ranging from about 2 minutes to an hour. Using Monte Carlo simulations, we find that the variability of Sgr A* in this data set is consistent with models based on correlated noise with power spectra having frequency dependent power law slopes between 2.0 to 3.0, consistent with those reported for AGN light curves. Of particular interest are periods of ~20 min, corresponding to a quasi-periodic signal claimed based upon previous near-infrared observations and interpreted as the orbit of a 'hot spot' at or near the last stable orbit of a spinning black hole. We find no significant periodicity at any time scale probed in these new observations for periodic signals. This study is sensitive to periodic signals with amplitudes greater than 20% of the maximum amplitude of the underlying red noise component for light curves with duration greater than ~2 hours at a 98% confidence limit.

Reconstructing Green's function by correlation of the coda of the correlation (C3) of ambient seismic noise

Abstract: Analysis of long-range correlation of the microseisms has been shown to provide reliable measurements of surface wave speeds that can be used for seismic imaging and monitoring. In the case of an even distribution of noise sources, it has been theoretically demonstrated that the correlation is the exact Green's function, including all types of waves. This method is limited in its application by the actual source distribution. In practice, the azimuthal distribution of energy flux of the noise is dominated by some particular directions resulting in a clear azimuthal dependence of the quality of the reconstruction of Rayleigh waves, with a poor reconstruction in some azimuths. To solve this problem, we use noise correlations measured on the entire network. We consider two stations, A and B, for which the Rayleigh waves could not be discerned in the in the correlation of continuous records of ambient noise. We computed all correlations between the station A (respectively B) and all the 150 other stations located at regional distances. Theoretically, these virtual seismograms contain direct waves and coda, although they are clearly contaminated by the influence of the imperfect ambient noise field and most are inadequate for direct analysis. We used these correlation functions as equivalents to seismograms produced by sources acting at the 150 stations locations and recorded in A (respectively B). We select time windows in those virtual seismograms that correspond to coda and compute correlations between them. This metacorrelation is found to exhibit the surface wave part of the Green's function that was not visible in the raw correlation of ambient noise. We illustrate the legitimacy of the reconstruction by comparison with raw noise correlations. This procedure can be used to assess seismic velocity between stations, even in presence of a directive and poorly oriented ambient noise. The result shows that in spite of the small signal-to-noise ratios often seen in correlations of ambient noise, especially at large lag time corresponding to coda, their codas are better equipartitioned than was the ambient noise upon which they were based. They are therefore presumably multiply scattered and contain information on both direct surface waves and also on more complex travel paths.

Dual-Frequency Laser at 1.5 $\mu$ m for Optical Distribution and Generation of High-Purity Microwave Signals

Abstract: We describe the stabilization of the beatnote of an Er,Yb:glass dual-frequency laser at 1.5 mum with and without an external microwave reference. In the first case, a classical optical phase-locked loop (OPLL) is used, and absolute phase noise levels as low as -117 dBrad2/Hz at 10 kHz from the carrier are reported. In the second case one or two fiber-optic delay lines are used to lock the frequency of the beatnote. Absolute phase noise levels as low as -107 dBrad2/Hz at 10 kHz from the carrier are measured, fairly independant of the beatnote frequency varying from 2 to 6 GHz. An analysis of the phase noise level limitation is presented in the linear servo-loop theory framework. The expected phase noise level calculated from the measurement of the different noise sources fits well with the predictions.

Temperature Dependence of Electric Field Noise above Gold Surfaces

Abstract: Electric field noise from fluctuating patch potentials is a significant problem for a broad range of precision experiments, including trapped ion quantum computation and single spin detection. Recent results demonstrated strong suppression of this noise by cryogenic cooling, suggesting an underlying thermal process. We present measurements characterizing the temperature and frequency dependence of the noise from 7 to 100 K, using a single Sr+ ion trapped 75 mum above the surface of a gold plated surface electrode ion trap. The noise amplitude is observed to have an approximate 1/f spectrum around 1 MHz, and grows rapidly with temperature as T;{beta} for beta from 2 to 4. The data are consistent with microfabricated cantilever measurements of noncontact friction but do not extrapolate to the dc measurements with neutral atoms or contact potential probes.

A semi-empirical model for two-level system noise in superconducting microresonators

Abstract: We present measurements of the low--temperature excess frequency noise of four niobium superconducting coplanar waveguide microresonators, with center strip widths $s_r$ ranging from 3 $\mu$m to 20 $\mu$m. For a fixed internal power, we find that the frequency noise decreases rapidly with increasing center strip width, scaling as $1/s_r^{1.6}$. We show that this geometrical scaling is readily explained by a simple semi-empirical model which assumes a surface distribution of independent two-level system fluctuators. These results allow the resonator geometry to be optimized for minimum noise.

Contribution of dielectrics to frequency and noise of NbTiN superconducting resonators

Abstract: We study NbTiN resonators by measurements of the temperature dependent resonance frequency and frequency noise. Additionally, resonators are studied covered with SiOx dielectric layers of various thicknesses. The resonance frequency develops a nonmonotonic temperature dependence with increasing SiOx layer thickness. The increase in the noise is independent of the SiOx thickness, demonstrating that the noise is not dominantly related to the low temperature resonance frequency deviations.

Coherent optical frequency transfer over 50-km physical distance using a 120-km-long installed telecom fiber network.

Abstract: Optical frequency at 1542 nm was coherently transferred over a 120-km-long installed telecom fiber network between two cities (Tsukuba and Tokyo) in Japan separated by more than 50 km. The phase noise induced by the fiber length fluctuations was actively reduced by using a fiber stretcher and an acousto-optic modulator. The fractional frequency instability of the one-way transmitted light was reduced down to less than 8.0 x 10(-16) at an averaging time of 1s, which is limited by the theoretical limit deduced from the length and the intrinsic noise of the fiber.

ACOUSTICS2008/3539 LES prediction of wall-pressure fluctuations and noise of a low-speed airfoil

Abstract: The wall-pressure fluctuations and noise of a low-speed airfoil are computed using large-eddy simulation (LES). The results are compared with experimental measurements made in an open-jet anechoic wind-tunnel at Ecole Centrale de Lyon. To account for the eect of the jet on airfoil loading, a RANS calculation is conducted in the full wind-tunnel configuration, which provides velocity boundary conditions for the LES in a smaller domain within the potential core of the jet. The flow field is characterized by an attached laminar boundary layer on the pressure side and a transitional and turbulent boundary layer on the suction side. The predicted unsteady surface pressure field shows reasonable agreement with the experimental data in terms of frequency spectra and coherence in the trailing-edge region. In the nose region, characterized by unsteady separation and transition to turbulence, the wall-pressure fluctuations are highly sensitive to small perturbations and dicult to predict or measure with certainty. The LES, in combination with the Ffowcs Williams and Hall solution to the Lighthill equation, also predicts well the radiated trailing-edge noise. A finite-chord correction is derived and applied to the noise prediction, which is shown to improve the overall agreement with the experimental sound spectra.

Dual-Beam Delay Calibration for VERA

Abstract: We present the technique of instrumental delay calibration for the dual-beam system of VLBI Exploration of Radio Astrometry (VERA), namely, the horn-on-dish method, in which artificial noise sources are mounted on the antenna feedome base and a wide-band radio noise is injected into the dual-beam receivers after reflection by a subreflector. We introduce the basic concept of calibration with the horn-on-dish method, and also present results of the experiments to evaluate its calibration accuracy. Detailed comparisons between model path calculations and measured paths from the noise sources show that the horn-on-dish method can calibrate the dual-beam delay difference in the antenna structure and receiver within an � 0.1 mm level. We estimated that the systematic error in

Transmission gaps and Fano resonances in an acoustic waveguide: analytical model

Abstract: A simple acoustic device consisting of two dangling side resonators grafted at two sites on a slender tube is designed possibly to obtain transmission stop bands (where the propagation of longitudinal acoustic waves is forbidden). In contrast to all known systems of this kind, a spectral transmission gap of nonzero width occurs here even with this simple structure. This is obtained by combining appropriately the zeros of transmission of the side resonators. Sharp resonant states inside the gaps can be achieved without introducing any defects in the structure. This results from an internal resonance of the structure when such a resonance is situated in the vicinity of a zero of transmission or placed between two zeros of transmission, the so-called Fano resonances. A general analytical expression for the transmission coefficient is given for various systems of this kind within the framework of the Green's function method. The amplitude and the phase of the transmission are discussed as a function of frequency and it is shown that the width of the stop bands is very sensitive to the number of side resonators. These results should have important consequences for the suppression of low-frequency noise and for designing filters.

Noise-induced mixed-mode oscillations in a relaxation oscillator near the onset of a limit cycle.

Abstract: A detailed asymptotic study of the effect of small Gaussian white noise on a relaxation oscillator undergoing a supercritical Hopf bifurcation is presented. The analysis reveals an intricate stochastic bifurcation leading to several kinds of noise-driven mixed-mode oscillations at different levels of amplitude of the noise. In the limit of strong time-scale separation, five different scaling regimes for the noise amplitude are identified. As the noise amplitude is decreased, the dynamics of the system goes from the limit cycle due to self-induced stochastic resonance to the coherence resonance limit cycle, then to bursting relaxation oscillations, followed by rare clusters of several relaxation cycles (spikes), and finally to small-amplitude oscillations (or stable fixed point) with sporadic single spikes. These scenarios are corroborated by numerical simulations.

Contribution of dielectrics to frequency and noise of NbTiN superconducting resonators

Abstract: We study NbTiN resonators by measurements of the temperature dependent resonance frequency and frequency noise. Additionally, resonators are studied covered with SiOx dielectric layers of various thicknesses. The resonance frequency develops a nonmonotonic temperature dependence with increasing SiOx layer thickness. The increase in the noise is independent of the SiOx thickness, demonstrating that the noise is not dominantly related to the low temperature resonance frequency deviations.

Temperature dependence of the frequency and noise of superconducting coplanar waveguide resonators

Abstract: We present measurements of the temperature and power dependence of the resonance frequency and frequency noise of superconducting niobium thin-film coplanar waveguide resonators carried out at temperatures well below the superconducting transition (Tc=9.2 K). The noise decreases by nearly two orders of magnitude as the temperature is increased from 120 to 1200 mK, while the variation of the resonance frequency with temperature over this range agrees well with the standard two-level system (TLS) model for amorphous dielectrics. These results support the hypothesis that TLSs are responsible for the noise in superconducting microresonators and have important implications for resonator applications such as qubits and photon detectors.

ACOUSTICS2008/460 Reconstruction of Rayleigh-Lamb dispersion spectrum based on noise obtained from an air-jet forcing

Abstract: The time-domain cross-correlation of incoherent and random noise recorded by a series of passive sensors contains the impulse response of the medium between these sensors. By using noise generated by a can of compressed air sprayed on the surface of a plexiglass plate, we are able to reconstruct not only the time of flight but the whole waveforms between the sensors. From the reconstruction of the direct A0 and S0 waves, we derive the dispersion curves of the flexural waves, thus estimating the mechanical properties of the material without a conventional electromechanical source. The dense array of receivers employed here allow a precise frequency-wavenumber study of flexural waves, along with a thorough evaluation of the rate of convergence of the correlation with respect to the record length, the frequency, and the distance between the receivers. The reconstruction of the actual amplitude and attenuation of the impulse response is also addressed in this paper [Larose et al, J. Acoust. Soc. Am 122(2007)].

Dynamics of Quantum Noise in a Tunnel Junction under ac Excitation

Abstract: We report the first measurement of the dynamical response of shot noise (measured at frequency omega) of a tunnel junction to an ac excitation at frequency omega0. The experiment is performed in the quantum regime, variant Planck's over 2piomega approximately variant Planck's over 2piomega0>>kBT at very low temperature T=35 mK and high frequency omega0/2pi=6.2 GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.

Does a Model-Scale Nozzle Emit the Same Jet Noise as a Jet Engine?

Abstract: A careful experimental study has been carried out to verify whether a model-scale nozzle produces the same jet noise as a jet engine, with the twin goals of 1) validating the practice of carrying out jet noise research at model scale for full-scale applications, and 2) aiding the identification of jet noise in measured total spectra from jet engines, which have contributions from several noise sources. A special test was carried out to measure pure jet noise from a low-bypass ratio jet engine at all the angles. Aeroacoustic measurements were made from model-scale nozzles at the Boeing Low-Speed Aeroacoustic Facility at the same operating conditions as the jet engine. Many issues that are pertinent for the comparison of scaled model data with engine data have been investigated thoroughly. These include the requirements of the instrumentation system for model and engine tests, suitable methodology for the calculation of the atmospheric attenuation coefficients, propagation effects, data repeatability, scaling of jet noise spectra, effects of the disparate Reynolds numbers and mixing perimeters between model nozzles and engines, etc. The data indicate that the weather corrections are accurate even at very high frequencies of interest in model tests. A new phenomenon of nonlinear propagation effects is identified for subsonic heated jets when the convective Mach number exceeds unity. The convective Mach number is shown to be a better indicator of nonlinear processes than high sound amplitudes,as has been believed in the past As demonstrated, with the proper scaling factors, both narrowband and one-third-octave jet spectra can be collapsed. The concept of increasing the shear perimeter for achieving better mixing and hence reducing noise has no merit, even at supercritical pressure ratios. Excellent agreement between scaled model and engine spectra is demonstrated at all angles and frequencies for a variety of power levels; thus, it is possible to acquire high-quality spectra at high frequencies, provided proper care is taken. Finally, the major conclusion from the preceding results is that jet noise research carried out at model scale is applicable to jet engines.

Avalanche Noise Characteristics in Submicron InP Diodes

Abstract: We report excess noise factors measured on a series of InP diodes with varying avalanche region thickness, covering a wide electric field range from 180 to 850 kV/cm. The increased significance of dead space in diodes with thin avalanche region thickness decreases the excess noise. An excess noise factor of F = 3.5 at multiplication factor M = 10 was measured, the lowest value reported so far for InP. The electric field dependence of impact ionization coefficients and threshold energies in InP have been determined using a non-local model to take into account the dead space effects. This work suggests that further optimization of InP separate absorption multiplication avalanche photodiodes (SAM APDs) could result in a noise performance comparable to InAlAs SAM APDs.

Probing noise in flux qubits via macroscopic resonant tunneling.

Abstract: Macroscopic resonant tunneling between the two lowest lying states of a bistable rf SQUID is used to characterize noise in a flux qubit. Measurements of the incoherent decay rate as a function of flux bias revealed a Gaussian-shaped profile that is not peaked at the resonance point but is shifted to a bias at which the initial well is higher than the target well. The rms amplitude of the noise, which is proportional to the dephasing rate 1/tauphi, was observed to be weakly dependent on temperature below 70 mK. Analysis of these results indicates that the dominant source of low energy flux noise in this device is a quantum mechanical environment in thermal equilibrium.

Recent results for plasma antennasa)

Abstract: Plasma antennas are just as effective as metal antennas. They can transmit, receive, and reflect radio waves just as well as metal antennas. In addition, plasma generated noise does not appear to be a problem.

Two microphone noise reduction system

Abstract: A two microphone noise reduction system is described. In an embodiment, input signals from each of the microphones are divided into subbands and each subband is then filtered independently to separate noise and desired signals and to suppress non-stationary and stationary noise. Filtering methods used include adaptive decorrelation filtering. A post-processing module using adaptive noise cancellation like filtering algorithms may be used to further suppress stationary and non-stationary noise in the output signals from the adaptive decorrelation filtering and a single microphone noise reduction algorithm may be used to further provide optimal stationary noise reduction performance of the system.

Seasonal Anisotropy in Short-Period Seismic Noise Recorded in South Asia

Abstract: We present an analysis of seismic noise recorded during 1995–2004 by a medium-aperture, short-period seismic array located in Chiang Mai, Thailand (CMAR). We calculated frequency-wavenumber spectra for nearly 1000 randomly selected time windows, each with a length of 160 sec. At frequencies above about 1.4 Hz the noise is unorganized and the wavenumber spectra are isotropic and diffuse; however, at lower frequencies three robust wavenumber peaks exist. Two of the peaks have phase velocities centered near 4.0 km/sec, consistent with higher-mode Rayleigh waves, while the third peak has much higher apparent velocity (>25 km/sec), consistent with body waves that have interacted with the Earth’s core ( PKP , PcP ). All three peaks are strongly seasonal with annual power variations of 10–20 dB, and all show excellent correlation in their putative source regions with ocean wave heights derived from TOPEX/POSEIDON satellite tracks. To the best of our knowledge, this is the first time such a high-velocity component of seismic noise has been consistently observed. The presence of this high-velocity peak raises the possibility of using ambient noise to image the Earth’s lower mantle and core.

Phonon-plasmon coupled-mode lifetime in semiconductors

Abstract: Raman scattering measurements of the lifetime of hot phonons in GaN show a decrease with increasing electron concentration. Density-dependent lifetimes have also been deduced from noise measurements of AlGaN/GaN channels. We suggest that the results from Raman scattering can be understood by the frequency dependence of the anharmonic interaction for coupled modes and that the results from noise measurements can be understood qualitatively by the anharmonic interaction plus the migration of coupled modes.

Secure Chaotic Transmission on a Free-Space Optics Data Link

Abstract: In this paper, we numerically demonstrate secure data transmission, using synchronized ldquotwinrdquo semiconductor lasers working in the chaotic regime, which represent the transmitter and receiver of a cryptographic scheme, compatible with free-space optics technology for line-of-sight communication links. Chaotic dynamics and synchronization are obtained by current injection into the laser pair of a common, chaotic driving-signal. Results of simulations are reported for the configuration in which the chaotic driving-current is obtained by photodetection of the emission of a third laser (driver), chaotic by delayed optical feedback in a short cavity scheme, selected with different parameters with respect to the laser pair. The emissions of the synchronized, matched lasers are highly correlated, whereas their correlation with the driver is low. The digital message modulates the pumping current of the transmitter. Message recovery is performed by subtracting the chaos, locally generated by the synchronized receiver laser, from the signal obtained by photodetection (at the receiver side) of the chaos-masked message transmitted in free space. Simulations have been performed with the Lang-Kobayashi model, keeping into account both attenuation of the optical signal in a line-of-sight configuration, and noise. Security has been investigated and demonstrated by considering the effect, on synchronization and message recovery, of the parameter mismatch between transmitter and receiver.

Short Note Seasonal Anisotropy in Short-Period Seismic Noise Recorded in South Asia

Abstract: We present an analysis of seismic noise recorded during 1995-2004 by a medium-aperture, short-period seismic array located in Chiang Mai, Thailand (CMAR). We calculated frequency-wavenumber spectra for nearly 1000 randomly selected time windows, each with a length of 160 sec. At frequencies above about 1.4 Hz the noise is unorganized and the wavenumber spectra are isotropic and dif- fuse; however, at lower frequencies three robust wavenumber peaks exist. Two of the peaks have phase velocities centered near 4:0 km=sec, consistent with higher- mode Rayleigh waves, while the third peak has much higher apparent velocity (>25 km=sec), consistent with body waves that have interacted with the Earth's core (PKP, PcP). All three peaks are strongly seasonal with annual power variations of 10- 20 dB, and all show excellent correlation in their putative source regions with ocean wave heights derived from TOPEX/POSEIDON satellite tracks. To the best of our knowledge, this is the first time such a high-velocity component of seismic noise has been consistently observed. The presence of this high-velocity peak raises the possibility of using ambient noise to image the Earth's lower mantle and core.