Showing papers on "Noise (radio) published in 2004"

Emergence of broadband Rayleigh waves from correlations of the ambient seismic noise.

Abstract: [1] We demonstrate that the coherent information about the Earth structure can be extracted from the ambient seismic noise We compute cross-correlations of vertical component records of several days of seismic noise at different pairs of stations separated by distances from about one hundred to more than two thousand kilometers Coherent broadband dispersive wavetrains clearly emerge with group velocities similar to those predicted from the global Rayleigh-wave tomographic maps that have been constrained using ballistic surface waves Those results show that coherent Rayleigh waves can be extracted from the ambient seismic noise and that their dispersion characteristics can be measured in a broad range of periods This provides a source for new types of surface-wave measurements that can be obtained for numerous paths that could not be sampled with the ballistic waves and, therefore, can significantly improve the resolution of seismic images

Bolometer noise: nonequilibrium theory

Abstract: New theoretical results for noise in cryogenic bolometers are derived. Johnson noise is reduced by as much as 60% by electrothermal feedback from the bias supply. Phonon noise in the thermal link is reduced by as much as 30% relative to the usual equilibrium formula. Photon noise in the Rayleigh-Jeans limit is computed with attention to the attenuation of the photon correlations in the light beam. Basic results on bolometer responsivity, time constant, and thermal properties are presented in a new and convenient form. Excess 1/f and contact shot noise are also discussed.

The color of environmental noise

Abstract: Biological populations are strongly influenced by the random variation in their environment. The spectrum of frequencies in noise is particularly important to dynamics and persistence. Here we present an analysis of the variance spectra of a wide variety of long-term time series of environmental variables. Spectra were well approximated by the inverse power law 1/fβ within the appropriate range of frequencies f; however, the majority of spectra were “flattened” at low frequencies. With some qualification we found the spectral exponents (β) to corroborate an earlier suggestion that terrestrial noise tends to be “white” (β < 0.5), while marine environments tend to be “red” (β ≈ 1) or “brown” (β ≈ 2). As well, we found a tendency for whiter noise in temperate latitudes than in either high or low latitudes. These results have wide-ranging consequences for ecosystem fragility and species conservation.

Excitations of incoherent spin-waves due to spin-transfer torque.

Abstract: The possibility of exciting microwave oscillations in a nanomagnet by a spin-polarized current, as predicted by Slonczewski and Berger, has recently been demonstrated. This observation opens important prospects of applications in radiofrequency components. However, some unresolved inconsistencies are found when interpreting the magnetization dynamics within the coherent spin-torque model. In some cases, the telegraph noise caused by spin-currents could not be quantitatively described by that model. This has led to controversy about the need for an effective magnetic temperature model. Here we interpret the experimental results of Kiselev et al. using micromagnetic simulations. We point out the key role played by incoherent spin-wave excitation due to spin-transfer torque. The incoherence is caused by spatial inhomogeneities in local fields generating distributions of local precession frequencies. We observe telegraph noise with gigahertz frequencies at zero temperature. This is a consequence of the chaotic dynamics and is associated with transitions between attraction wells in phase space.

Electromagnetic interference (EMI) reduction from printed circuit boards (PCB) using electromagnetic bandgap structures

Abstract: As digital circuits become faster and more powerful, direct radiation from the power bus of their printed circuit boards (PCB) becomes a major concern for electromagnetic compatibility engineers. In such multilayer PCBs, the power and ground planes act as radiating microstrip patch antennas, where radiation is caused by fringing electric fields at board edges. In this paper, we introduce an effective method for suppressing PCB radiation from their power bus over an ultrawide range of frequencies by using metallo-dielectric electromagnetic band-gap structures. More specifically, this study focuses on the suppression of radiation from parallel-plate bus structures in high-speed PCBs caused by switching noise, such as simultaneous switching noise, also known as Delta-I noise or ground bounce. This noise consists of unwanted voltage fluctuations on the power bus of a PCB due to resonance of the parallel-plate waveguiding system created by the power bus planes. The techniques introduced here are not limited to the suppression of switching noise and can be extended to any wave propagation between the plates of the power bus. Laboratory PCB prototypes were fabricated and tested revealing appreciable suppression of radiated noise over specific frequency bands of interest, thus, testifying to the effectiveness of this concept.

Excitations of incoherent spin-waves due to spin-transfer torque

Abstract: As predicted by Slonczewski and Berger, the possibility of exciting microwave oscillations in a nanomagnet by a spin-polarized current has been recently demonstrated. This observation opens very important perspectives of applications in RF components. However, some unresolved inconsistencies are found when interpreting the magnetization dynamics results within the coherent spin-torque model (CSM). In some cases, the telegraph noise caused by spin-currents could not be described quantitatively by the CSM. This led to controversies about the need of an effective magnetic temperature model (ETM). Here we interpret the experimental results of Kiselev et al. [Nature 425, 380 (2003)] using micromagnetic simulations. We point out the key role played by incoherent spin-waves excitation due to spin-transfer effects. The incoherence is caused by the spatial inhomogeneities of the local fields, generating a distribution of local precession frequencies. It results in telegraph noise at zero temperature associated with transitions between attraction wells in phase space.

Spectroscopy of spontaneous spin noise as a probe of spin dynamics and magnetic resonance

Abstract: Not all noise in experimental measurements is unwelcome. Certain fundamental noise sources contain valuable information about the system itself-a notable example being the inherent voltage fluctuations (Johnson noise) that exist across any resistor, which allow the temperature to be determined. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations. For example, statistical fluctuations of N paramagnetic spins should generate measurable noise of order N spins, even in zero magnetic field. Here we exploit this effect to perform perturbation-free magnetic resonance. We use off-resonant Faraday rotation to passively detect the magnetization noise in an equilibrium ensemble of paramagnetic alkali atoms; the random fluctuations generate spontaneous spin coherences that precess and decay with the same characteristic energy and timescales as the macroscopic magnetization of an intentionally polarized or driven ensemble. Correlation spectra of the measured spin noise reveal g-factors, nuclear spin, isotope abundance ratios, hyperfine splittings, nuclear moments and spin coherence lifetimes-without having to excite, optically pump or otherwise drive the system away from thermal equilibrium. These noise signatures scale inversely with interaction volume, suggesting a possible route towards non-perturbative, sourceless magnetic resonance of small systems.

Circuit and method for suppression of electromagnetic coupling and switching noise in multilayer printed circuit boards

Abstract: Apparatus for suppressing noise and electromagnetic coupling (3402) in the printed circuit board (3400) of an electronic device (3404) including an upper conductive plate (3420) and an array of conductive coplanar patches positioned a distance t2 from the upper conductive plate. The distance t2 is chosen to optimize capacitance between the conductive coplanar patches and the upper conductive plate for suppression of noise or electromagnetic coupling. The apparatus further includes a lower conductive plate (3422) at a distance tl from the array of conductive coplanar patches and conductive rods (3126) extending from respective patches to the lower conductive plate.

Low-frequency magnetic and resistance noise in magnetic tunnel junctions

Abstract: We have studied the voltage fluctuations of current-biased, micron-scale magnetic tunnel junctions. We find that the spectral power density is $1/f$-like at low frequencies and becomes frequency independent at high frequencies. The frequency-independent background noise is due to Johnson-Nyquist noise and shot noise mechanisms. The nature of the $1/f$ noise has its origin in two different mechanisms. In the magnetic hysteresis loops this noise power is strongly field-dependent and is due to thermal magnetization fluctuations in both the ``free'' and ``fixed'' magnetic layers. We attribute these magnetic fluctuations to thermally excited hopping of magnetic domain walls between pinning sites. At high temperatures, this magnetic noise is found to track the dc resistance susceptibility but it is not in quantitative agreement with the fluctuation dissipation relation, indicating that the magnetic structure is not in equilibrium. A second mechanism for the $1/f$ noise, connected with defects in the tunnel barrier but not related to the overall magnetization fluctuations, was found at fields for which the magnetic structure in the free and fixed layers is well aligned. We attribute this noise to electron trapping processes having thermally activated kinetics and a broad distribution of activation energies. Below $\ensuremath{\sim}25\mathrm{K}$ the noise power is temperature independent suggesting that the kinetics are dominated by tunneling. Our results show that the thermal stability of both the magnetic layers and the quality of the tunnel barrier are important factors in reducing the low-frequency noise in magnetic tunnel junctions.

Confusion Noise from LISA Capture Sources

Abstract: Captures of compact objects (COs) by massive black holes (MBHs) in galactic nuclei will be an important source for LISA, the space-based gravitational-wave (GW) detector. However, a large fraction of captures will not be individually resolvable--either because they are too distant, have unfavorable orientation, or have too many years to go before final plunge--and so will constitute a source of "confusion noise," obscuring other types of sources. Here we estimate the shape and overall magnitude of the spectrum of confusion noise from CO captures. The overall magnitude depends on the capture rates, which are rather uncertain, so we present results for a plausible range of rates. We show that the impact of capture confusion noise on the total LISA noise curve ranges from insignificant to modest, depending on these rates. Capture rates at the high end of estimated ranges would raise LISA's overall (effective) noise level by at most a factor \sim 2. While this slightly elevated noise level would somewhat decrease LISA's sensitivity to other classes of sources, overall, this would be a pleasant problem for LISA to have: It would also imply that detection rates for CO captures were at nearly their maximum possible levels (given LISA's baseline design). This paper includes several other results that should be useful in further studies of LISA capture sources, including (i) a calculation of the total GW energy output from generic inspirals into Kerr MBHs, and (ii) an approximate GW energy spectrum for a typical capture, and (iii) an estimate showing that in the population of detected capture sources, roughly half the white dwarfs and a third of the neutron stars will be detected when they still have >~10 years to go before final plunge.

The oceanic excitation hypothesis for the continuous oscillations of the Earth

Abstract: Continuous oscillations of the Earth are observed for frequencies between 2 and 7 mHz at almost every seismically quiet site in the world. These oscillations ride on a broad noise peak which spans the frequency band from 3 to 15 mHz and reaches its maximum at 7-9 mHz. We propose an oceanic excitation hypothesis which explains both the modal oscillations and the broad noise peak, specifically the action of oceanic infragravity waves on the solid Earth. Using the estimated amplitudes of oceanic infragravity waves from observation, although they are limited in number at the moment, we show that there is sufficient energy in these waves to excite observed seismic signals; for a given time window, the contribution from a small area (which may be as small as 100 km x 100 km) is all that is required to explain the seismic observations. The advantage of this oceanic mechanism over the previously proposed atmospheric mechanism is in the simultaneous explanation of the above two features in seismograms, whereas the atmospheric hypothesis has only explained the modal oscillations. The oceanic mechanism naturally explains the predominant 6-month periodicity as a result of semi-hemispheric ocean-wave activities in the Northern and Southern hemispheres, showing a good match between seismic data and satellite ocean-wave data both in the amplitude and phase of seasonal variation. Our Earth seems to be filled with ubiquitous propagating Rayleigh waves, generated directly by oceanic infragravity waves, for the frequency band 3-15 mHz.

Characterization and reduction of unexplained noise in superconducting transition-edge sensors

Abstract: The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance.

Systematic analysis of equatorial noise below the lower hybrid frequency

Abstract: . We report results of a systematic analysis of a large number of observations of equatorial noise between the local proton cyclotron frequency and the local lower hybrid frequency. The analysis is based on the data collected by the STAFF-SA instruments on board the four Cluster spacecraft. The data set covers their first two years of measurement in the equatorial magnetosphere at radial distances between 3.9 and 5 Earth radii. Inspection of 781 perigee passages shows that the occurrence rate of equatorial noise is approximately 60%. We identify equatorial noise by selecting data with nearly linearly polarized magnetic field fluctuations. These waves are found within 10° of the geomagnetic equator, consistent with the published past observations. Our results show that equatorial noise has the most intense magnetic field fluctuations among all the natural emissions in the given interval of frequencies and latitudes. Electric field fluctuations of equatorial noise are also more intense compared to the average of all detected waves. Equatorial noise thus can play a non-negligible role in the dynamics of the internal magnetosphere.

Testing alternative theories of gravity using LISA

Abstract: We investigate the possible bounds which could be placed on alternative theories of gravity using gravitational wave detection from inspiralling compact binaries with the proposed LISA space interferometer. Specifically, we estimate lower bounds on the coupling parameter ω of scalar–tensor theories of the Brans–Dicke type and on the Compton wavelength of the graviton λg in hypothetical massive graviton theories. In these theories, modifications of the gravitational radiation damping formulae or of the propagation of the waves translate into a change in the phase evolution of the observed gravitational waveform. We obtain the bounds through the technique of matched filtering, employing the LISA sensitivity curve generator (SCG), available online. For a non-spinning neutron star on a quasi-circular inspiral into a non-spinning 103M⊙ black hole in the Virgo Cluster, in a two-year integration, we find a lower bound ω > 3 × 105. For lower-mass black holes, the bound could be as large as 2 × 106. The bound is independent of LISA arm length, but is inversely proportional to the LISA position noise error, under the assumption that position error noise dominates laser shot noise. Lower bounds on the graviton Compton wavelength ranging from 1015 km to 5 × 1016 km can be obtained from one-year observations of massive binary black-hole inspirals at cosmological distances (3 Gpc) for masses ranging from 104 to 107M⊙. For the highest-mass systems (107M⊙), the bound is proportional to (LISA arm length)1/2 and to (LISA acceleration noise)−1/2. For the others, the bound is independent of these parameters because of the dominance of white-dwarf confusion noise in the relevant part of the frequency spectrum. These bounds improve and extend earlier work which used analytic formulae for the noise curves.

Simultaneous measurement of surface shape and variation in optical thickness of a transparent parallel plate in wavelength-scanning Fizeau interferometer

Abstract: Wavelength-scanning interferometry permits the simultaneous measurement of variations in surface shape and optical thickness of a nearly parallel plate. Interference signals from both surfaces of the test plate can be separated in frequency space; however, these frequencies are shifted from the expected frequency by the refractive-index dispersion of the test plate and any nonlinearity that is due to wavelength scanning. Conventional Fourier analysis is sensitive to this detuning of the signal frequency and suffers from multiple-beam interference noise. We propose new wavelength-scanning algorithms that permit a large tolerance for dispersion of the test plate and nonlinearity of scanning. Two 19-sample algorithms that suppress multiple-interference noise up to the second order of the reflectance of the test plate are presented. Experimental results show that the variation in surface shape and optical thickness of a glass parallel plate of 250-mm diameter was measured with a resolution of 1–2 nm rms.

Thermoelastic dissipation in inhomogeneous media: loss measurements and displacement noise in coated test masses for interferometric gravitational wave detectors

Abstract: The displacement noise in the test-mass mirrors of interferometric gravitational wave detectors is proportional to their elastic dissipation at the observation frequencies. In this paper, we analyze one fundamental source of dissipation in thin coatings, thermoelastic damping associated with the dissimilar thermal and elastic properties of the film and the substrate. We obtain expressions for the thermoelastic dissipation factor necessary to interpret resonant loss measurements, and for the spectral density of displacement noise imposed on a Gaussian beam reflected from the face of a coated mass. The predicted size of these effects is large enough to affect the interpretation of loss measurements, and to influence design choices in advanced gravitational wave detectors.

Enhancement of stability in randomly switching potential with metastable state

Abstract: The overdamped motion of a Brownian particle in randomly switching piece-wise metastable linear potential shows noise enhanced stability (NES): the noise stabilizes the metastable system and the system remains in this state for a longer time than in the absence of white noise. The mean first passage time (MFPT) has a maximum at a finite value of white noise intensity. The analytical expression of MFPT in terms of the white noise intensity, the parameters of the potential barrier, and of the dichotomous noise is derived. The conditions for the NES phenomenon and the parameter region where the effect can be observed are obtained. The mean first passage time behaviors as a function of the mean flipping rate of the potential for unstable and metastable initial configurations are also analyzed. We observe the resonant activation phenomenon for initial metastable configuration of the potential profile.

Silicon carbide for high resolution X-ray detectors operating up to 100°C

Abstract: This work presents experimental results on the possibility of high resolution X-ray spectroscopy in a wide temperature range, from room temperature up to 100°C without any cooling system, using Silicon Carbide (SiC) detectors. This capability arises from the very low noise of SiC detectors at high temperature because of their extremely low-leakage current density ( 20 pA/cm 2 at 24°C and 1 nA/cm 2 at 107°C with mean electric fields of 120 kV/cm ). Spectra of 241 Am acquired by a pixel SiC detector are reported with equivalent noise energies of 315 eV FWHM at 27°C and 797 eV FWHM at 100°C. The contributions of the different noise sources of the detector and of the front-end electronics are determined and analyzed. The potential for SiC X-ray detectors and open issues in SiC technology are highlighted.

Noise performance of the radio-frequency single-electron transistor

Abstract: We have analyzed a radio-frequency single-electron-transistor (RF-SET) circuit that includes a high-electron-mobility-transistor (HEMT) amplifier, coupled to the single-electron-transistor (SET) via an impedance transformer. We consider how power is transferred between different components of the circuit, model noise components, and analyze the operating conditions of practical importance. The results are compared with experimental data on SETs. Good agreement is obtained between our noise model and the experimental results. Our analysis shows, also, that the biggest improvement to the present RF-SETs will be achieved by increasing the charging energy and by lowering the HEMT amplifier noise contribution.

A comparison of the limits to the performance of thermal and photon detector imaging arrays

Abstract: Photon detectors are fundamentally limited by generation-recombination noise arising from photon exchange with a radiating background. Thermal detectors are fundamentally limited by temperature fluctuation noise arising from radiant power exchange with a radiating background. These two classes of detectors have differing dependencies of their detectivities upon their operating temperature and that of the radiating background. The limits favor photon detectors at shorter wavelengths (LWIR) and lower operating temperatures. Thermal detectors are favored at longer wavelengths (VLWIR) and higher operating temperatures.

Pattern Formation and Spatial Correlation Induced by the Noise in Two Competing Species

Abstract: We analyze the spatio-temporal patterns of two competing species in the presence of two white noise sources: an additive noise acting on the interaction parameter and a multiplicative noise which affects directly the dynamics of the species densities. We use a coupled map lattice (CML) with uniform initial conditions. We find a nonmonotonic behavior both of the pattern formation and the density correlation as a function of the multiplicative noise intensity.

Contrast sensitivity loss with aging: sampling efficiency and equivalent noise at different spatial frequencies.

Abstract: The relative contributions of optical and neural factors to the decrease in visual function with aging were investigated by measurement of contrast detection at three different spatial frequencies, in the presence of external noise, on young and older subjects. Contrast detection in noise functions allows two parameters to be measured: sampling efficiency, which indicates neural changes, and equivalent noise, which demonstrates optical effects. Contrast thresholds were measured in the presence of four levels (including zero) of externally added visual noise. Measurements were obtained from eight young and eight older visually normal observers. Compared with young subjects, older subjects showed significantly (p < 0.05) lower sampling efficiencies at spatial frequencies of 1 and 4 cycles per degree (c/deg) and significantly higher equivalent noise levels for gratings of 10 c/deg. Neural and optical factors affect contrast sensitivity loss with aging differently, depending on the spatial frequency tested, implying the existence of different mechanisms.

Jet noise prediction of cold and hot subsonic jets using large-eddy simulation

Abstract: The results of four large-eddy simulations of heated and unheated subsonic jets are presented. The computations were performed using one million grid points on jets at operating conditions originally investigated by Tanna in the late 1970s [Tanna, H. K., iAn Experimental Study of Jet Noise Part I: Turbulent Mixing Noise,i J. Sound Vib., Vol. 50, No. 3, pp. 405n428, 1977]. Two acoustic Mach numbers are investigated (U j=a1= 0:5 and 0:9) at cold (constant stagnation temperature) and heated conditions (T j=T1= 1:8 and T j=T1= 2:7). Changes in the jet mean elds with temperature and Mach number are found to be in agreement with published data. The far-eld overall sound pressure levels are shown to be consistent with experimental data and spectral comparisons indicate that the simulation sound elds are in agreement with measurements. The limited resolution of the computations is shown to impact the radiated sound by yielding e ectively low-pass lter ed versions of the experimental spectra.

A Review of rapid x-ray variability in x-ray binaries

Abstract: A comprehensive review of aperiodic rapid X-ray variability (QPO and noise) in X-ray binaries with neutron stars and black holes.

Phase, timing, and amplitude noise on supercontinua generated in microstructure fiber

Abstract: During supercontinuum formation in nonlinear fiber the presence of a noise seed on the input laser pulse can lead to significant excess noise on the generated output supercontinuum electric field. We relate pulse-averaged moments of this electric-field noise to the measured RF spectrum of the frequency comb formed by the supercontinuum. We present quantitative numerical results for the fundamental phase, timing, and amplitude noise on the frequency comb resulting from input quantum noise, including the scaling of the noise with different experimental parameters. This fundamental noise provides a lower limit to the phase stability of frequency combs that originate from microstructure fiber.

Modified kappa-epsilon Turbulence Model for Calculating Hot Jet Mean Flows and Noise

Abstract: The κ-e model has no provision to include the effect of the presence of a large-density gradient when used to compute the mean flow and noise of very hot jets. As a result, the model is found to give good predictions only when the jet is cold or moderately hot. Stability consideration indicates that a large density difference across a shear layer would generate strong spatial Kelvin-Helmholtz instabilities. Such instabilities would, invariably, lead to intense mixing in the jet shear layer. This, in turn, gives rise to an increase in turbulence intensity and jet spreading rate. A modification to the κ-e model is proposed to mimic these effects

MOSFET 1/f noise measurement under switched bias conditions

Abstract: Klumperink et al, have recently had a number of publications on the low frequency noise of MOSFETs under switched gate bias conditions. Since this is an important consideration in the low frequency noise in analog circuits with switching, we have investigated the signal processing technique used in some detail with respect to the data presented in the IEEE Electron Device Letters (vol.2, no.1, p. 43-46, 2000). No consideration was given to phase noise, a mixing with and modulation of the switched bias drain current by 1/f noise, in the analysis of the data. This can result in a response on the spectrum analyzer which corresponds very closely to the experimental data where the switched bias off gate voltage is near the threshold voltage. At low frequencies there will be just 1/f noise, then a plateau caused by the sum of 1/f noise and phase noise, a peak corresponding to the fundamental component of the switched bias, and at higher frequencies a phase noise in excess of the 1/f noise.

Reductions in Multi-Component Jet Noise by Water Injection

Abstract: An experimental investigation was performed in the NASA Langley Low Speed Aeroacoustics Wind Tunnel to determine the extent of jet exhaust noise reduction that can be obtained using water injection in a hot jet environment. The effects of water parameters such as mass flow rate, injection location, and spray patterns on suppression of dominant noise sources in both subsonic and supersonic jets were determined, and extrapolations to full-scale engine noise reduction were made. Water jets and sprays were injected in to the shear layers of cold and hot circular jets operating at both subsonic and supersonic exhaust conditions. Use of convergent-divergent and convergent nozzles (2.7in. D) allowed for simulations of all major jet noise sources. The experimental results show that water injection clearly disrupts shock noise sources within the jet plume, with large reductions in radiated shock noise. There are smaller reductions in jet mixing noise, resulting in only a small decrease in effective perceived noise level when projections are made to full scale. The fact that the measured noise reduction in the direction upstream of the nozzle was consistently larger than in the noisier downstream direction contributed to keeping effective perceived noise reductions small. Variations in the operation of the water injection system clearly show that injection at the nozzle exit rather than further downstream is required for the largest noise reduction. Noise reduction increased with water pressure as well as with its mass flow, although the type of injector had little effect.