Showing papers on "Noise (radio) published in 2007"

Low-noise fiber-laser frequency combs (Invited)

Abstract: We discuss experimental and theoretical aspects of a low-noise fiber-laser frequency comb, including the experimental configuration and the major contributions to the frequency noise and linewidth of the individual comb modes. Intracavity noise sources acting on the mode-locked laser determine the free-running comb linewidth and include environmental changes, pump noise, and amplified spontaneous emission (ASE). Extracavity noise sources acting outside of the laser typically determine the signal-to-noise ratio on the comb lines and include environmental effects, shot noise, and noise generated during supercontinuum generation. Feedback strongly suppresses these intracavity noise contributions, yielding a system that operates with comb linewidths and timing jitter below the quantum limit set by the intracavity ASE. Finally, we discuss correlations in the residual noise across a phase-locked comb.

Three-Year Wilkinson Microwave Anisotropy Probe (WMAP)* Observations: Beam Profiles, Data Processing, Radiometer Characterization, and Systematic Error Limits

Abstract: The WMAP satellite has completed 3 years of observations of the cosmic microwave background radiation. The 3yeardataproductsincludeseveralsetsof fullskymapsoftheStokesI,Q,andUparametersinfivefrequencybands, spanning 23Y94 GHz, and supporting items such as beam window functions and noise covariance matrices. The processing used to produce the current sky maps and supporting products represents a significant advancement over the first-year analysis and is described herein. Improvements to the pointing reconstruction, radiometer gain modeling, window function determination, and radiometer spectral noise parameterization are presented. A detailed description of the updated data processing that produces maximum likelihood sky map estimates is presented, along withthemethodsusedtoproducereducedresolutionmapsandcorrespondingnoisecovariancematrices.Finally,two methods used to evaluate the noise of the full resolution sky maps are presented along with several representativeyear-to-yearnulltests,demonstratingthatskymapsproducedfromdatafromdifferentobservationalepochsare consistent. Subject headingg cosmic microwave background — instrumentation: detectors — space vehicles: instruments

Active Control of a Mach 0.9 Jet for Noise Mitigation Using Plasma Actuators

Abstract: longer time before decaying gradually. The saturation and decay of the seeded perturbations moved farther upstream as their Strouhal number was increased. Seeded perturbations with higher azimuthal modes exhibited faster decay. Particle image velocimetry results showed that when exciting the jet’s preferred-mode instability at lower azimuthal modes, the jet potential core was shortened and the turbulent kinetic energy was increased significantly. At higher Strouhal numbers and higher azimuthal modes, forcing had less of an impact on the mean velocity and turbulent kinetic energy. Far-field acoustic results showed a significant noise increase (2 to 4 dB) when thejetisexcitedaroundthejet’spreferred-modeinstabilityStrouhalnumber(StDF 0:2–0:5),inagreementwiththe results in the literature. Noise reduction of 0.5 to over 1.0 dB is observed over a large excitation Strouhal number range; this reduction seems to peak around StDF 1:5 to 2.0 at a 30-deg angle, but around StDF 3:0 to 3.5 at a 90deg angle. Although forcing the jet with higher azimuthal modes is advantageous for noise mitigation at a 30-deg angleandlowerStrouhalnumbers,theeffectisnotasclearathigherforcingStrouhalnumbersandata90-degangle.

1/f Flux Noise in Josephson Phase Qubits

Abstract: We present a new method to measure 1/f noise in Josephson quantum bits (qubits) that yields low-frequency spectra below 1 Hz. A comparison of the noise taken at positive and negative bias of a phase qubit shows the dominant noise source to be flux noise and not junction critical-current noise, with a magnitude similar to that measured previously in other systems. Theoretical calculations show that the level of flux noise is not compatible with the standard model of noise from two-level state defects in the surface oxides of the films.

High speed engine gas thermometry by Fourier-domain mode-locked laser absorption spectroscopy.

Abstract: We present a novel method for low noise, high-speed, real-time spectroscopy to monitor molecular absorption spectra. The system is based on a rapidly swept, narrowband CW Fourier-domain mode-locked (FDML) laser source for spectral encoding in time and an optically time-multiplexed split-pulse data acquisition system for improved noise performance and sensitivity. An acquisition speed of ∼100 kHz, a spectral resolution better than 0.1 nm over a wavelength range of ∼1335–1373 nm and a relative noise level of ∼5 mOD (∼1% minimum detectable base-e absorbance) are achieved. The system is applied for crank-angle-resolved gas thermometry by H2O absorption spectroscopy in an engine motoring at 600 and 900 rpm with a precision of ∼1%. Influences of various noise sources such as laser phase and intensity noise, trigger and synchronization jitter in the electronic detection system, and the accuracy of available H2O absorption databases are discussed.

Traveltime measurements from noise correlation: stability and detection of instrumental time-shifts

Abstract: We test the feasibility of using Green's functions extracted from records of ambient seismic noise to monitor temporal changes in the Earth crust properties by repeated measurements at regional distances. We use about 11 yr of continuous recordings to extract surface waves between three pairs of stations in California. The correlations are computed in a moving 1-month window and we analyse the temporal evolution of measured interstation traveltimes. The comparison of the arrival times in the positive and negative correlation time of Rayleigh and Love waves allows us to separate time-shifts associated with any form of physical change in the medium, those resulting from clock drift or other instrumental errors, and those due to change in the localization of the noise sources. This separation is based on the principle of time symmetry. When possible, we perform our analysis in two different period bands: 5–10 and 10–20 s. The results indicate that significant instrumental time errors (0.5 s) are present in the data. These time-shifts can be measured and tested by closure relation and finally corrected independently of any velocity model. The traveltime series show a periodic oscillation that we interpret as the signature of the seasonal variation of the region of origin of the seismic noise. Between 1999 and 2005, the final arrival time fluctuations have a variance of the order of 0.01 s. This allows us to measure interstation traveltimes with errors smaller than 0.3 per cent of the interstation traveltime and smaller than 1 per cent of the used wave period. This level of accuracy was not sufficient to detect clear physical variation of crustal velocity during the considered 11 yr between the three stations in California. Such changes may be more easily detectable when considering pairs of stations more closely located to each other and in the vicinity of tectonically active faults or volcanoes.

Tandem Cylinder Noise Predictions

Abstract: In an effort to better understand landing-gear noise sources, we have been examining a simplified configuration that still maintains some of the salient features of landing-gear flow fields. In particular, tandem cylinders have been studied because they model a variety of component level interactions. The present effort is directed at the case of two identical cylinders spatially separated in the streamwise direction by 3.7 diameters. Experimental measurements from the Basic Aerodynamic Research Tunnel (BART) and Quiet Flow Facility (QFF) at NASA Langley Research Center (LaRC) have provided steady surface pressures, detailed off-surface measurements of the flow field using Particle Image Velocimetry (PIV), hot-wire measurements in the wake of the rear cylinder, unsteady surface pressure data, and the radiated noise. The experiments were conducted at a Reynolds number of 166 105 based on the cylinder diameter. A trip was used on the upstream cylinder to insure a fully turbulent shedding process and simulate the effects of a high Reynolds number flow. The parallel computational effort uses the three-dimensional Navier-Stokes solver CFL3D with a hybrid, zonal turbulence model that turns off the turbulence production term everywhere except in a narrow ring surrounding solid surfaces. The current calculations further explore the influence of the grid resolution and spanwise extent on the flow and associated radiated noise. Extensive comparisons with the experimental data are used to assess the ability of the computations to simulate the details of the flow. The results show that the pressure fluctuations on the upstream cylinder, caused by vortex shedding, are smaller than those generated on the downstream cylinder by wake interaction. Consequently, the downstream cylinder dominates the noise radiation, producing an overall directivity pattern that is similar to that of an isolated cylinder. Only calculations based on the full length of the model span were able to capture the complete decay in the spanwise correlation, thereby producing reasonable noise radiation levels.

Experimental Study of the Spectral Properties of Near-Field and Far-Field Jet Noise

Abstract: The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ Mj ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound source localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical...

Feedforward Interference Cancellation in Radio Receiver Front-Ends

Abstract: An interference cancellation technique is described for improving the dynamic range of receivers. A feedforward approach is used to attenuate large interferers before the down-conversion mixer in a receiver. This is accomplished with no measurable impact on the in-band noise performance. Techniques to cancel interference within a narrowband and also in multiple bands are described. Simulation results and measurements from a discrete prototype system are used to validate the approach.

Signal-to-Noise Performance of Two Analog Photonic Links Using Different Noise Reduction Techniques

Abstract: We demonstrate two analog photonic links that use different noise reduction techniques to achieve high gain and low noise figure without electronic amplification. Both links use a high-power, low-noise master oscillator power amplifier as the optical source, a balanced-bridge dual-output LiNbO3 Mach-Zehnder modulator with a record low Vpi = 1.33 V at 12 GHz, and either one or two high-power rear-illuminated photodetectors. In the first link, both outputs of the quadrature-biased modulator are used to illuminate two photodetectors configured for laser noise cancellation, yielding record high gain (> 17.0 dB) and low noise figure ( 12.7 dB) but also record low noise figure (< 5.7 dB) across this same frequency band.

Combining adaptive noise and look-ahead in local search for SAT

Abstract: The adaptive noise mechanism was introduced in Novelty+ to automatically adapt noise settings during the search [4]. The local search algorithm G2WSAT deterministically exploits promising decreasing variables to reduce randomness and consequently the dependence on noise parameters. In this paper, we first integrate the adaptive noise mechanism in G2WSAT to obtain an algorithm adaptG2WSAT, whose performance suggests that the deterministic exploitation of promising decreasing variables cooperates well with this mechanism. Then, we propose an approach that uses look-ahead for promising decreasing variables to further reinforce this cooperation. We implement this approach in adaptG2WSAT, resulting in a new local search algorithm called adaptG2WSATP. Without any manual noise or other parameter tuning, adaptG2WSATP shows generally good performance, compared with G2WSAT with approximately optimal static noise settings, or is sometimes even better than G2WSAT. In addition, adaptG2WSATP is favorably compared with state-of-the-art local search algorithms such as R+adaptNovelty+ and VW.

The theory of a continuous damped vibration absorber to reduce broad-band wave propagation in beams

Abstract: In order to attenuate structural waves in beams, a damped mass-spring absorber system is considered that is attached continuously along the beam length. Compared with other measures, such as impedance changes or tuned neutralisers applied at a single point, it is effective for excitation at any location along the beam. Although it is a tuned system, it can also be designed to be effective over a broad frequency range by the use of a high damping loss factor and multiple tuning frequencies. It has the advantage over constrained layer damping treatments that it can be effective even when the structural wavelength is long. The parameters controlling its behaviour are investigated and simple formulae developed, allowing optimisation of its performance. A particular application is the reduction of noise from a railway track, which requires the attenuation of structural waves along the rail to be increased typically in the frequency range 500 to 2000 Hz

Theoretical Evaluation of Several Possible Along-Track InSAR Modes of TerraSAR-X for Ocean Current Measurements

Abstract: The German satellite TerraSAR-X, scheduled for launch in late 2006, will permit high-resolution ocean current measurements by along-track interferometric SAR (along-track InSAR) in various experimental modes of operation, using different sections of its X-band SAR antenna array with a total length of 4.8 m as individual receive antennas. Depending on antenna and receive-chain settings, effective InSAR time lags of about 0.17 to 0.29 ms can be realized in combination with different noise levels, single-look resolutions, swath widths, and incidence angles. We give an overview of the characteristics of the possible InSAR modes and evaluate their suitability for current measurements on the basis of simulated data products. Our results indicate that the quality of interferometric stripmap data from TerraSAR-X will be clearly superior to the quality of the existing data acquired over the Dutch coast during the Shuttle Radar Topography Mission; accurate current retrievals can be expected at effective spatial resolutions on the order of 500 m. However, in modes using a multiplexed single receive chain, the effective swath width of stripmap data will be limited to only 15 km, while dual receive-chain operation offers a swath width of 30 km for stripmap data and promises a reasonable data quality even for ScanSAR data with a maximum swath width of 100 km. Finally, we consider fundamental relations between along-track baseline, instrument noise, and resulting InSAR phase noise to discuss the potential for current measuring performance improvements of TerraSAR-X follow-on satellites

Influence of the seismic noise characteristics on noise correlations in the Baltic shield

Abstract: It has recently been shown that correlations of seismic noise can contain significant information about the Green's function along the station profile. Using an array of 38 temporary broad-band stations located in Finland between 1998 September and 1999 March, we study the resulting 703 noise correlations to understand how they are influenced by the directivity of the noise field. The latter information is obtained through f-k analysis of data from two permanent seismic arrays in Germany and Norway and from a subset of stations of the array in Finland. Both types of analysis confirm that the characteristics of the seismic noise is strongly frequency-dependent. At low frequencies (0.02–0.04 Hz), we observe diffuse noise and/or randomly distributed sources. In contrast, the noise is strongly direction-dependent and not fully diffuse in the intermediate period ranges (0.04–0.25 Hz) which correspond to the first and second microseismic peak, created at the Irish and Scottish coast and the western coast of Norway. In this frequency interval the noise is sufficiently close to a plane wave to introduce systematic errors in group velocities for station pairs which are not parallel to the direction of the dominant incident noise. Phase velocities calculated by slant stack over many traces are however correct, independently of profile direction. In the high-frequency band (0.25–1.0 Hz), the situation is a mix between the low-frequency and the intermediate frequency cases. Average phase velocities and individual group velocities from well-oriented profiles are in excellent agreement with results from Rayleigh wave studies of the area.

Terahertz wave emission and detection using photoconductive antennas made on low-temperature-grown InGaAs with 1.56μm pulse excitation

Abstract: Photoconductive antennas made on low-temperature-grown Be doped InxGa1−xAs (0.45⩽x⩽0.53) have been investigated focusing on the terahertz emission properties. In the antenna of x=0.45, the resistance as high as 3MΩ enabled us to increase the bias field up to 60kV∕cm, and the terahertz waves emitted from the antenna were significantly enhanced. In addition, terahertz waves with the spectral range over 2.5THz and the peak to noise ratio of 45dB were generated and detected using only 1.56μm pulses.

Theory of collective firing induced by noise or diversity in excitable media

Abstract: Large variety of physical, chemical, and biological systems show excitable behavior, characterized by a nonlinear response under external perturbations: only perturbations exceeding a threshold induce a full system response (firing). It has been reported that in coupled excitable identical systems noise may induce the simultaneous firing of a macroscopic fraction of units. However, a comprehensive understanding of the role of noise and that of natural diversity present in realistic systems is still lacking. Here we develop a theory for the emergence of collective firings in nonidentical excitable systems subject to noise. Three different dynamical regimes arise: subthreshold motion, where all elements remain confined near the fixed point; coherent pulsations, where a macroscopic fraction fire simultaneously; and incoherent pulsations, where units fire in a disordered fashion. We also show that the mechanism for collective firing is generic: it arises from degradation of entrainment originated either by noise or by diversity.

Excess Avalanche Noise in $\hbox{In}_{0.52}\hbox{Al}_{0.48}\hbox{As}$

Abstract: Avalanche multiplication and excess noise arising from both electron and hole injection have been measured on a series of In0.52Al0.48As p+-i-n+ and n +-i-p+ diodes with nominal avalanche region widths between 0.1 and 2.5 mum. With pure electron injection, low excess noise was measured at values corresponding to effective k=beta/alpha between 0.15 and 0.25 for all widths. Enabled ionization coefficients were deduced using a non-local ionization model utilizing recurrence equation techniques covering an electric field range from approximately 200 kV/cm to 1 MV/cm

Influence of lasers propagation delay on the sensitivity of atom interferometers

Abstract: In atom interferometers based on two photon transitions, the delay induced by the difference of the laser beams paths makes the interferometer sensitive to the fluctuations of the frequency of the lasers. We first study, in the general case, how the laser frequency noise affects the performance of the interferometer measurement. Our calculations are compared with the measurements performed on our cold atom gravimeter based on stimulated Raman transitions. We finally extend this study to the case of cold atom gradiometers.

Low Noise 1 THz–1.4 THz Mixers Using Nb/Al-AlN/NbTiN SIS Junctions

Abstract: We present the development of a low noise 1.2 THz and 1.4 THz SIS mixers for heterodyne spectrometry on the Stratospheric Observatory For Infrared Astronomy (SOFIA) and Herschel Space Observatory. This frequency range is above the limit for the commonly used Nb quasi particle SIS junctions, and a special type of hybrid Nb/AlN/NbTiN junctions has been developed for this project. We are using a quasi-optical mixer design with two Nb/AlN/NbTiN junctions with an area of 0.25. The SIS junction tuning circuit is made of Nb and gold wire layers. At 1.13 THz the minimum SIS receiver uncorrected noise temperature is 450 K. The SIS receiver noise corrected for the loss in the LO coupler and in the cryostat optics is 350-450 K across 1.1-1.25 THz band. The receiver has a uniform sensitivity in a full 4-8 GHz IF band. The 1.4 THz SIS receiver test at 1.33-1.35 THz gives promising results, although limited by the level of available LO power. Extrapolation of the data obtained with low LO power level shows a possibility to reach 500 K DSB receiver noise using already existing SIS mixer.

Low frequency noise characterizations of ZnO nanowire field effect transistors

Abstract: We fabricated ZnO nanowire field effect transistors (FETs) and systematically characterized their low frequency (f) noise properties. The obtained noise power spectra showed a classical 1∕f dependence. A Hooge’s constant of 5×10−3 was estimated from the gate dependence of the noise amplitude. This value is within the range reported for complementary metal-oxide semiconductor (CMOS) FETs with high-k dielectrics, supporting the concept that nanowires can be utilized for future beyond-CMOS electronic applications from the point of view of device noise properties. ZnO FETs measured in a dry O2 environment displayed elevated noise levels that can be attributed to increased fluctuations associated with O2− on the nanowire surfaces.

Autocorrelation and Cross-Correlation Estimators of Polarimetric Variables

Abstract: Herein are proposed novel estimators of differential reflectivity ZDR and correlation coefficient ρhv between horizontally and vertically polarized echoes. The estimators use autocorrelations and cross correlations of the returned signals to avoid bias by omnipresent but varying white noise. These estimators are considered for implementation on the future polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) network. On the current network the reflectivity factor is measured at signal-to-noise ratios (SNRs) as low as 2 dB and the same threshold is expected to hold for the polarimetric variables. At such low SNR and all the way up to SNR = 15 dB, the conventional estimators of differential reflectivity and the copolar correlation coefficient are prone to errors due to uncertainties in noise levels caused by instability of radar devices, thermal radiations of precipitation and the ground, and wideband radiation of electrically active clouds. Noise variations at SNR less than 15 dB can bias...

Microscopic Model of Critical Current Noise in Josephson Junctions

Abstract: We present a simple microscopic model to show how fluctuating two-level systems in a Josephson junction tunnel barrier of thickness $L$ can modify the potential energy of the barrier and produce critical current noise spectra. We find low frequency $1/f$ noise that goes as ${L}^{5}$. Our values are in good agreement with recent experimental measurements of critical current noise in $\mathrm{Al}/{\mathrm{AlO}}_{x}/\mathrm{Al}$ Josephson junctions. We also investigate the sensitivity of the noise on the nonuniformity of the tunnel barrier.

Fast quantum noise in the Landau-Zener transition

Abstract: We show by direct calculation starting from a microscopic model that the two-state system with time-dependent energy levels in the presence of fast quantum noise obeys the master equation. The solution of master equation is found analytically and analyzed in a broad range of parameters. The fast transverse noise affects the transition probability during much longer time (the accumulation time) than the longitudinal one. The action of the fast longitudinal noise is restricted by the shorter Landau-Zener time, the same as in the regular Landau-Zener process. The large ratio of time scales allows solving the Landau-Zener problem with longitudinal noise only, and then solving the same problem with the transverse noise only and matching the two solutions. The correlation of the longitudinal and transverse noise renormalizes the Landau-Zener transition matrix element and can strongly enhance the survival probability, whereas the transverse noise always reduces it. If the noise is fast, its intensity at which the multiquantum processes become essential corresponds to a deeply adiabatic regime. We briefly discuss possible applications of the general theory to the molecular magnets.

Density and Magnetic Field Signatures of Interplanetary 1/f Noise

Abstract: We investigate the occurrence of 1/f noise in the interplanetary density and the magnetic field at varying heliocentric latitudes. The characteristic spectral amplitudes can be found in Ulysses density and magnetic data in the expected frequency ranges at all available latitudes, ranging from the ecliptic plane to more than 80°. Average spectra indicate a latitudinal variation, with a 1/f density signal becoming more pronounced in higher latitude bands. Azimuthal spectral analysis of solar magnetogram data using the SOHO Michelson Doppler Interferometer also shows 1/f noise in the photospheric magnetic field, most clearly at high latitude. Accordingly, we discuss possibilities that the 1/f signal arises at varying altitudes, possibly surviving coronal dynamics. This raises questions that may be addressed in future studies using spectroscopic, white light, and radio scintillation data.

Use of Ambient Noise: From Spectral Amplitude Variability to H/V Stability

Abstract: The study of the variation over time of both spectral amplitudes and H/V curves, has been performed on three different sites, two close to cities and one in the countryside, during periods varying from week to over a month. It demonstrates the robustness of the H/V technique to give consistent peak frequency values. In particular, H/V peak frequencies, either fundamental (f0) or natural (fx, x•1), are not affected by weather nor the level of human activity. However, while fundamental H/V peak amplitudes are stable, they proved rather unstable for natural (secondary) peak. Spectral amplitude curves are very variable but follow human activity cycles from week-week end and day-night variations down to a very small scale, such as lunch breaks. Finally, the frequency limit between anthropic noise and natural noise, commonly taken at 1 Hz, is not straightforward and is varying from site to site from 0.7–0.8 Hz up to 2–3 Hz.

Steady-State Noise Analysis of Spontaneous and Stimulated Brillouin Scattering in Optical Fibers

Abstract: In this paper, we present a steady-state theoretical and experimental analysis of the noise resulting from spontaneous and stimulated Brillouin scattering in an optical fiber. Wave equations are derived and solved that describe the evolution of the pump and Stokes waves along the fiber. Experiments and numerical results demonstrate the validity of the theory, either when a Stokes wave is launched at the end of the fiber or when the noise in the Stokes wave is spontaneously generated

Noise correlations in a Coulomb-blockaded quantum dot.

Abstract: We report measurements of current noise auto- and cross correlation in a tunable quantum dot with two or three leads. As the Coulomb blockade is lifted at finite source-drain bias, the autocorrelation evolves from super- to sub-Poissonian in the two-lead case, and the cross correlation evolves from positive to negative in the three-lead case, consistent with transport through multiple levels. Cross correlations in the three-lead dot are found to be proportional to the noise in excess of the Poissonian value in the limit of weak output tunneling.