Flat Frequency Spectrum Research Articles

Metrological Qualification of PD Analysers for Insulation Diagnosis of HVDC and HVAC Grids.

On-site partial discharge (PD) measurements have turned out to be a very efficient technique for determining the insulation condition in high-voltage electrical grids (AIS, cable systems, GIS, HVDC converters, etc.); however, there is not any standardised procedure for determining the performances of PD measuring systems. In on-line and on-site PD measurements, high-frequency current transformers (HFCTs) are commonly used as sensors as they allow for monitoring over long distances in high-voltage installations. To ensure the required performances, a metrological qualification of the PD analysers by applying an evaluation procedure is necessary. A novel evaluation procedure was established to specify the quantities to be measured (electrical charge and PD repetition rate) and to describe the evaluation tests considering the measured influence parameters: noise, charge amplitude, pulse width and time interval between consecutive pulses. This procedure was applied to different types of PD analysers used for off-line measurements, sporadic on-line measurements and continuous PD monitoring. The procedure was validated in a round-robin test involving two metrological institutes (RISE from Sweden and FFII from Spain) and three universities (TUDelft from the Netherlands, TAU from Finland and UPM from Spain). With this round-robin test, the effectiveness of the proposed qualification procedure for discriminating between efficient and inappropriate PD analysers was demonstrated. Furthermore, it was shown that the PD charge quantity can be properly determined for on-line measurements and continuous monitoring by integrating the pulse signals acquired with HFCT sensors. In this case, these sensors must have a flat frequency spectrum in the range between several tens of kHz and at least two tens of MHz, where the frequency pulse content is more significant. The proposed qualification procedure can be useful for improving the future versions of the technical specification TS IEC 62478 and the standard IEC 60270.

Efficient Cherenkov-type optical-to-terahertz converter with terahertz beam combining.

A nonlinear structure for efficient Cherenkov-type terahertz emission from ultrashort laser pulses is proposed, modeled, and experimentally demonstrated. The structure comprises a thin (a few tens of micrometers thick) layer of lithium niobate sandwiched between two silicon prisms. A focused-to-a-line laser pulse propagates in the layer and generates a Cherenkov wedge of terahertz radiation in the prisms. The radiation experiences total internal reflection in the prisms and emerges into free space as two adjacent beams collinear to the pump laser beam. The structure can generate a centimeter-wide terahertz beam with high transverse uniformity and a flat frequency spectrum. An optical-to-terahertz conversion efficiency as high as 0.35% is achieved with 10-µJ laser pulses. It can be further enhanced by reducing the thickness of the lithium niobate layer.

Software development firmware system for broken rotor bar detection and diagnosis of induction motor through current signature analysis

The induction motors (IMs) are undoubtedly the most used machines in industries because of the advantages they offer such as simplicity, service continuity and low cost. Due to wear and tear, the motor suffers different types of mechanical and electrical failures. Depending on the criticality of the plant motors, it could be necessary to implement predictive techniques in order to detect the faults before they can cause unnecessary downtime. Therefore, in this paper, the research approach was to develop a low cost measurement system based on a micro controller platform for machine diagnosis. The FRDM K64F developing board was selected as the most suitable for satisfying the system conditions, and it was used to collect induction motor`s current data. In order to validate the accuracy of the developed system, the Frequency Transfer Functions (FRF) of the developed measurement system and the standard system (NI USB-6009) were compare. It showed a flat frequency spectrum from 0 to 1 KHz, with small fluctuations of about 0.25 dB standard deviation. A fully automated test bench was implemented, which allows to perform all the measurement tests with the IMs, and in this case, the detection and diagnosis of broken bars. Around 240 tests were performed with varying loads, different rotation speeds, and with different severity damage levels in the rotor. The data analysis procedure for broken rotor bar detection and motor diagnosis was performed by the Motor Current Signature Analysis (MCSA), FFT and Enveloped Analysis (EA). Finally, the research approach was successfully accomplished, by the team by developing a software firmware measurement ultra-low cost development platform for machine diagnosis. It was also developed a proper antialiasing filter to reduce industrial noise. The effectiveness of the proposed system is detecting a weak fault in a noise signal. It was found out a new consistent and robust parameter called the pole pass frequency (fpsf), which could be used as a diagnosis parameter for detection of broken rotor bars faults, with their damage severity degree. The detected parameter can be found around 2.6 Hz, and it increases in amplitude with increasing damage severity.

Sound Source and Loudspeaker Base Angle Dependency of Phantom Image Elevation Effect

Early studies found that, when identical signals were presented from two loudspeakers equidistant from the listener, the resulting phantom image was elevated in the median plane and the degree of the elevation increased with the loudspeaker base angle. However, sound sources used in such studies were either unknown or limited to noise signals. In order to investigate the dependencies of the elevation effect on sound source and loudspeaker base angle in details, the present study conducted listening tests using eleven natural sources and four noise sources with different spectral and temporal characteristics for seven loudspeaker base angles between 0° and 360°. The elevation effect was found to be significantly dependent on the sound source and base angle. Results generally suggest that the effect is stronger for sources with transient nature and a flat frequency spectrum than for continuous and low-frequency-dominant sources. Theoretical reasons for the effect are also discussed based on head-related transfer function measurements. It is proposed that the perceived degree of elevation would be determined by a relative cue related to the spectral energy distribution at high frequencies, but by an absolute cue associated with the acoustic crosstalk and torso reflections at low frequencies.

Comparison of Deconvolution Filters for Photoacoustic Tomography.

In this work, we compare the merits of three temporal data deconvolution methods for use in the filtered backprojection algorithm for photoacoustic tomography (PAT). We evaluate the standard Fourier division technique, the Wiener deconvolution filter, and a Tikhonov L-2 norm regularized matrix inversion method. Our experiments were carried out on subjects of various appearances, namely a pencil lead, two man-made phantoms, an in vivo subcutaneous mouse tumor model, and a perfused and excised mouse brain. All subjects were scanned using an imaging system with a rotatable hemispherical bowl, into which 128 ultrasound transducer elements were embedded in a spiral pattern. We characterized the frequency response of each deconvolution method, compared the final image quality achieved by each deconvolution technique, and evaluated each method’s robustness to noise. The frequency response was quantified by measuring the accuracy with which each filter recovered the ideal flat frequency spectrum of an experimentally measured impulse response. Image quality under the various scenarios was quantified by computing noise versus resolution curves for a point source phantom, as well as the full width at half maximum (FWHM) and contrast-to-noise ratio (CNR) of selected image features such as dots and linear structures in additional imaging subjects. It was found that the Tikhonov filter yielded the most accurate balance of lower and higher frequency content (as measured by comparing the spectra of deconvolved impulse response signals to the ideal flat frequency spectrum), achieved a competitive image resolution and contrast-to-noise ratio, and yielded the greatest robustness to noise. While the Wiener filter achieved a similar image resolution, it tended to underrepresent the lower frequency content of the deconvolved signals, and hence of the reconstructed images after backprojection. In addition, its robustness to noise was poorer than that of the Tikhonov filter. The performance of the Fourier filter was found to be the poorest of all three methods, based on the reconstructed images’ lowest resolution (blurriest appearance), generally lowest contrast-to-noise ratio, and lowest robustness to noise. Overall, the Tikhonov filter was deemed to produce the most desirable image reconstructions.

A two-stage optical frequency comb generator based on Mach-Zehnder modulator and Re-circulating frequency shifter

We propose a novel scheme to generate optical frequency comb by using a two-stage generator based on Mach-Zehnder modulator and re-circulating frequency shifter. Based on the Mach-Zehnder modulator a light source including 2 flat and stable frequency spectrum lines is generated at the first stage, and the light source is then used as the seed light in the re-circulating frequency shifter loop at the second stage. The scheme can greatly reduce the accumulated noise of the system. Through theoretical analysis and simulation it is shown that the proposed theoretical model of the two-stage system is proved in good agreement with simulation results.

Phase Rotation for Constructing Uniform Frequency Spectrum in IFDMA Communication

In this paper, a scheme for constructing the flat frequency spectrum of interleaved frequency division multiple access (IFDMA) is proposed. Since IFDMA is one of the single carrier modulation schemes, the frequency spectrum components are fluctuated and depend on the information data sequence. Even if IFDMA modulation scheme makes frequency spectrum dispersive for obtaining frequency diversity gain, frequency diversity gain is reduced by the fluctuation of frequency spectrum. In addition, in decision directed channel estimation (DDCE), which achieves good channel estimation accuracy in fast fading environment, the accuracy of channel transfer function estimated at the significant attenuated frequency component is much degraded. In the proposed technique, random phase sequence is multiplied to the information data sequence for constructing the flat frequency spectrum. As a result, the frequency diversity gain is enlarged and the accuracy of channel estimation by DDCE is improved. Furthermore we propose the blind estimation technique for the random phase sequence selected by transmitter. We show the effects of the proposed scheme by computer simulation.

Phase-dependent stimulation effects on bursting activity in a neural network cortical simulation

A neural network simulation with realistic cortical architecture has been used to study synchronized bursting as a seizure representation. This model has the property that bursting epochs arise and cease spontaneously, and bursting epochs can be induced by external stimulation. We have used this simulation to study the time-frequency properties of the evolving bursting activity, as well as effects due to network stimulation. The model represents a cortical region of 1.6 mm x 1.6mm, and includes seven neuron classes organized by cortical layer, inhibitory or excitatory properties, and electrophysiological characteristics. There are a total of 65,536 modeled single compartment neurons that operate according to a version of Hodgkin-Huxley dynamics. The intercellular wiring is based on histological studies and our previous modeling efforts. The bursting phase is characterized by a flat frequency spectrum. Stimulation pulses are applied to this modeled network, with an electric field provided by a 1mm radius circular electrode represented mathematically in the simulation. A phase dependence to the post-stimulation quiescence is demonstrated, with local relative maxima in efficacy occurring before or during the network depolarization phase in the underlying activity. Brief periods of network insensitivity to stimulation are also demonstrated. The phase dependence was irregular and did not reach statistical significance when averaged over the full 2.5s of simulated bursting investigated. This result provides comparison with previous in vivo studies which have also demonstrated increased efficacy of stimulation when pulses are applied at the peak of the local field potential during cortical after discharges. The network bursting is synchronous when comparing the different neuron classes represented up to an uncertainty of 10 ms. Studies performed with an excitatory chandelier cell component demonstrated increased synchronous bursting in the model, as predicted from experimental work. This large-scale multi-neuron neural network simulation reproduces many aspects of evolving cortical bursting behavior as well as the timing-dependent effects of electrical stimulation on that bursting.

Thermal Effects on Cavitation Instabilities in Helical Inducers

The main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility at Alta S.p.A. are presented. The experiments have been carried out on two different axial inducers (a three-bladed aluminum-made inducer of simple helical geometry and a prototype of the axial inducer of the Vulcain MK1 engine liquid oxygen turbopump) to characterize the instabilities affecting the pumps in a wide range of flow conditions; some experiments have also been carried out at higher temperatures to investigate the possible influence of the thermal cavitation effects on the observed phenomena. The transparent inlet section of the facility has been instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet, and middle of the axial chord of the blades. For each axial station, at least two transducers were mounted at a given angular spacing to cross correlate their signals for coherence and phase analysis. The most interesting instabilities have been detected on the three-bladed inducer, including a cavitation surge, a rotating stall, and an auto-oscillation leading to a violent surge-mode instability. Some of these instabilities have also been found to be slightly affected by temperature. On the other hand, very few oscillating phenomena have been detected on the MK1 inducer, with a practically flat frequency spectrum at flow coefficients near its nominal operating point.

Low frequency noise enhances cortisol among noise sensitive subjects during work performance

Salivary free cortisol concentration, rated stress and annoyance were determined in 32 subjects before, during and after carrying out a battery of performance tasks for 2 hours during exposure to ventilation noise, with dominant low frequencies (low frequency noise) or a flat frequency spectrum (reference noise). Both noises had a level of 40 dBA. All subjects were studied on two occasions and were exposed to both noises in strict rotation. Subjects were categorised as high- or low-sensitive to noise in general and low frequency noise in particular on the basis of questionnaires. Cortisol concentrations during the task were not significantly modulated by the noises or related to noise sensitivity alone. The normal circadian decline in cortisol concentration was however significantly attenuated in subjects high-sensitive to noise in general, when they were exposed to the low frequency noise. This noise was rated as more annoying and more disruptive to working capacity than the reference noise. The study showed physiological evidence of increased stress related to noise sensitivity and noise exposure during work. This is the first study to demonstrate an effect of moderate levels of noise on neuroendocrine activity. The impact of long-term exposure to moderate noise levels, and particularly low frequency noise, in the workplace deserves further investigation.

ORIGINATIONS AND EXTINCTIONS: ANALYSES OF FOSSIL DATA AND SIMULATIONS

We analyse the fossil data of Benton1 with and without interpolation schemes. By Fourier transform analysis, we find a frequency dependence of the amplitude of 1/f for the various interpolation schemes used in the past. We illustrate that shuffling the interpolated data changes the spectra only slightly. On the other hand, an identical analysis performed on the raw (uninterpolated) fossil data gives a flat frequency spectrum. We conclude that the 1/f behavior is an artifact of the interpolation schemes. We next introduce a simulation of extinctions driven only by interactions between two trophic levels. Fourier transform analysis of the simulation data shows a frequency dependence of 1/f. When the data are grouped into a form resembling the fossil record the frequency dependence vanishes, giving a flat spectrum. Our simulation produces a frequency spectrum that agrees with the observed fossil record.

The nonlinear acoustic response of thermally buckled plates

Simplified procedures for the analyses of random thermal acoustic responses of aircraft structures are derived using equivalent lineanization and the Fokker–Plank equation. Experimental results are obtained for an aluminum plate with combined radiant heating and base excitation. Base excitation can provide a flat frequency spectrum of uniform pressure excitation whereas loudspeaker devices will give standing wave and coupling effects. The nonlinear dynamic behaviour of the acoustic response of thermally buckled plates from the theoretical results agreed with those from the experimental results. The most important dynamic behaviour is the presence of low frequency dynamic snap-through motion which changed the static equilibrium state of the buckled plate. Also the nonlinear dynamic behaviour changed from the hardening spring type to the softening spring type due to the snap through motion. It is found that the Fokker–Planck equation gives an accurate prediction of the intermittent snapthrough response where as the equivalent linearization give a good prediction for continuous snap-through response.

Technique for the generation and frequency compensation of bandlimited white noise and its application in studies of masked hearing thresholds

Masking noise is often used in hearing tests to create a floor effect in the presence of ambient noise or to examine specific features of the auditory system (e.g., the critical bandwidth). One of the chief requirements of the masking noise is that it possess a flat frequency spectrum within some user-defined bandwidth. Generation of suitable masking noise is complicated by the frequency response of the sound projector, which may possess a frequency-dependent transmitting sensitivity and/or exhibit resonances within the desired frequency range. At low frequencies acoustic standing waves may also alter the noise frequency spectrum. To overcome these limitations, a technique has been developed to generate bandlimited noise whose frequency content is compensated in order to flatten peaks or valleys in the measured frequency spectrum. Compensation is performed by passing white noise through a digital filter whose coefficients are determined from previous measurements of the acoustic system frequency response. The system has been implemented using a personal computer with commercial hardware and custom software. The method has been used to quickly generate bandlimited Gaussian and uniform white noise for studies of masked underwater hearing thresholds in marine mammals. [Work supported by ONR and the NRC Research Associateship Program.]

Phenomenology of the normal and superconducting states of a marginal Fermi liquid (invited)

A metal with scattering from a flat frequency spectrum of particle-hole pairs is a ‘‘marginal’’ Fermi liquid, with infinite quasiparticle lifetime but a logarithmically vanishing spectral weight. In the superconducting state, the scattering spectrum should acquire a gap, and the quasiparticle character will be restored at low energies. Unless the scattering spectrum results from interband processes, the Drude-like component of the optical conductivity will then have an onset at 4Δ in the clean limit, twice the superconducting gap. Other experimental ramifications, in particular for tunnelling, NMR relaxation rate, thermal conductivity, and photoemission spectra are considered.

Complex-valued ghost cancellation reference signal for TV broadcasting

A complex-valued ghost cancellation reference (CVGCR) signal for TV broadcasting is described. The CVGCR has the following advantages over all other TV GCRs: a perfectly flat frequency spectrum; an ideal zero autocorrelation function; high signal energy; and arbitrarily long length. Because of these advantages, the complex-valued reference signal has already been successfully applied to modem channel identification. A mathematical description for transmitting a CVGCR over a TV channel with a modulation system is given. Some simulation results are presented. >

Broad-Band and Narrow-Band Measurements

EMC specifications consider two kinds of signals (broad band and narrow band). Special care is to be taken to determine the nature of the signal if meaningful measurements are desired. Tests specified in MIL-STD-462 and in other EMC specifications to decide if there are narrow-band or broad-band emissions are not conclusive, and many times it is difficult to make a decision about the nature of the signal. The decision is less difficult to make when bandwidth effects are understood. This paper tries to show the implications of performing EMC measurements using different receiver bandwidths. It appears that broad-band measurements are valid only in the special case in which the signal has a continuous flat frequency spectrum; in the other cases their meaning is rather questionable. In the conclusion some remarks are made in order to clarify this difficult situation.

Frequency analysis of human involuntary eye movement.

A discussion is given of considerations involved in forming a frequency spectrum of a signal such as human fixation eye movements, in which an impulsive signal (saccadic movements) and a noise-like signal (tremor movements) are present together. A method is outlined which enables the spectrum of each component to be determined. Results are presented of human eye movement frequency spectra and it is shown that the tremor movements alone are adequate to prevent the fading of vision under conditions of retinal image stabilisation. An interpretation of the observed frequency spectra is given in terms of a model, which assumes that the dynamics of the eye muscle system are linear and that the active state input producing tremor has a flat frequency spectrum. From this it is deduced that the eye behaves as an overdamped second order system with time constants of 0.002 and 0.02 seconds. The active state input involved in production of an involuntary saccade is shown to consist of an impulse function with exponential rise and decay.

Physical Sources of Noise

Johnson noise is caused by the fluctuation of charges or polarizable molecules in lossy materials. Its magnitude can best be calculated by means of statistical mechanics, which tells us that each degree of freedom must have associated with it an energy of ? kT. This leads to the usual expressions for Johnson noise, including the fact that the available thermal noise power, from a resistor, a lossy network, a lossy dielectric, or an antenna is always kTB. In the case of a resistor, network, or dielectric, T is the temperature of the lossy material. In the case of an antenna, T is the average temperature of the environment which the antenna sees. Consistent with the power kTB available from an antenna, there is a particular density of radiation in space which is a function of temperature and frequency. Noise figure is defined in terms of Johnson noise. Shot noise, due to the discrete nature of electron flow, is generally distinct from Johnson noise, although in some electron devices the expression for the noise in the electron flow has the same form as that for Johnson noise. When the noise in electron flow is greater or less than pure shot noise, the motions of the electrons must be in some degree correlated. In an electron stream of low noise, the random interception of a fraction of the electron flow can reduce the correlation and increase the noise. Johnson noise and shot noise have a flat frequency spectrum.