Average Spectral Densities Research Articles

Extrapolation of transformer equipment noise from excessive background noise environment

Dynamic, high relative background noise environments present challenging measurement conditions for discerning the noise level contributions for equipment of interest, particularly in circumstances where equipment cannot ideally be shut down for baseline measurement comparison. This paper considers the evaluation of large transformer noise emissions contributions in excessive background noise conditions for purposes of property line compliance assessment, exemplifying such an occurrence. While often insufficient alone, typical field measurements may be accompanied by fundamental, but powerful, narrowband signal analysis techniques to estimate and demonstrate equipment noise contributions, especially for sources containing prominent tonal components, such as those that are common to electrical equipment. This simple and straightforward approach focusing on calculation of average spectral densities and use of spectrograms, supplemented by other analysis tools including three-dimensional environmental noise modeling as was employed in this case study, offers an efficient and effective equipment noise assessment strategy that may be readily applied by the practitioner.

Evaluation of an automated pipeline for large-scale EEG spectral analysis: the National Sleep Research Resource

Study objectivesWe present an automated sleep electroencephalogram (EEG) spectral analysis pipeline that includes an automated artifact detection step, and we test the hypothesis that spectral power density estimates computed with this pipeline are comparable to those computed with a commercial method preceded by visual artifact detection by a sleep expert (standard approach). MethodsEEG data were analyzed from the C3-A2 lead in a sample of polysomnograms from 161 older women participants in a community-based cohort study. We calculated the sensitivity, specificity, accuracy, and Cohen's kappa measures from epoch-by-epoch comparisons of automated to visual-based artifact detection results; then we computed the average EEG spectral power densities in six commonly used EEG frequency bands and compared results from the two methods using correlation analysis and Bland–Altman plots. ResultsAssessment of automated artifact detection showed high specificity [96.8%–99.4% in non-rapid eye movement (NREM), 96.9%–99.1% in rapid eye movement (REM) sleep] but low sensitivity (26.7%–38.1% in NREM, 9.1–27.4% in REM sleep). However, large artifacts (total power > 99th percentile) were removed with sensitivity up to 87.7% in NREM and 90.9% in REM, with specificities of 96.9% and 96.6%, respectively. Mean power densities computed with the two approaches for all EEG frequency bands showed very high correlation (≥0.99). The automated pipeline allowed for a 100-fold reduction in analysis time with regard to the standard approach. ConclusionDespite low sensitivity for artifact rejection, the automated pipeline generated results comparable to those obtained with a standard method that included manual artifact detection. Automated pipelines can enable practical analyses of recordings from thousands of individuals, allowing for use in genetics and epidemiological research requiring large samples.

Beta band stability over time correlates with Parkinsonian rigidity and bradykinesia

Abnormal oscillatory activity in the basal ganglia is increasingly implicated in the pathophysiology of Parkinson's disease. Such activity is recorded in patients in the form of oscillations in the local field potential (LFP) picked up in the subthalamic nucleus. Previous studies have focused on correlations between features of the time averaged power or amplitude spectrum of the LFP and the clinical state, either off medication or in response to levodopa. However, average spectral densities do not take account of time variant spectral properties and we hypothesised that these dynamic properties of the spectrum of the LFP would contain additional information about clinical state. Here we assess the variability in LFP amplitude over time using the coefficient of variation (CV), evaluating this with regard to clinical state off medication and in response to levodopa in two datasets. The CV of activity in the high beta frequency band was found to be correlated with clinical state off levodopa (rho=−0.59, p<0.001) and this was shown to be complementary, rather than redundant, to spectral amplitude in a multiple regression analysis, selective for rigidity–bradykinesia and highly focal. Similarly, a strong correlation was found between change in clinical scores and change in high beta CV following levodopa (rho=−0.66, p=0.004). This too was selective for rigidity–bradykinesia and non-redundant to spectral power in a multiple regression model. Our results indicate that temporal stability in the beta band is correlated with rigidity–bradykinesia. It is suggested that loss of beta reactivity is deleterious to basal ganglia function over and above any concomitant change in absolute level of beta synchrony. The CV of LFP beta band amplitude may potentially provide an additional index of clinical state suitable for feedback control in closed loop stimulation therapy.

Computation of the Average and Harmonic Noise Power-Spectral Density in Switched-Capacitor Circuits

Switched-capacitor (SC) circuits are periodically time-varying circuits and the noise at the output of these circuits is cyclostationary. This noise is therefore characterized by the average and harmonic spectral densities. We extend the method proposed in a previous paper to compute the average and harmonic noise-spectral densities in periodically varying circuits. We derive expressions for the average and harmonic spectral densities and use the mixed-frequency-time technique for the computation. The results for the average spectral density are compared with published results. The contribution of the harmonic spectral densities to the average noise-spectral density at the output of a cascaded block is estimated.

High-speed digital and analog performance of low-noise integrated MSM-HEMT photoreceivers

The digital and analog performance of high-speed, monolithically integrated, low-noise metal-semiconductor-metal MSM-HEMT photoreceivers is reported. The MSM photodetectors were vertically integrated with the HEMT's using a stacked layer structure, with the HEMT layers beneath the MSM layers. The photoreceivers were implemented using a HEMT-based low-noise three-stage transimpedance amplifier. The amplifier transimpedance was 803 /spl Omega/, and the photoreceivers exhibited -3 dB bandwidths up to 7.2 GHz. Analog noise performance was characterized, and average input-referred noise current spectral densities of 6.5 and 7.3 pA/Hz/sup 1/2 / were found for receivers with 1and 1.5-/spl mu/m MSM inter-electrode spacings, respectively. The digital performance of the photoreceivers was examined at bit rates up to 10 Gb/s, and open eye patterns obtained. The measured sensitivity for a bit-error rate of 10/sup -9/ was found to be -16.9 and -13.1 dBm at 5 and 8 Gb/s, respectively, for photoreceivers with a 1.5 /spl mu/m inter-electrode spacing, and -10.7 dBm at 10 Gb/s for photoreceivers with a 1-/spl mu/m inter-electrode spacing. To the authors' knowledge, this is the first report of multigigabit measured sensitivities of MSM-HEMT photoreceivers on InP substrates.

Design, fabrication, and performance of high-speed monolithically integrated InAlAs/InGaAs/InP MSM/HEMT photoreceivers

A detailed study of the performance of monolithically integrated photoreceivers based on metal-semiconductor-metal (MSM) photodetectors (PD's) and HEMT's is undertaken. Two different stacked-layer approaches to integrating MSM-PD's with HEMT's are investigated, and the performance of detectors and HEMT's for each approach is compared. The structure with the MSM layers grown on top of the HEMT layers exhibited the best overall performance. A physics-based MSM model is developed and incorporated into microwave circuit design software; excellent agreement between circuit simulations and measured frequency responses is demonstrated. To evaluate the effects of MSM electrode geometry and detector area on photoreceiver performance, photoreceivers with MSM interelectrode spacings of 1, 1.5, and 2 /spl mu/m were fabricated and characterized. The electrical amplifier used in the photoreceivers is a two-stage, variable-transimpedance amplifier with a common-gate HEMT as the feedback path. By adjusting the DC voltage applied to the gate of this feedback HEMT, transimpedances ranging from 55.8 to 38.1 dB/spl Omega/, with corresponding -3 dB cutoff frequencies from 6.3 to 18.5 GHz, were measured experimentally. Excellent noise performance has been measured, with average input noise current spectral densities of 7.5, 8, and 12 pA/Hz/sup 1/2/ obtained for bandwidths of 6.3, 8, and 13.7 GHz, respectively. A packaged receiver has been tested at 5 Gb/s and an open eye pattern obtained.

Lightning electromagnetic radiation field spectra in the interval from 0.2 to 20 MHz

Average energy spectral densities are presented for the fast transitions in most of the components that produce large radiation field impulses from cloud‐to‐ground lightning: first and subsequent return strokes; stepped, dart‐stepped, and “chaotic” leaders; and “characteristic” cloud pulses. A disagreement in the previous literature about the spectral energy radiated by return strokes at high frequencies is noted and explained. We show that our spectral amplitudes are not seriously distorted by propagation over less than 35 km of seawater, although as much as 45 km of such propagation does appear to produce significant attenuation above about 10 MHz. First and subsequent return strokes produce identical spectra between 0.2 and 20 MHz. The spectra of stepped and dart‐stepped leader steps are nearly identical and are very similar to that of characteristic pulses. The spectra of leader steps also match return stroke spectra above 2–3 MHz after the former are increased by about 7 dB. The shapes of individual spectra do not depend on their amplitude, so the shapes of the average spectra are probably not distorted by the trigger thresholds used in the data acquisition. Return strokes are the strongest sources of radiation from cloud‐to‐ground lightning in the 0.2‐ to 20‐MHz frequency range, although certain intracloud processes are stronger radiators above about 8 MHz.

Average electric wave spectra in the plasma sheet: Dependence on ion density and ion beta

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer and the plasma instrument on board the AMPTE/IRM satellite, more than 50,000 ten‐second‐averaged electric wave spectra were analyzed in order to establish typical spectra for periods of high and low ion density and high and low ion β. The general spectral slope of the spectra in the plasma sheet follows an f−2 law. Ion β has a stronger influence on the spectral form than the ion density. Highest average spectral densities are obtained in the low‐β plasma sheet boundary layer, where the spectrum is that of broadband electrostatic noise extending to frequencies near and above the upper hybrid frequency. Lowest wave intensities are encountered in the high‐β inner central plasma sheet. The outer central plasma sheet has generally low wave intensities and is dominated by electron cyclotron odd half‐harmonics and electron plasma wave emissions. Lower electron cyclotron harmonics are found in the high‐β cases in all three regions of the plasma sheet while higher odd half‐harmonics dominate the low‐β and low‐density inner central plasma sheet.

A thermal activation model for noise in the dc SQUID

A thermal activation model is described for the dc SQUID. The equations of motion for the junction phase differences are shown to develop in time like the coordinates of a particle performing Brownian motion in a viscous medium in a two-dimensional potential field. Expressions are derived relating the average voltage, transfer function, and current and voltage noise spectral densities to the features of the potential determined by the device parameters. Comparison with a numerical simulation is presented. Calculations of the current noise as a function of loop inductance and critical current asymmetry are performed. An anomalously large current noise is predicted at certain values of the device characteristics. The correlation spectral density is also calculated as a function of loop inductance, and related to the optimal source resistance for a tuned SQUID amplifier. A theory of low-frequency noise sources in the SQUID is developed in a fashion compatible with the thermal activation model. Equilibrium temperature fluctuations as a possible source of 1/f noise in the SQUID are discussed. A scheme for optimizing the resolution at low frequencies is presented. Proper exploitation of low-capacitance Pb-PbOx-Pb junction technology is shown to increase the resolution at 1 Hz by at least a factor of 8, provided that the temperature fluctuations are the dominant source of noise.

An Investigation of Broad Band Random Vibration Simulation

The paper is concerned with the simulation of the action of a force environment on a multi-mode structure by means of a substituted force environment and is relevant to vibration testing of shell-like structures. The use of broad-band average spectral densities as response parameters in the simulation is justified. It is assumed that only simulation in some average sense of the modal responses in frequency bands can be contemplated because of the numbers of modes involved and their indeterminacy. By introducing a simple probabilistic model of the structure-environment interactions, some general results concerning the accuracy of simulation are deduced. Predictions of the simulation accuracy are shown to be possible if the relevant parameters can be estimated. This is confirmed by an experimental investigation.