Frequency Dependence Of Noise Research Articles

Frequency Characteristics of Hydrogen‐Air Fuel Cell Electrochemical Noise

AbstractElectrochemical noise (ECN) of operating polymer membrane hydrogen‐air fuel cell was measured in a wide frequency band. Spectral power densities of the noise were calculated to consider its frequency characteristics. Air flow speed on the cathode side was sinusoidal modulated to investigate the response of fuel cell noise to external conditions changing. It was shown that properly operated fuel cell does not possess self‐resonance or peaks in electrochemical noise frequency dependence.

An Efficient and Optimal Filter for Identifying Point Sources in Millimeter/Submillimeter Wavelength Sky Maps

ABSTRACTA new technique for reliably identifying point sources in millimeter/submillimeter wavelength maps is presented. This method accounts for the frequency dependence of noise in the Fourier domain as well as nonuniformities in the coverage of a field. This optimal filter is an improvement over commonly-used matched filters that ignore coverage gradients. Treating noise variations in the Fourier domain as well as map space is traditionally viewed as a computationally intensive problem. We show that the penalty incurred in terms of computing time is quite small due to casting many of the calculations in terms of FFTs and exploiting the absence of sharp features in the noise spectra of observations. Practical aspects of implementing the optimal filter are presented in the context of data from the AzTEC bolometer camera. The advantages of using the new filter over the standard matched filter are also addressed in terms of a typical AzTEC map.

Analysis of a Power Line Communication System Over a Non-white Additive Gaussian Noise Channel and Performance Improvement Using Diversity Reception

Performance improvement of a power line communication system is presented here considering the noise as a cyclostationary non-white Gaussian random process. Performance of a power line communication system is severely deteriorated by fading and multipath effect. The impulsive noise has been considered time variant, has short duration, random occurrence with a high power spectral density (PSD). It causes bit error in the signal. Using orthogonal frequency division multiplexing (OFDM) technique, the effect of impulsive noise and fading can be improved greatly. An analytic approach is presented to evaluate the performance of a power line communication link in the presence of the above limitations. The simulation results show that there is deterioration in system BER due to time and frequency dependence of noise and the degradation is found to be significant at higher bit rates and bandwidth. The system suffers penalty in receiver sensitivity due to non-white nature of the noise process. In this paper, an analytical approach using diversity reception is carried out to examine the performance improvement of a power line channel in fading and impulsive noise. The system bit error rate (BER) is compared numerically for both binary phase shift keying (BPSK) and OFDM system. The BER results show that there is significant improvement in OFDM. Also the performance is remarkably upgraded using diversity reception.

Resistivity noise in crystalline magnetic nanowires and its implications to domain formation and kinetics

We have investigated the time-dependent fluctuations in electrical resistance, or noise, in high quality crystalline magnetic nanowires within nanoporous templates. The noise increases exponentially with increasing temperature and magnetic field, and has been analyzed in terms of domain wall depinning within the Neel-Brown framework. The frequency-dependence of noise also indicates a crossover from nondiffusive kinetics to long-range diffusion at higher temperatures, as well as a strong collective depinning, which need to be considered when implementing these nanowires in magnetoelectronic devices.

Low-frequency noise in analytical soil measurements

Fourier analysis of the real-time quality-control charts for colorimetric, photometric, and pH soil measurements shows 1/fn (n=1±0.05) noise intensity dependence on frequency, in correspondence to the earlier test for atomic absorption measurements. In addition, the prevailing character of the sample preparation in the general noise frequency dependence is clearly demonstrated. Some important analytical problems are addressed where knowledge of the specific character of the time variation in the analytical data sets is essential.

Frequency-dependent shot noise in long disordered superconductor-normal-metal-superconductor contacts.

The shot noise in long diffusive superconductor-normal-metal-superconductor contacts is calculated using the semiclassical approach. At low frequencies and for purely elastic scattering, the voltage dependence of the noise is of the form S(I) = (4Delta+2eV)/3R. The electron-electron scattering suppresses the noise at small voltages resulting in vanishing noise yet infinite dS(I)/dV at V = 0. The distribution function of electrons consists of a series of steps, and the frequency dependence of noise exhibits peculiarities at omega = neV, omega = neV-2Delta, and omega = 2Delta-neV for integer n.

Semiclassical theory of shot noise in disordered superconductor–normal-metal contacts

We present a semiclassical theory of shot noise in diffusive superconductor - normal metal contacts. At subgap voltages, we reproduce the doubling of shot noise with respect to conventional normal-metal contacts, which is interpreted in terms of an energy balance of electrons. Above the gap, the voltage dependence of the noise crosses over to the standard one with a voltage-independent excess noise. The semiclassical description of noise leads to correlations between currents at different electrodes of multiterminal SN contacts which are always of fermionic type, i.e. negative. Using a quantum extension of the Boltzmann - Langevin method, we reproduce the peculiarity of noise at the Josephson frequency and obtain an analytical frequency dependence of noise at above-gap voltages.

Bias, frequency, and area dependencies of high frequency noise in AlGaAs/GaAs HBT's

The noise figure of the heterojunction bipolar transistor (HBT) in the microwave frequency range is studied, and an improved physical noise model is developed. Unlike the conventional high frequency noise model, which considers only the bias current dependence, the present model includes both the effects of voltage and current on the noise behavior. In addition, the frequency- and area-dependent natures of the HBT noise at very high frequencies are incorporated in the model. It is found that the voltage dependence of the high frequency noise in the HBT results from the self-heating effect, which gives rise to a higher HBT lattice temperature than the ambient temperature. Also, the free-carrier transport delay time must be considered to properly model the frequency dependence of noise since the inverse of this time is comparable with the frequency. Furthermore, the area dependence of noise is dominated by changes in the base resistance and emitter-base junction capacitance. Results for the minimum noise factor calculated from the model compare favorably with those obtained from measurements.

The frequency and temperature dependence of noise in YBa2Cu3O7 multijunction flux-flow amplifiers

Measurements of the absolute noise power of high Tc multijunction flux-flow amplifiers in the frequency range 0.1 Hz–60 kHz are reported. The noise is found to have maxima at the operating points corresponding to optimum transresistance and gain, and is consistent with existing models of critical current fluctuations as found in single grain boundary junctions. The frequency dependence of the noise power is usually close to 1/f, although a Lorentzian term is sometimes observed. The normalized values of measured critical current fluctuations are approximately temperature independent as previously found for single junctions prepared by similar techniques.

Current fluctuations in strongly electron-correlated one-dimensional double-barrier heterostructures.

Based upon Keldysh nonequilibrium Green functions, a theoretical formalism is developed to study the current-noise characteristics of one-dimensional double-barrier heterostructures. The effects of the Coulomb blockade on the I-V curve and on the current fluctuation is studied. For the static case a reduction of the full shot noise is observed. A connection between the noise frequency dependence and the transit time of the electron going through the structure is established.

Lattice structure, flux dynamics, and low frequency noise in high temperature superconductors

Noise sources associated with the dynamics of motion of the flux bundles and its lattice interactions in YBaCuO high-Tc superconductors (HTS) have been examined. A model is proposed where, in the layered structure of the material, the flux bundles or fluxoids exhibit a continuous crossover from a motion over activation barriers to diffusive transfer between pinning sites. The results of the model were compared to various experimental measurements on thin films YBaCuO with distinct low frequency behavior. In one of the experiments the low frequency 1f noise spectral power could, in general, be reproduced. In another experiment, agreement between measurements and theoretical predictions was achieved when a defect density functional D(n) (where n is the density of defects in a single plane) was derived. In the flux flow regime, D(n) has the simple form D(n) = (δ/ξ)(1n12ξ)δ−1 where ξ is the superconducting coherence length in the Cu-O planes and δ = (UoKT) is the pinning energy of a flux line normalized to its thermal energy. These results are clear indications that the noise frequency dependence is strongly related to the dynamics of motion of free vortices and the number fluctuation of bound vortices.

Johnson noise in one- and two-dimensional distributed resistor-capacitor systems

Johnson noise is often the dominant noise source in microelectronic devices. While the magnitude and frequency dependence of noise within a network of discrete resistors and capacitors is well documented, there appears to be a lack of analogous results for distributed resistor-capacitor systems. This latter model is quite relevant to microelectronic devices since one finds that long resistive conductors are capacitively coupled with varying strength to other elements along the entire conductor length. We solve both the one-and two-dimensional distributed resistor-capacitor systems and find both the autocorrelation function of the potential at any point and the frequency spectrum of the associated mean-square potential fluctuations. We note that the one-dimensional result exhibits an extended ‘‘1/f’’ region in the power spectrum at high frequencies and speculate that all systems with macroscopic behavior governed by a diffusion equation may share this frequency dependence if the diffusivity depends appropriately on spatial frequency. The two-dimensional system solution shows us that Johnson noise within the epitaxial layer of a two-dimensional image sensor will not dominate other imager noise sources.

Hot electron noise in n-InSb

Results of Monte Carlo calculation of hot-electron noise temperature are presented for n-InSb at 85 K. A method for the eatimation of correlation time for velocity fluctuations and calculation of frequency dependence of noise is also given. The results of the calculations are found to agree closely with the available experimental data.

Electron Cyclotron Drift Instability Experiment

The electron cyclotron drift instability has been observed in a controlled, steady-state experiment. Enhanced noise bands are seen in the first two Bernstein wave dispersion branches. Dispersion, noise frequency dependence upon magnetic field and ion beam velocity, and saturation are demonstrated.

PROPERTIES OF CADMIUM SULPHIDE PHOTOCONDUCTIVE CELLS

Measurement of the noise spectra of cadmium sulphide photoconductive cells from 10 to 10 Mc/sec has failed to reveal a transition from the 1/f to the 1/f2 noise frequency dependence. This clearly demonstrates a complete lack of dependence of the 1/f noise spectrum on the photoconductive time constant, which is approximately 30 milliseconds.No noise correlation has been observed, indicating a minority carrier lifetime of less than 10−7 seconds.The extreme difference between the minority carrier lifetime and the photoconductive time constant is explained by assuming that most of the recombination takes place at the surface of the film, but that the rate of recombination is controlled by diffusion. Slow diffusion of the minority carriers is a consequence of their very short lifetimes.The noise power and time constants of CdS have been observed to depend on the value of the series resistance used with the cell. The two effects are believed to be related and to arise from the presence of intercrystalline barriers.