Showing papers on "Noise (radio) published in 1998"

Broadband seismology and noise under the ocean

Abstract: Most of our understanding of the Earth's interior has been derived from measurements from global seismic networks, although no network has ever been truly “global” because some 71% of the Earth's surface is underwater. The resulting gaps in coverage produce a biased and incomplete image of the Earth. Work has begun toward establishing permanent observatories on the deep seafloor, although the technical difficulties remain severe. Will these stations be useful, and where and how shall they be established? Data from seafloor observatories will be of poorer quality than continental site data because the sea surface is an important and local source of broadband noise. This noise is derived from wind and waves through direct forcing at long periods and by nonlinear coupling to elastic waves at short periods. Our understanding of the generation and propagation of seismic noise and of wind and wave climatology can be used to predict the temporal and geographical variability of the noise spectrum and to assess likely sites for permanent seafloor observatories. High noise levels near 1 Hz may raise detection limits for short-period, teleseismic arrivals above mb = 7.5, limiting the usefulness of many seafloor sites. Noise levels in deep boreholes will be 10 dB quieter than those at the seafloor, but sensors buried short distances below the seafloor may also provide comparable noise levels and fidelity. The retrieval of data from permanent seafloor observatories remains an unsolved problem, but long-term temporary arrays of ocean bottom seismometers are now being used in regional scale experiments using earthquakes as sources. Such experiments are likely to be less successful in the Pacific basin than in either the Indian Ocean or North Atlantic Ocean because of higher noise levels.

Integrated satellite interferometry: Tropospheric noise, GPS estimates and implications for interferometric synthetic aperture radar products

Abstract: Interferometric synthetic aperture radar (INSAR), like other astronomic and space geodetic techniques, is limited by the spatially and temporally variable delay of electromagnetic waves propagating through the neutral atmosphere. Statistical analysis of these variations, from a wide variety of instruments, reveals a power law dependence on frequency that is characteristic of elementary (Kolmogorov) turbulence. A statistical model for a major component of the delay fluctuations, the “wet” component, has previously been developed by Treuhaft and Lanyi [1987] for very long baseline interferometry. A continuous Global Positioning System (GPS) network is now in place in southern California that allows estimation of, along with geodetic parameters, the total delay due to the atmosphere above each site on a subhourly basis. These measurements are shown to conform to the Treuhaft and Lanyi (TL) statistical model both temporally and spatially. The TL statistical model is applied to the problem of INSAR and used to produce the covariance between two points separated in time and/or space. The error, due to the atmospheric variations, for SAR products such as topography and surface deformation is calculated via propagation of errors. There are two methods commonly cited to reduce the effect of atmospheric distortion in products from SAR interferometry, stacking and calibration. Stacking involves averaging independent interferograms to reduce the noise. Calibration involves removing part (or all) of the delay using data from an independent source such as total zenith delay estimates from continuous GPS networks. Despite the relatively poor spatial density of surface measurements, calibration can be used to reduce noise if the measurements are sufficiently accurate. Reduction in tropospheric noise increases with increasing number of measurement points and increasing accuracy up to a maximum of √N, where N is the number of points. Stacking and calibration are shown to be complementary and can be used simultaneously to reduce the noise to below that achievable by either method alone.

Jet Noise: Since 1952

Abstract: Jet noise research was initiated by Sir James Lighthill in 1952. Since that time, the development of jet noise theory has followed a very tortuous path. This is, perhaps, not surprising for the understanding of jet noise is inherently tied to the understanding of turbulence in jet flows. Even now, our understanding of turbulence is still tenuous. In the fifties, turbulence was regarded as consisting of a random assortment of small eddies. As a result, the primary focus of jet noise research was to quantify the noise from fine-scale turbulence. This line of work persisted into the eighties. The discovery of large turbulence structures in free shear flows in the early seventies led some investigators to begin questioning the validity of the then established theories. Some went further to suggest that, for high-speed jets, it was the large turbulence structures/instability waves of the flow that were responsible for the dominant part of jet mixing noise. Development of a quantitative theory of noise from large turbulence structures/instability waves took place during the next 15 years. Precision instrumentation and facilities for jet noise measurements became available in the mid-eighties. This permitted a large bank of high-quality narrow band jet noise data to be gathered over the subsequent years. Recent analysis of these data has provided irrefutable evidence that jet noise, in fact, is made up of two basic components; one from the large turbulence structures/instability waves, the other from the fine-scale turbulence. This is true even for subsonic jets. In this paper, some of the crucial research results of the past 44 years, that form the basis of our present understanding of jet noise generation and propagation, are discussed.

Noise-supported travelling waves in sub-excitable media

Abstract: The detection of weak signals of nonlinear dynamical systems in noisy environments may improve with increasing noise, reaching an optimal level before the signal is overwhelmed by the noise This phenomenon, known as stochastic resonance1,2, has been characterized in electronic3, laser4, magnetoelastic5, physical6 and chemical7 systems Studies of stochastic resonance and noise effects in biological8,9 and excitable dynamical systems10,11,12,13 have attracted particular interest, because of the possibility of noise-supported signal transmission in neuronal tissue and other excitable biological media Here we report the positive influence of noise on wave propagation in a photosensitive Belousov–Zhabotinsky14,15,16,17 reaction The chemical medium, which is sub-excitable and unable to support sustained wave propagation, is illuminated with light that is spatially partitioned into an array of cells in which the intensity is randomly varied Wave propagation is enhanced with increasing noise amplitude, and sustained propagation is achieved at an optimal level Above this level, only fragmented waves are observed

Conducted electromagnetic emissions in induction motor drive systems. I. Time domain analysis and identification of dominant modes

Abstract: Stray components distributed in a pulsewidth modulation (PWM) drive system form parts of resonant circuits which can be excited to produce radio frequency (RF) noise driven by the pulsed switching action of the power devices. The dynamic response of such circuits is complex. It is essential to identify the dominant oscillation modes in the system so that electromagnetic interference (EMI) reduction techniques can be effectively implemented. This paper (Part I) investigates the mechanisms of conducted EMI emissions associated with a typical PWM inverter induction motor drive system. A numerical model, which includes the high-frequency effects within the machine, is established to evaluate the emissions in the time domain. The dominant high-frequency current paths are identified, and this allows the oscillation frequencies to be predicted from knowledge of the component values. The analysis is confirmed using laboratory measurements. Simplified frequency domain methods for direct calculation of the emission spectra based on the dominant high-frequency current paths are discussed in Part II.

Observations of shock waves in cloud cavitation

Abstract: This paper describes an investigation of the dynamics and acoustics of cloud cavitation, the structures which are often formed by the periodic breakup and collapse of a sheet or vortex cavity. This form of cavitation frequently causes severe noise and damage, though the precise mechanism responsible for the enhancement of these adverse effects is not fully understood. In this paper, we investigate the large impulsive surface pressures generated by this type of cavitation and correlate these with the images from high-speed motion pictures. This reveals that several types of propagating structures (shock waves) are formed in a collapsing cloud and dictate the dynamics and acoustics of collapse. One type of shock wave structure is associated with the coherent collapse of a well-defined and separate cloud when it is convected into a region of higher pressure. This type of global structure causes the largest impulsive pressures and radiated noise. But two other types of structure, termed 'crescent-shaped regions' and 'leading-edge structures' occur during the less-coherent collapse of clouds. These local events are smaller and therefore produce less radiated noise but the interior pressure pulse magnitudes are almost as large as those produced by the global events. The ubiquity and severity of these propagating shock wave structures provides a new perspective on the mechanisms reponsible for noise and damage in cavitating flows involving clouds of bubbles. It would appear that shock wave dynamics rather than the collapse dynamics of single bubbles determine the damage and noise in many cavitating flows.

Effect of sea roughness on bistatically scattered range coded signals from the Global Positioning System

Abstract: A series of aircraft experiments was performed using a specialized GPS receiver and a nadir-oriented left hand circularly polarized antenna. This apparatus received reflections of the GPS signals from water surfaces under a variety of sea states. The cross-correlation between the reflected signal and a reference pseudo-random noise code was recorded as a function of the relative time delay. The shape of this function showed a dependence on the roughness of the reflecting surface. This dependence generally followed that predicted by theory for bistatic scattering of range-coded signals. Use of this information as a remote sensing technique for the determination of sea state is discussed.

Dynamics of noise-induced heating in atom traps

Abstract: A Fokker-Planck equation is derived for the energy distribution of atoms in a three-dimensional harmonic trap with fluctuations in the spring constant and the equilibrium position. Using this model, we predict trap lifetimes based on the measurable noise spectra of the fluctuations. The energy distributions evolve into a single eigenmode where the apparent temperature of the distribution remains constant while the population decays as a consequence of the energy input. The method of analysis and the corresponding results are applicable to any optical, magnetic, or ion trap that is approximately harmonic, and offer useful insights into both noise-induced and optical heating processes. @S1050-2947~98!05211-1# PACS number~s!: 32.80.Pj I. INTRODUCTION Stable atom traps have diverse applications in quantum optics ranging from studies of Bose-Einstein condensates ~BEC’s !@ 1‐3# in magnetic traps to quantum computing @4# in ion traps. Optical far-off-resonance traps have been demonstrated for confining arbitrary atomic ground states in nearly identical potentials @5,6# and for optical lattices @7#. Multiple spin condensates have been stored in a shallow faroff-resonance trap to study interactions in the quantum degenerate regime @8#. These traps also offer an attractive possibility for investigating weakly interacting atomic fermions where multiple atomic states are required for S-wave scattering @9#. In all of these applications, achievement of long storage times is of great importance. For some time it has been appreciated that fluctuations in the trap parameters can cause atom heating, and that the resulting trap loss limits the maximum storage period. This problem has been circumvented in BEC experiments by using magnetic traps with relatively low trap resonance frequencies, so that fluctuations in the potential are only weakly transmitted to the atoms. Consequently, adequate mechanical and power supply stability are not too difficult to achieve. In contrast, far-off-resonance optical traps with high resonance frequencies can be very sensitive to laser intensity fluctuations and beam-pointing noise which cause trap fluctuations and subsequent heating. However, the conditions on the trap stability needed to achieve long storage times have not been carefully studied. In a recent paper, we considered fluctuations in a far-offresonance optical trap using a simple harmonic-oscillator model @10#. Heating rates were estimated in terms of the intensity and position noise power spectra measured for an argon ion laser. Intensity noise causes fluctuations in the spring constant and results in exponential heating, while beam-pointing noise causes fluctuations in the center of the trap and leads to heating at a constant rate. It was shown that achieving heating time constants well beyond 10 sec imposes stringent requirements on the trap stability. Since this model is applicable to any harmonic trap, the results apply equally well to magnetic, optical, and ion traps, and provide estimates of the expected heating time scales. In this paper, we apply the fluctuating harmonic-oscillator model to noise-induced trap dynamics by deriving a FokkerPlanck equation for the energy distribution of atoms in a three-dimensional fluctuating trap. Numerical modeling for a trap with a finite depth yields estimates of the atom loss rate and the average energy for a number of loading conditions. We begin by reviewing the results of our previous paper for the heating rates, and then derive an approximate FokkerPlanck equation that describes the evolution of the atom energy distribution in the trap.

Logistic Avalanche Processes, Elementary Time Structures, and Frequency Distributions in Solar Flares

Abstract: We analyze the elementary time structures (on timescales of ? 0.1-3.0 s) and their frequency distributions in solar flares using hard X-ray (HXR) data from the Compton Gamma Ray Observatory (CGRO) and radio data from the radio spectrometers of Eidgenoessische Technische Hochschule (ETH) Zurich. The four analyzed data sets are gathered from over 600 different solar flares and include about (1) 104 HXR pulses at ?25 and ?50 keV, (2) 4000 radio type III bursts, (3) 4000 pulses of decimetric quasi-periodic broadband pulsation events, and (4) 104 elements of decimetric millisecond spike events. The time profiles of resolved elementary time structures have a near-Gaussian shape and can be modeled with the logistic equation, which provides a quantitative measurement of the exponential growth time ?G and the nonlinear saturation energy level WS of the underlying instability. Assuming a random distribution (Poisson statistics) of saturation times tS (with an e-folding constant tSe), the resulting frequency distribution of saturation energies WS or peak energy dissipation rates FS = (dW/dt) -->t = tS has the form of a power-law function, i.e., N(F -->S)F$−{α}S$ --> , where the power-law index ? is directly related to the number of e-folding amplifications by the relation ? = (1 + ?G/tSe). The same mathematical formalism is used to generate power-law distributions, as in Rosner & Vaiana, but the distribution of energies released in elementary flare instabilities is not related to the global energy storage process. We assume Poissonian noise for the unamplified energy levels in unstable flare cells, implying an exponential frequency distribution of avalanche energies WS or fluxes FS in the absence of coherent amplifications. Also, in the case of coherent amplification, the Poissonian noise introduces exponential rollovers of the power law at the low and high ends of the frequency distributions. We fit both power-law and exponential functions to the observed frequency distributions of elementary pulse fluxes N(F) in each flare separately. For HXR pulses, one-half of the flares are better fitted with power-law frequency distributions, demanding coherent amplification of the underlying energy dissipation mechanism, e.g., current exponentiation occurring in the magnetic tearing instability. The majority of type III burst flares are best fitted with power-law distributions, consistent with the interpretation in terms of beam-driven coherent emission. The frequency distributions of decimetric pulsations and decimetric millisecond spikes are found to fit exponential functions, in contrast to the expected power laws for coherent emission mechanisms generally proposed for these radio burst types. A coherent emission mechanism can be reconciled with the observed exponential frequency distributions only if nonlinear saturation occurs at a fixed amplification factor for all elementary pulses or spikes, for example, if it is produced by an oscillatory compact source (in the case of decimetric pulsations) or by a fragmented source with similar spatial cell sizes (in the case of decimetric millisecond spikes).

Evidence for Scale-Scale Correlations in the Cosmic Microwave Background Radiation

Abstract: We perform a discrete wavelet analysis of the Cosmic Background Explorer differential microwave radiometer (DMR) 4-yr sky maps and find a significant scale-scale correlation on angular scales from about 11\ifmmode^\circ\else\textdegree\fi{} to 22\ifmmode^\circ\else\textdegree\fi{}, only in the DMR face centered on the north galactic pole. This non-Gaussian signature does not arise either from the known foregrounds or the correlated noise maps, nor is it consistent with upper limits on the residual systematic errors in the DMR maps. Either the scale-scale correlations are caused by an unknown foreground contaminate or systematic errors on angular scales as large as 22\ifmmode^\circ\else\textdegree\fi{}, or the standard inflation plus cold dark matter paradigm is ruled out at the $g99%$ confidence level.

Accuracy and noise in optical Doppler tomography studied by Monte Carlo simulation.

Abstract: A Monte Carlo model has been developed for optical Doppler tomography (ODT) within the framework of a model for optical coherence tomography (OCT). A phantom situation represented by blood flowing in a horizontal 100 microm diameter vessel placed at 250 microm axial depth in 2% intralipid solution was implemented for the Monte Carlo simulation, and a similar configuration used for experimental ODT measurements in the laboratory. Simulated depth profiles through the centre of the vessel of average Doppler frequency demonstrated an accuracy of 3-4% deviation in frequency values and position localization of flow borders, compared with true values. Stochastic Doppler frequency noise was experimentally observed as a shadowing in regions underneath the vessel and also seen in simulated Doppler frequency depth profiles. By Monte Carlo simulation, this Doppler noise was shown to represent a nearly constant level over an investigated 100 microm interval of depth underneath the vessel. The noise level was essentially independent of the numerical aperture of the detector and angle between the flow velocity and the direction of observation, as long as this angle was larger than 60 degrees. Since this angle determines the magnitude of the Doppler frequency for backscattering from the flow region, this means that the signal-to-noise ratio between Doppler signal from the flow region to Doppler noise from regions underneath the flow is improved by decreasing the angle between the flow direction and direction of observation. Doppler noise values from Monte Carlo simulations were compared with values from statistical analysis.

Observation of noise and dissipation effects on dynamical localization

Abstract: We observe the effects of noise and dissipation on dynamical localization. Our system consists of cold cesium atoms in a pulsed standing wave of light, and is an experimental realization of the $\ensuremath{\delta}$-kicked rotor. We compare the effects of amplitude noise with those of spontaneous scattering. The experimental signature in both cases is an increased growth in energy, with qualitatively similar momentum distributions.

Asymmetry in Velocity and Intensity Helioseismic Spectra: A Solution to a Long-standing Puzzle

Abstract: We give an explanation for the opposite sense of asymmetry of the solar acoustic mode lines in velocity and intensity oscillation power spectra, thereby solving the half-decade-old puzzle of Duvall and coworkers. The solution came after comparing the velocity and intensity oscillation data of medium angular degree l obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory with the theoretical power spectra. We conclude that the solar noise in the velocity and intensity spectra is made up of two components: one is correlated to the source that is responsible for driving the solar p-modes, and the other is an additive uncorrelated background. The correlated component of the noise affects the line profiles. The asymmetry of the intensity spectrum is reversed because the correlated component is of a sufficiently large level, while the asymmetry of the velocity spectrum remains unreversed because the correlated component is smaller. This also explains the high-frequency shift between velocity and intensity at and above the acoustic cutoff frequency. A composite source consisting of a monopole term (mass term) and a dipole term (force due to Reynolds stress) is found to explain the observed spectra when it is located in the zone of superadiabatic convection at a depth of 75±50 km below the photosphere.

Thermal radiation and amplified spontaneous emission from a random medium

Abstract: We compute the statistics of thermal emission from systems in which the radiation is scattered chaotically, by relating the photocount distribution to the scattering matrix---whose statistical properties are known from random-matrix theory. We find that the super-Poissonian noise is that of a blackbody with a reduced number of degrees of freedom. The general theory is applied to a disordered slab and to a chaotic cavity, and is extended to include amplifying as well as absorbing systems. We predict an excess noise of amplified spontaneous emission in a random laser below the laser threshold.

Analysis of the accuracy of a destriping method for future cosmic microwave background mapping with the PLANCK SURVEYOR satellite

Abstract: A major problem in cosmic microwave back- ground radiation (CMBR) anisotropy measurements is the presence of low-frequency noise in the data streams. This noise arises from thermal instabilities of optical elements or of the thermal bath, gain instabilities and 1=f noise in the electronics, and other poorly understood processes. If improperly monitored or processed, this excess low- frequency noise might lead to striping in the maps, com- promising the success of the experiment. In this paper, we show that a simple destriping method will clean the maps obtained with the High Frequency Instrument of the PLANCK SURVEYOR mission of any signicant ad- ditional noise from low-frequency drifts, provided that the knee frequency of the low frequency noise is less than the spinning frequency of the satellite, i.e. fknee 0:017 Hz. For the High Frequency Instrument of PLANCK, the nom- inal knee frequency of the noise is fknee' 0:01 Hz or less, and thus no signicant striping nor increase of the noise rms is expected due to low-frequency drifts. In addition, we show that even if the knee frequency of the low fre- quency noise were somewhat higher than the spinning fre- quency of the satellite one could estimate and remove the striping with a excellent accuracy.

Method and system for determining position of mobile radio terminals

Abstract: A method and apparatus are disclosed for determining more accurately, in a radio environment with time dispersion, the distance between a radio receiver (100) and radio transmitter (130), by special processing of received radio signals that have been transmitted repeatedly from the same radio transmitter (130) and are possibly subject to multipath propagation. The Time of Arrival (TOA) of the received radio signals is repeatedly estimated using channel power profiles (step 402). A TOA value near the minimum occurring TOA is selected (step 402) wherein each estimated TOA is derived from incoherent integration of a randomly chosen number of the received bursts having the same known bit sequence (step 404), in order to eliminate the influence of noise.

Fluctuational phase-flip transitions in parametrically driven oscillators

Abstract: We analyze the rates of noise-induced transitions between period-two attractors. The model investigated is an underdamped oscillator parametrically driven by a field at nearly twice the oscillator eigenfrequency. The activation energy of the transitions is analyzed as a function of frequency detuning and field amplitude scaled by the damping and nonlinearity parameters of the oscillator. Both fourth- and sixth-order nonlinearities are taken into account. The parameter ranges where the system is bistable and tristable are investigated. Explicit results are obtained in the limit of small damping, or equivalently, strong driving, including scaling near bifurcation points. @S1063-651X~98!15405-3# functional gives the exponent in the expression for the es- cape rate. An advantageous feature of period doubling in an under- damped oscillator is that it occurs for comparatively small amplitudes F of the driving force, where the nonlinearity of the oscillator is still small: the anharmonic part of the poten- tial energy is much less than the harmonic one, v 0 q 2 /2, where q is the oscillator coordinate. In this case, the quanti- ties of interest are the amplitude and phase of the vibrations at the frequency v F/2'v 0 . They vary only a little over the time ;v F1 . The corresponding dynamics is affected by Fourier components of the noise within a narrow band cen- tered at v F/2. Essentially, this means that, in the analysis of the dynamics of slow variables, the noise may be assumed to be white. A similar situation arises @12# in the problem of transitions between the stable states of forced vibrations of a resonantly driven underdamped oscillator. Below, in Sec. II, we discuss the phase portrait of a driven Duffing oscillator ~with the fourth-order nonlinearity! in the rotating frame. We then derive the properties of noise for slow variables. For low noise intensities, we formulate and solve numerically the variational problem for the activation energy of escape from period-two states. In Sec. III, explicit expressions are provided for the escape rates in the vicinities of the bifurcation points where there emerge period-two at- tractors ~a supercritical bifurcation! or unstable period-two states. The analysis in Sec. IV refers to comparatively strong driving, where the motion in slow variables is underdamped. Explicit analytical results for escape activation energies are obtained in limiting cases and compared with numerical re- sults. In Sec. V the role of sixth-order nonlinearity is dis- cussed, and the activation energies are found near bifurcation points, and also in the range where sixth-order nonlinearity is strong and the motion in slow variables is underdamped. Section VI contains concluding remarks.

Conquering noise in deep-submicron digital ICs

Abstract: As feature sizes decrease and clock frequencies increase, noise is becoming a greater concern in digital IC design. The authors describe a verification metric, noise stability, which guarantees functionality in the presence of noise, and a CAD technique, static noise analysis, for applying this metric on a chipwide basis.

Origin of conductivity and low-frequency noise in reverse-biased GaN p-n junction

Abstract: We study the origins of conductivity and low-frequency noise in GaN p-n junctions under reverse bias. Carrier hopping through defect states in the space charge region is identified as the main mechanism responsible for low bias conductivity. Threading dislocations appear the most likely source of such defect states. At higher bias hopping is supplemented with Poole–Frenkel emission. A relatively high level of 1/f-like noise is observed in the diode current. The bias and temperature dependencies of the noise current are investigated.

Phase noise in CMOS differential LC oscillators

Abstract: An analysis of phase noise in differential cross-coupled tuned tank voltage controlled oscillators is presented. The effect of active device noise sources as well as the noise due to the passive elements are taken into account. The predictions are in good agreement with the measurements for different tail currents and supply voltages. The effect of the complementary cross-coupled pair is analyzed and verified experimentally. A 1.8 GHz LC oscillator with a phase noise of -121 dBc/Hz at 600 kHz is demonstrated, dissipating 6 mW of power using spiral inductors.

Nanometer Scale Dielectric Fluctuations at the Glass Transition

Abstract: Using noncontact scanning probe microscopy techniques, dielectric properties were studied on 50 nm length scales in polyvinyl-acetate films in the vicinity of the glass transition. Low frequency $(1/f)$ noise observed in the measurements was shown to arise from thermal fluctuations of the electric polarization. Anomalous variations observed in the noise spectrum provide direct evidence for cooperative nanoregions with heterogeneous kinetics. The cooperative length scale was determined. Heterogeneity was long lived only well below the glass transition for faster than average processes.

Noise-induced coherent oscillations in randomly connected neural networks

Abstract: We study the dynamics of randomly connected excitatory networks of excitable spike-response neuron models. Large networks can exhibit a nonmonotonic collective response to a stimulation when the coupling strength between neurons lies within an appropriate range. With such a coupling, noise imposed upon neurons induces synchronization of the units and oscillations of the network activity, consisting of a succession of bursts. Furthermore, the regularity of this rhythmic activity goes through a maximum as the noise amplitude is increased. This nonmonotonic dependence on the noise amplitude relies on the fact that noise acts in two antagonistic ways. Noise of low amplitude shortens the interval between two successive bursts, leading to an increase of the dynamics regularity, whereas noise of strong amplitude deteriorates the regularity of the dynamics during a burst. In order to study the influence of the noise amplitude and the coupling on the generation of collective oscillations quantitatively, we consider a simpler network model of excitable units. We derive a discrete map, including the noise amplitude and the coupling strength as parameters, which describes the network dynamics in the limit of a large number of neurons. This map reproduces all characteristic features of the activity dynamics obtained with simulated networks. From the analysis of the bifurcation structure of this map, we obtain parameter regions where noise-induced oscillations occur. Using this map we also study the effect of the network connectivity on the generation of oscillations. We show that such noise-induced coherent oscillations in fully connected networks are related to special initial conditions, and are sensitive to perturbations, whereas they can be the only asymptotically stable regime in sparsely connected networks.

Noise in Al single electron transistors of stacked design

Abstract: We have fabricated and examined several Al single electron transistors whose small islands were positioned on top of a counterelectrode and hence did not come into contact with a dielectric substrate. The equivalent charge noise figure of all transistors turned out to be surprisingly low, (2.5–7)×10−5e/Hz at f=10 Hz. Although the lowest detected noise originates mostly from fluctuations of background charge, the noise contribution of the tunnel junction conductances was, on occasion, found to be dominant.

Noise Induced Pattern Transition and Spatiotemporal Stochastic Resonance

Abstract: The influence of parametric noise on the spatiotemporal dynamics of a two-variable reaction-diffusion model, describing pattern formation in excitable media, is investigated. When a control parameter is perturbed by stochastic force with suitable intensity, noise induced pattern transition between ``single'' spiral wave and ``double'' spiral waves, which does not occur in the absence of noise, is observed. If this parameter also varies periodically in time around the bifurcation point, we find that the output signal-to-noise ratios show maxima with the variation of noise intensity, indicating the occurrence of spatiotemporal stochastic resonance.

Influence of Nozzle Geometry on the Noise of High-Speed Jets

Abstract: The effectiveness of jet noise reduction by the use of different nozzle exit geometry is examined. Because there will be thrust loss associated with a nozzle of complex geometry, consideration is confined to practical configurations with reasonably small thrust loss. Only jets with a single stream are considered. The nozzle configurations examined are circular, elliptic, and rectangular. Plug nozzles as well as a suppressor nozzle are included. It is shown that the measured turbulent mixing noise of the jets from these nozzles consists of two independent components. The noise spectrum of each component is found to fit the shape of a seemingly universal similarity spectrum. It is also found that the maximum levels of the fitted noise power spectra of the jets are nearly the same. This finding suggests that nozzle geometry modification may not be an effective method for jet noise suppression.