Power Spectral Density Of Fluctuations Research Articles

Narrow-linewidth swept laser phase reconstruction and noise measurement technology and its applications.

A dynamic noise characterization technique for measuring narrow-linewidth frequency-sweep lasers based on phase reconstruction method is proposed. The phase and the frequency fluctuation power spectral density (PSD) of the swept optical field within a specific time window are recovered mainly by demodulating the differential phase information of the 120-degree phase difference interferometer. Then the details of the laser noise characteristics and the performance evolution law of the frequency sweep process can be observed by investigating the calculated frequency fluctuation PSD. Moreover, the integration time linewidth and Lorentzian linewidth of the swept frequency field can be obtained by introducing the integral algorithm even beyond the limit of PSD physical resolution. Meanwhile, the power of this method is verified by applying it to a kHz-linewidth frequency swept laser source based on high-order modulation-sideband injection-locking. The results show many features of the laser such as specific noise peaks and the laser characteristic evolution rules which could not be measured by other traditional methods.

전열관군에서 양력 변동의 PSD 특성 연구

전열관군에서 양력 변동의 PSD 특성 연구

Suppression of Low-Frequency Electronic Noise in Polymer Nanowire Field-Effect Transistors.

The authors report on the reduction of low-frequency noise in semiconductor polymer nanowires with respect to thin-films made of the same organic material. Flicker noise is experimentally investigated in polymer nanowires in the range of 10-10(5) Hz by means of field-effect transistor architectures. The noise in the devices is well described by the Hooge empirical model and exhibits an average Hooge constant, which describes the current power spectral density of fluctuations, suppressed by 1-2 orders of magnitude compared to thin-film devices. To explain the Hooge constant reduction, a resistor network model is developed, in which the organic semiconducting nanostructures or films are depicted through a two-dimensional network of resistors with a square-lattice structure, accounting for the different anisotropy and degree of structural disorder of the active nanowires and films. Results from modeling agree well with experimental findings. These results support enhanced structural order through size-confinement in organic nanostructures as effective route to improve the noise performance in polymer electronic devices.

Power Spectral Density Estimation for Wireless Fluctuation Enhanced Gas Sensor Nodes

Fluctuation enhanced sensing (FES) is a promising method to improve the selectivity and sensitivity of semiconductor and nanotechnology gas sensors. Most measurement setups include high cost signal conditioning and data acquisition units as well as intensive data processing. However, there are attempts to reduce the cost and energy consumption of the hardware and to find efficient processing methods for low cost wireless solutions. In this paper, we propose highly efficient signal processing methods to analyze the power spectral density of fluctuations. These support the development of ultra-low-power intelligent fluctuation enhanced wireless sensor nodes while several further applications are also possible.

Fluctuations of the number of adsorbed molecules in biosensors due to stochastic adsorption–desorption processes coupled with mass transfer

We derived a simple theory of fluctuations of the equilibrium number of adsorbed molecules in biosensors, caused by the stochastic nature of adsorption–desorption (AD) processes coupled with mass transfer. The two-compartment model is used for approximation of the spatial dependence of analyte concentration in the reaction chamber, which is justified when a thin layer depleted of the analyte exists close to the surface on which the binding reaction occurs. By using the obtained analytical expression for the power spectral density of fluctuations we perform for the first time the quantitative analysis of the influence of the mass transfer on the fluctuations spectrum. For realistic parameter values, the influence of mass transfer proved to be significant, causing the increase in the fluctuations level of up to two orders of magnitude compared to the rapid mixing case. The dependences of the mass transfer influenced fluctuations spectrum on various parameters of the analyte–receptor binding process are also systematically investigated. The presented theoretical model of fluctuations enables good estimation of the AD noise, which affects the total noise and the minimal detectable signal of biosensors. It provides the guidelines for improvement of the limits of detection, and for optimization of detection methods. The theory is also proposed as a basis for development of highly sensitive methods for analyte detection and characterization of biomolecular binding processes, based on the measured fluctuations spectrum. It is applicable for various types of sensors whose operation principle relies on the adsorption process of analyte molecules.

KEPLER OBSERVATIONS OF RAPID OPTICAL VARIABILITY IN ACTIVE GALACTIC NUCLEI

Over three quarters in 2010–2011, Kepler monitored optical emission from four active galactic nuclei (AGNs) with ∼30 minute sampling, >90% duty cycle, and ≲0.1% repeatability. These data determined the AGN optical fluctuation power spectral density (PSD) functions over a wide range in temporal frequency. Fits to these PSDs yielded power-law slopes of −2.6 to −3.3, much steeper than typically seen in the X-rays. We find evidence that individual AGNs exhibit intrinsically different PSD slopes. The steep PSD fits are a challenge to recent AGN variability models but seem consistent with first-order magnetorotational instability theoretical calculations of accretion disk fluctuations.

X‐Ray Variability Characteristics of the Seyfert 1 Galaxy NGC 3783

We have characterized the energy-dependent X-ray variability properties of the Seyfert 1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-Ray Timing Explorer data. The high-frequency fluctuation power spectral density function (PSD) slope is consistent with flattening toward higher energies. Light-curve cross-correlation functions yield no significant lags, but peak coefficients generally decrease as energy separation of the bands increases on both short and long timescales. We have measured the coherence between various X-ray bands over the temporal frequency range of 6 × 10-8-1 × 10-4 Hz; this range includes the temporal frequency of the low-frequency PSD break tentatively detected by Markowitz et al. and includes the lowest temporal frequency over which coherence has been measured in any active galactic nucleus to date. Coherence is generally near unity at these temporal frequencies, although it decreases slightly as energy separation of the bands increases. Temporal frequency-dependent phase lags are detected on short timescales; phase lags are consistent with increasing as energy separation increases or as temporal frequency decreases. All of these results are similar to those obtained previously for several Seyfert galaxies and stellar mass black hole systems. Qualitatively, these results are consistent with the variability models of Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly propagating variations in the local mass accretion rate.

Effect of stress on variability of systemic hemodynamics in rats of various genetic strains.

Power spectral density of heart rate fluctuations in the range of 0.02-5.00 Hz in August rats was lower than in Wistar rats. Changes in mean blood pressure and heart rate during stress (15-min immobilization) were similar in animals of both strains. As differentiated from Wistar rats, power spectral density of fluctuations in August rats considerably decreased after stress. August rats were characterized by low spectral power at rest and high resistance to the arrhythmogenic effect of 10-min acute myocardial ischemia.

Volume versus surface origin of 1/f noise in metals.

We have examined the question of whether excess low-frequency (1/f) noise in metal films arises in the bulk or at the sample surface. We measured the noise in continuous Cr films with 80 A\r{}2600 A\r{}, where t is the film thickness. Establishing that the relative noise intensity ${S}_{\ensuremath{\rho}}$(f)/${\ensuremath{\rho}}^{2}$\ensuremath{\propto}1/t supports the hypothesis that the noise is produced throughout the bulk; here, f is the frequency, \ensuremath{\rho} is the resistivity, and ${S}_{\ensuremath{\rho}}$(f) is the power spectral density of fluctuations in \ensuremath{\rho}. By considering a model of noise from the surface shunted by an underlying noise-free conductor volume, we devised a quantitative test which we used to refute the possibility that the noise arises only at the surface of our metal films.

Two-Group Theory of Space Dependent At-Power Reactor Noise

Theory of space-dependent at-power reactor noise is developed in order to evaluate the effect of spatial higher harmonics and feedback on the power spectral density of fluctuations. Their analyses in the framework of two-group diffusion model are important to understand the basic pattern of the power spectral density in normal operating condition. The stochastic integro-differential equations for fluctuations, which is a modified one-group model having the temperature feedback of fuel and coolant, are solved accordingly to Saito's solving procedure with use of the prompt spread approximation. All the transfer functions, which are relevant to contributing the power spectral density of fluctuation, are obtained in the form of modal expansion. The following information can be extracted from the obtained transfer functions: (1) the effect of the spatial higher harmonics on the profiles of the transfer functions plays an important role in connection with the global and local concept; (2) when feedback is relevant, the fluctuations can not be decomposed into two parts, viz. the global and the local components. Consequently, the explicit one-to-one correspondence between the noise components and the global and local concept can not be established.

Two-group theory of space dependent at-power reactor noise.

Theory of space-dependent at-power reactor noise is developed in order to evaluate the effect of spatial higher harmonics and feedback on the power spectral density of fluctuations. Their analyses in the framework of two-group diffusion model are important to understand the basic pattern of the power spectral density in normal operating condition. The stochastic integro-differential equations for fluctuations, which is a modified one-group model having the temperature feedback of fuel and coolant, are solved accordingly to Saito's solving procedure with use of the prompt spread approximation. All the transfer functions, which are relevant to contributing the power spectral density of fluctuation, are obtained in the form of modal expansion. The following information can be extracted from the obtained transfer functions: (1) the effect of the spatial higher harmonics on the profiles of the transfer functions plays an important role in connection with the global and local concept; (2) when feedback is relevant, the fluctuations can not be decomposed into two parts, viz. the global and the local components. Consequently, the explicit one-to-one correspondence between the noise components and the global and local concept can not be established.

Sensitivity of the Total Power Radiometer with Periodic Absolute Calibration

The total power radiometer is an attractive choice for imaging applications due to its high sensitivity and simple configuration. However, available theoretical results are inadequate to allow an accurate radiometer performance prediction in the presence of receiver gain fluctuations and other receiver characteristics with nonuniform fluctuation power spectra. An improved analysis of the total power radiometer Delta T is presented in terms of the receiver output fluctuation power spectral density and a transfer function due to postdetection filtering and periodic calibration. Verification of this analysis is obtained by measuring the fluctuation power spectrum of a 94-GHz receiver and comparing the predicted Delta T with a direct measurement. Numerical results including application to an example radiometer system are presented. These results indicate that the total power radiometer should function well in short integration time, periodically calibrated radiometer systems.

Crack propagation due to random thermal fluctuations: Effect of temporal incoherence

Metal components in LMFBR's are subjected to random temperature fluctuations. These are caused by the turbulence in the liquid sodium coolant and the existence of jets at differing inlet temperatures. Here an LEFM approach to crack propagation is used. The metal surface temperature is characterised by the power spectral density of the fluctuations and these fluctuations are assumed to be a Gaussian stochastic process. This enables the frequency distribution of peaks of the stress intensity factor to the determined—and hence the crack propagation rate. The results depend on the thermal and mechanical boundary conditions and on two parameters derived from the thermal fluctuation power spectral density—the microscale and the mean square stress intensity function. Only thin components, such as tubes or plates, are considered explicitly in this paper, but the method is of general application.