Maximum Frequency Content Research Articles

Time-Window-Based Discrete-Time Fourier Series for Electromagnetic Transients in Power Systems

This paper approaches the solution of electromagnetic transients in the frequency domain from the point of view of choosing a time window with sufficient width to allow the time constants of the transients to decay to a small value. This criterion, and the required maximum frequency content of the simulation, fully determine the simulation parameters. Satisfying this criterion, a very simple methodology, like the discrete time Fourier series (DTFS), can provide very accurate results without the inherent difficulties of time-warping solutions based on the numerical Laplace transform (NLT). Even though the NLT can give very accurate results, it requires expert knowledge to correctly specify the damping factors, sampling schemes, and windowing functions. The DTFS solution is compared with the NLT and with analytical solution for a circuit with lumped elements, and with the most-accepted frequency-dependent transmission line models in the Electromagnetic Transients Program: the JMARTI (fdLINE) model and the Universal Line Model for a double-circuit transmission line.

Envelope-Tracking Adaptive Integral Method for Band-Pass Transient Scattering Analysis

An envelope-tracking (ET) algorithm is presented for efficiently solving integral equations pertinent to the analysis of transient scattering from perfect electrically conducting surfaces. The algorithm solves space-time samples of the complex envelope of the surface current density by implicit time marching; thus, the time-step size is inversely proportional to the bandwidth—not the maximum frequency content—of the fields of interest. At each time step, anterpolation, propagation, interpolation, and near-zone correction stages are performed to compute the right-hand side of a matrix equation, which involves 4-D space-time FFTs, and to accelerate its iterative solution, which involves 3-D space FFTs. These FFTs dominate the computational costs of the method: they require $O({N_{\mathrm{C}}} [\log^2 {N_{\mathrm{g}}} + {\bar{N}_{\mathrm{I}}} \log {N_{\mathrm{C}}}])$ operations per time step; here, $N_{\mathrm{C}}$ is the number of nodes on an auxiliary regular grid, $N_{\mathrm{g}}$ is determined by the number of time steps that fields take to travel the maximum distance between two points on the scattering surface, and $\bar{N}_{\mathrm{I}}$ is the average number of iterations needed for the iterative solution to converge at each time step. Moreover, the algorithm is systematically compared to its time- and frequency-domain counterparts. To this end, radar profiles of benchmark targets are computed, a time-domain dB-error norm is proposed, the algorithms’ parameters are optimized subject to at most 2% error in the radar profile according to this norm, and the computational costs are measured, as target size, shape, and excitation pulse are varied. The results show that the algorithm outperforms its time-domain counterpart and is competitive with its frequency-domain counterpart.

An Envelope-Tracking Hybrid Field-Circuit Simulator for Narrowband Analysis of Nonlinearly Loaded Wire Antennas

An envelope-tracking hybrid field-circuit simulator is proposed for efficiently analyzing narrowband scattering from wire antennas loaded with nonlinear devices. The simulator models the interactions of fields with wires and lumped elements by coupling and simultaneously solving the thin-wire electric field integral equation and Kirchhoff's equations, respectively. The coupled nonlinear system of equations is iteratively solved by a time marching scheme that represents the fields, voltages, and currents of interest (signals) as a truncated series of harmonic sinusoids (carriers) multiplied with complex-valued time-varying coefficients (envelopes). Unlike time-domain simulators, which sample the signals at a rate proportional to their maximum frequency content, the proposed envelope-tracking simulator samples the envelopes at a rate proportional to their maximum bandwidth; thus, it requires significantly fewer time-steps when solving narrowband problems. Moreover, the envelope-tracking simulator is generally more accurate than its time-domain counterpart because of smaller integration and interpolation errors. Numerical results demonstrate that the proposed simulator improves the tradeoff between accuracy and computation time, especially when analyzing antennas excited by narrowband signals or loaded with weakly nonlinear devices.

Contrasting behavior between dispersive seismic velocity and attenuation: Advantages in subsoil characterization

A careful look into the pertinent models of poroelasticity reveals that in water-saturated sediments or soils, the seismic (P and S wave) velocity dispersion and attenuation in the low field-seismic frequency band (20-200 Hz) have a contrasting behavior in the porosity-permeability domain. Taking advantage of this nearly orthogonal behavior, a new approach has been proposed, which leads to unique estimates of both porosity and permeability simultaneously. Through realistic numerical tests, the effect of maximum frequency content in data and the integration of P and S waves on the accuracy and robustness of the estimates are demonstrated.

Spectral analysis of knee isokinetic extension curves of osteoarthritic patients

Isokinetic assessment of knee extensors muscle group function in patients suffering from knee osteoarthritis has exclusively been achieved using time-domain based variables. While useful, these analyses do not quantify the conspicuously irregular moment-time pattern exhibited by the involved knee when compared to the contralateral knee. The purpose of this investigation was to assess whether the frequency content of the isokinetic curve can quantify this latter phenomenon. Thirty two patients with knee osteoarthritis participated in this study. Each participant performed 5 maximal concentric knee extensions bilaterally at 60 � /s using an isokinetic dynamometer. For each time-domain extension curve, the power spectrum was calculated via Fast Fourier Transform and the maximum frequency content value contained within 99% of total signal power was extracted. Of the 32 participants, 26 exhibited higher frequency contents in the isokinetic curves obtained from their involved knee (p = 0.002, effect size = 0.35). The results suggest that the frequency content of iso kinetic moment-time curves may provide useful quantitative information regarding the smoothness of moment production, and as such, may be used in conjunction with traditional time-domain variables in the assessment of knee extensors function in patients suffering from knee osteoarthritis.

Seismic ground motion estimates for the M6.1 earthquake of July 26, 1963 at Skopje, Republic of Macedonia

The identification and characterization of active faults as earthquake sources are essential parts of seismic hazard assessment studies. In the area struck by the 1963 Skopje (M = 6.1) earthquake, we define an input fault model according to the proposed source mechanism, the magnitude, and the aftershocks distribution. We test different position for the nucleation point along the fault and we calculate maximum horizontal velocities for a maximum frequency content of 1 Hz. We compare the velocity field with the reported macroseismic intensity one in order to select the fault model that fits the observations best. Our results are in agreement with a seismic source modelled as a 15 × 8 km rectangular plane with magnitude equal to 5.9.

Benefit of higher closed�??loop bandwidths in ocular adaptive optics

We present an ocular adaptive optics system with a wavefront sampling rate of 240 Hz and maximum recorded closed-loop bandwidth close to 25 Hz, but with typical performances around 10 Hz. The measured bandwidth depended on the specific system configuration and the particular subject tested. An analysis of the system performance as a function of achieved bandwidth showed consistently higher Strehl ratios for higher closed-loop bandwidths. This may be attributed to a combination of limitations on the available technology and the dynamics of ocular aberrations. We observed dynamic behaviour with a maximum frequency content around 30 Hz.

Phase closure with a rotational shear interferometer

A simple and efficient way is proposed for achieving phase closure in an optical telescope (to enable recovering Fourier transform phases that would otherwise be corrupted by atmospheric and instrumental errors), by means of rotational shear interferometry. In a rotational shear interferometer, one images the telescope aperture onto the interferometer and then interferes the aperture with itself in a rotated orientation. To achieve the maximum frequency content permitted by the telescope, the shear has to be 180 deg, but better dynamic range and SNR are possible for lower frequencies. Drawbacks of the proposed method compared to radio astronomy are noted, and different approaches are indicated as to how to collect and use the phase closure data. Phase closure can be realized on existing telescopes and existing interferometers with special modifications. Although not all base lines are possible, the extra constraints provided by the closure phases greatly reduce the ambiguity now existing in phaseless image reconstruction.

Synthetics and theoretical seismology

In the near field, discrete‐finite wave number schemes are economic since they involve fewer wave numbers than most wave number integration schemes. The number of wave numbers is determined by the range and the location of artificial reflectors or fictitious sources inherent in discrete wave number techniques. The number and spacing of wave numbers in wave number integration schemes are determined by the desired accuracy.The vertical integration schemes used in the near field have been either spectral (Apsel, 1979, Bouchon, 1981) as in the regional techniques or finite‐element (Olson,1982) and finite‐difference in the time domain as in the Alexseev‐Mikhailenko method. The finite element schemes have the disadvantage in that the vertical step size is determined by the desired maximum frequency content, which in turn determines the time step required for stability. This time step is usually many times smaller than the time increment associated with the maximum frequency.

Time Domain Analysis of Machinery Vibration Signals Using Digital Techniques

The design and implementation of a general purpose processing system to analyze machinery vibration signals in the time domain are presented. The system comprises a data acquisition stage, where analog signals having a maximum frequency content up to 4 kHz and a dynamic range up to 72 dB, are converted into digital form and a processing stage, where modular software algorithms are employed to perform specific operations in an interactive environment. Processing capabilities encompass scaling, integration and the computation of statistical parameters such as means, RMS, standard deviation, maximum and minimum values. The input signal dynamic range and maximum frequency content are shown to be the governing criteria in the design and performance of the analysis system. The influence of hardware precision level and sampling rate on the overall accuracy of the obtained results is illustrated by digitizing standard waveforms under different processing parameters. Extraneous components introduced at the digitization and integration stages are identified and eliminated by employing a versatile digital filter algorithm. The performance of the system in processing acceleration signals from a 3 HP, 14000 RPM internal combustion engine is demonstrated and the different application aspects of such a facility in the machinery maintenance field are outlined.