Showing papers on "Frequency response published in 1990"

A low-order system frequency response model

Abstract: The authors present the derivation of a simple, low-order system frequency response (SFR) model that can be used for estimating the frequency behavior of a large power system, or islanded portion thereof, in response to sudden load disturbances. The SFR model is a simplification of other models used for this purpose, but it is believed to include the essential system dynamics. The SFR model is based on neglecting nonlinearities and all but the largest time constants in the equations of the generating units of the power system, with the added assumption that the generation is dominated by reheat steam turbine generators. This means that the generating unit inertia and reheat time constants predominate over the system average frequency response. Moreover, since only two time constants predominate, the resulting system response can be computed in closed form, thereby providing a simple, but fairly accurate, method of estimating the essential characteristics of the system frequency response. >

Measurement and analysis of the indoor radio channel in the frequency domain

Abstract: Using a network analyzer, several experiments for the frequency-domain characterization of the indoor radio channel in the 0.9-1.1-GHz band are performed. In the experiments, the frequency response measurements are taken at spatially distributed locations throughout the test area by fixing the receiver in a central location and moving the transmitter to different locations. The experiments are performed in a high-rise office building and a three-story building with offices and laboratories. For each experiment, the exponent of the power-distance relationship and the statistics of the 3-dB width of the frequency correlation function are determined from the frequency-domain data. The approximation to the impulse response of the channel is obtained from the inverse Fourier transform of the frequency response. An empirical exponential relationship between the 3-dB width of the frequency correlation function and the inverse of the RMS (root mean square) delay spread of the impulse response is derived. >

Design considerations for high-frequency continuous-time filters and implementation of an antialiasing filter for digital video

Abstract: An approach that has made possible the integration of video frequency continuous-time filters with wide dynamic range is discussed. The tuning scheme necessary to maintain the stable and accurate frequency response in the presence of temperature variations, process tolerance, and aging is described. Detailed design techniques specific to high-frequency operation are introduced to implement a 5-V, seventh-order elliptic analog magnitude filter for antialiasing in digital video applications. The filter, based on a G/sub m/-C technique, exhibits a dynamic range of 61 dB and dissipates a power of 75 mW. Ninety-two chips from various wafers and two different process runs were tested. Seventy-five percent of the fabricated chips were functional, and 63% of them met the commercial-grade specifications in spite of an error in the layout which forced the phase control circuitry to perform suboptimally. >

Determination of physiological levels of glucose in an aqueous matrix with digitally filtered Fourier transform near-infrared spectra.

Abstract: A procedure is described for the measurement of clinically relevant concentrations of glucose in aqueous solutions with near-infrared (NIR) absorbance spectroscopy. A glucose band centered at 4400 cm-1 is used for this analysis. NIR spectra are collected over the frequency range 5000-4000 cm-1 with a Fourier transform spectrometer. A narrow-band-pass optical interference filter is placed in the optical path of the spectrometer to eliminate light outside this restricted range. This configuration provides a 2.9-fold reduction in spectral noise by utilizing the dynamic range of the detector solely for light transmitted through the filter. In addition, a novel spectral processing scheme is described for extracting glucose concentration information from the resulting absorbance spectra. The key component of this scheme is a digital Fourier filter that removes both high-frequency noise and low-frequency base-line variations from the spectra. Numerical optimization procedures are used to identify the best location and width of a Gaussian-shaped frequency response function for this Fourier filter. A dynamic area calculation, coupled with a simple linear base-line correction, provides an integrated area from the processed spectra that is linearly related to glucose concentrations over the range 1-20 mM. The linear calibration model accurately predicted glucose levels in a series of test solutions with an overall mean percent error of 2.5%. Based on the uncertainty in the parameters defining the calibration model and the variability of the magnitudes of the integrated areas, an overall uncertainty of 7.8% was estimated for predicted glucose concentrations.

Mapping non-linear integro-differential equations into the frequency domain

Abstract: A recursive algorithm is derived to compute the generalized frequency response functions for a large class of non-linear integro-differential equations. Applications to Duffing's equation and a modified Van der Pol model are discussed.

Frequency response of linear time periodic systems

Abstract: A frequency response for linear time periodic (LTP) systems that exploits the one-to-one map induced by geometrically periodic signals is developed. This map is described by an integral operator, based on the GP (geometrically periodic) test input, and a generalized harmonic balance approach, based on an EMP (exponentially modulated periodic) input. The singular values or principal gains of the LTP operator are discussed, and the LTP principal gain diagram is described. Directional properties of the LTP operator are discussed, and notions of the domain and range spaces are presented. The framework of linear operators described has lead to the development of a comprehensive open-loop analysis theory for LTP systems, including a characterization of poles, transmission zeros, and their directional properties. >

Considerations for fast settling operational amplifiers

Abstract: The design considerations for fast-settling operational amplifiers (op amps) are significantly different between sampled-data switched-capacitor (SC) and conventional continuous-time applications. In SC circuits, the shape of the output voltage waveform of an op amp is of no consequence provided that the output settles to within a specified tolerance of its steady-state value prior to the next sampling instant. This feature allows for an optimum op amp frequency shaping to obtain a minimum small-signal settling time. The theory applies to any op amp that is well approximated by a two-pole model, including the conventional two-stage and single-stage folded-cascode topologies. As the commonly used equivalent-circuit Miller-effect model for frequency compensation has generally been improperly applied to two-stage transconductance amplifiers, it does not provide sufficient accuracy to achieve the optimum phase margin condition. Therefore, the use of equivalent-circuit models has been refined to provide greater accuracy and to eliminate some previous misconceptions. >

Realization of current mode all-pass networks using a current conveyor

Abstract: Two novel networks are proposed for realizing a current-mode all-pass filter with high output impedance, using a single current conveyor (CC II) and four passive elements. The networks can be used in cascade without requiring an impedance-matching device. Even with a nonideal CC II, the networks can realize all-pass networks with slightly altered values of the gain and the center frequency. >

Transit-time limited frequency response of InGaAs MSM photodetectors

Abstract: Calculations are reported of the transit-time limited frequency response of InGaAs interdigitated metal-semiconductor-metal (MSM) Schottky barrier photodetectors for 1.55- mu m wavelength incident radiation. The response is examined for light which is incident from above and a two-dimensional simulation based on carrier drift is used. It is shown how the impulse response of the device changes as the interdigital spacing and the InGaAs layer thickness vary over the range typically assumed by practical detectors. The device bandwidths are also computed, and the tradeoff which occurs between high-speed performance and high quantum efficiency is studied. >

Method and apparatus for changing brain wave frequency

Abstract: A method for changing brain wave frequency to a desired frequency determines a current brain wave frequency of a user, generates two frequencies with a frequency difference of a magnitude between that of the current actual brain wave frequency and the desired frequency but always within a predetermined range of the current actual brain wave frequency, and produces an output to the user corresponding to the two frequencies. One apparatus to accomplish the method has a computer processor, a computer memory, EEG electrodes along with an amplifier, a programmable timing generator responsive to the computer processor for generating the two frequencies, audio amplifiers and a beat frequency generator driving a visual frequency amplifier.

Modeling of frequency and temperature effects in GaAs MESFETs

Abstract: The small-signal conductance and large-signal I-V characteristics of conventional 1- mu m recessed-gate GaAs depletion-mode MESFET devices have been investigated. The small-signal saturation-region output conductance g/sub ds/ of a conventional GaAs MESFET is dependent on both frequency and temperature. These dependencies present serious difficulties in the design of many GaAs integrated circuits, since the small-signal voltage gain in analog circuits and the propagation delay in digital circuits depend on g/sub ds/, and no accurate simulation model is available. A semiempirical model for the frequency-dependent parameters in GaAs depletion-mode MESFETs is presented. An analytical formulation of temperature dependence is also included by an extension of the basic Curtice model. The resulting model is significantly more accurate than other models, which have not previously incorporated frequency- and temperature-dependent effects. >

Identification in H ∞ : a robustly convergent, nonlinear algorithm

Abstract: In this paper a system identification technique is developed which is compatible with current robust controller design methodologies. This technique is applicable to a broad class of stable, distributed, linear, shift-invariant systems. The information necessary for the application of this technique consists of a priori estimates on the relative stability and "steady state" gain of the unknown system together with a finite number of possibly corrupt frequency response estimates. Given this information an algorithm is specified which yields both an identified model and explicit H∞ norm error bounds. Several interesting properties of this algorithm are also discussed. Among them, the fact the algorithm is a nonlinear function of the frequency response data, and that it is robustly convergent with respect to the a priori information on relative stability and gain are singled out as characteristics which distinguish this algorithm from others currently under development by the authors.

Frequency response of electrochemical sensors to hydrodynamic fluctuations

Abstract: The response of mass transfer to a small mass sink to hydrodynamic fluctuations in the concentration boundary layer has been calculated as a function of frequency. The dimensionless local flux was expressed as a series expansion of the dimensionless local diffusion layer thickness η and the dimensionless local characteristic frequency ξ in the low frequency range, and as the asymptotic power law , in the high frequency range. The two solutions were shown to overlap fairly well for 6 [les ] ξ [les ] 13. The overall transfer function over the whole mass sink area involves a spatial distribution for which the low-frequency approximation applies at the upstream end and the high-frequency approximation applies downstream. The average response at frequency f varies as f−1.These theoretical predictions were tested electrochemically by using a rotating disk. The modulated limiting diffusion current due to a fast redox reaction at small circular microelectrodes embedded in the disk was measured as a function of the frequency of the modulation of the disk angular velocity.

Frequency analysis of time-interval-modulated switched networks

Abstract: A Fourier analysis of the time-interval-modulated switched networks is undertaken using time-varying system theory. The result is a linearized describing function approach which determines the small-signal control-to-output (fundamental Fourier component) frequency response. The proposed algorithm is general and exact, and the results are developed in closed form. The intended application area is that of switching DC-to-DC converters, where use of the algorithm is demonstrated in deriving an exact analytical expression for the control-to-output transfer function of pulse-width-modulated converters. Experimental results are presented, verifying the modeling technique. >

An efficient small signal frequency analysis method of nonlinear circuits with two frequency excitations

Abstract: One of the excitations, 'carrier', is a large signal and an arbitrary T-periodic function of time. The other excitation, 'signal', is considered as a small perturbation to the periodic steady-state response driven by the carrier. To find a small signal frequency response for the 'signal', the method uses variational equations around the periodic steady-state response. These linearized time-varying differential equations are solved in the frequency domain using a time discretization method. The method can be used to compute the small signal frequency responses of nonideal switched capacitor filters, mixers, and other nonlinear circuits, if the circuit has a stable periodic response. The formulation of the problem, computational complexity of the method, error analysis, sensitivity analysis, implementation of the method, and some applications are covered. >

Response of a planar microstrip line excited by an external electromagnetic field

Abstract: The voltages and currents induced by external electromagnetic fields on a planar microstrip line have been studied with the use of a distributed-source transmission-line model. The frequency response of the microstrip line has been analyzed by simulating the illuminating field with a plane wave arbitrarily incident on the line. The influence of the microstrip geometrical and electrical characteristics on the voltages and currents induced on the line has been examined, and indications for reducing the circuit susceptibility have been obtained. The model adopted can be used for studying the response of the line to any type of external field arbitrarily varying in space time. Numerical results show that for lines loaded with the characteristic impedance at both terminals, voltage amplitudes on the order of some millivolts and currents of some hundreds of microamperes can be induced at f=3 GHz by an incident plane wave with an electric-field intensity of 1 V/m and for various angles of incidence. The voltages and currents induced on a microstrip circuit can be reduced by using substrates of sufficiently high permittivity. >

A comprehensive approach for modeling and testing analog and mixed-signal devices

Abstract: An approach to optimizing the testing of analog and mixed-signal devices is presented. Once an accurate model has been developed, simple algebraic operations on the model can be used to select an optimum set of test points that will minimize the test effort and maximize the test confidence; estimate the parameters of the model from measurements made at the selected test points; predict the response of the device at all candidate test points as a basis for accepting or rejecting units; calculate the accuracy of the parameter estimates and response predictions on the basis of the random measurement error; and test the validity of the model, online, so that changes in the manufacturing process can be constantly monitored and the model can be updated. The authors show how each of these procedures can be performed using simple calls to routines that are available in both public domain and commercial linear algebra software packages. The approach is quite general and has been experimentally applied to the measurement of the frequency response of an amplifier-attenuator network, to fault diagnosis of a bandpass filter using time-domain measurements, and to efficient linearity tests of A/D and D/A converters. >

Damping phenomena in a wire rope vibration isolation system

Abstract: A study of the dynamic characteristics of a wire rope vibration isolation system constructed with helical isolators is presented. Emphasis is placed on the analytical modeling of damping mechanisms in the system. An experimental investigation is described in which the static stiffness curve, hysteresis curves, phase trajectories, and frequency response curves were obtained. A semi-empirical model having nonlinear stiffness, nth-power velocity damping, and variable Coulomb friction damping is developed and results are compared to experimental data. Conclusions about dynamic phenomena in the wire rope system are made based on the experimental and semi-empirical results.

Bi-impulse response design of isotropic quadratic filters

Abstract: The bi-impulse response of a 1-D or 2-D quadratic Volterra filter is introduced as a mathematical tool able to completely describe the nonlinear operator. The conditions that must be satisfied in order to obtain isotropic input/output relations are studied. The result is a formal framework that allows simple but effective operators (particularly for image enhancement and preprocessing) to be designed and a deeper insight into the properties of Volterra filters to be acquired. Examples of design and of the performance obtained by processing natural and synthetic images are presented. >

Linear and Nonlinear Encoding Properties of an Identified Mechanoreceptor on the Fly wing Measured with Mechanical Noise Stimuli

Abstract: The wing blades of most flies contain a small set of distal campaniform sensilla, mechanoreceptors that respond to deformations of the cuticle. This paper describes a method of analysis based upon mechanical noise stimuli which is used to quantify the encoding properties of one of these sensilla (the d-HCV cell) on the wing of the blowfly Calliphora vomitoria (L.). The neurone is modelled as two components, a linear filter that accounts for the frequency response and phase characteristics of the cell, followed by a static nonlinearity that limits the spike discharge to a narrow portion of the stimulus cycle. The model is successful in predicting the response of campaniform neurones to arbitrary stimuli, and provides a convenient method for quantifying the encoding properties of the sensilla. The d-HCV neurone is only broadly frequency tuned, but its maximal response near 150 Hz corresponds to the wingbeat frequency of Calliphora. In the range of frequencies likely to be encountered during flight, the d-HCV neurone fires a single phase-locked action potential for each stimulus cycle. The phase lag of the cell decreases linearly with increasing frequency such that the absolute delay between stimulus and response remains nearly constant. Thus, during flight the neurone is capable of firing one precisely timed action potential during each wingbeat, and might be used to modulate motor activity that requires afferent input on a cycle-by-cycle basis.

Time–frequency analysis of acoustic scattering from elastic objects

Abstract: Some characteristics of an insonified object are generally present in the signal carried by the returning scattering wave. As an echo is the result of the wave interaction with a material structure, such a response is distinctive for a body of given shape and composition. Traditionally, to express the information content in the echo signature, either the frequency response (transfer/form function) or the time signature (impulse response) is employed; however, for a detailed study of the scatterer’s structure, a joint time and frequency analysis is performed. The aim of this analysis is to develop a simple processing algorithm for extracting the prominent features, which can then be used to determine the physical parameters of the object. The approach is based on the modified version of the Wigner distribution function (WDF) and utilizes an image‐processing technique to depict the outstanding highlights of the scatterer’s response in a two‐dimensional time and frequency display. The physical parameters of the scatterer are then extracted from this diagram with the proper interpretation and understanding of the scattering phenomena. Several examples, based on data obtained from numerically simulated models and laboratory measurements for elastic spheres and shells, are used to illustrate the capability and proficiency of the algorithm. Separable traces in time‐frequency space are related to the specular reflection, creeping waves, and various reradiated elastic modes. From these, the general physical size, the corresponding wave speeds (elastic properties), and the density of the insonified object can be estimated.

High voltage oscilloscope probe with wide frequency response

Abstract: A high voltage probe comprises a standard high frequency coaxial cable terminated in its characteristic impedance through most of the high frequency range, and a circuit of resistors and capacitors that provide a fixed division ratio regardless of the input frequency.

Identification with non-parametric uncertainty

Abstract: An identification technique that is robust to nonparametric uncertainty (i.e. model mismatch) is presented. The identifier produces both a parameter set estimate and a frequency response set estimate. The estimates result from the inclusion of a model of the nonparametric uncertainty in that plant model. The frequency response set estimate is shown to always contain the frequency response of the plant, as long as certain design conditions are met. >

Frequency response theory for multilayer photodiodes

Abstract: An exact solution is developed for the frequency response of photodiodes composed of multiple spatially uniform layers. Each layer is analyzed separately to obtain a set of linear response coefficients. The response of the multilayer diodes is calculated using matrix algebra. Effects of carrier transit, electron and hole trapping, avalanche decay, and finite absorption length are included in the analysis. The results of R.B. Emmons (1967) and G. Lucovsky et al. (1958) for avalanche photodiodes (APDs) and p-i-n's, respectively, are obtained as special cases. The theory is illustrated by applying it to the separated absorption and multiplication. >

Interpretation of non-linear frequency response functions

Abstract: Analytical and graphical methods of interpreting generalised frequency response functions for non-linear systems are derived. It is shown that non-linear phenomena can be classified into intra-kernel and inter-kernel interference and that worst-case responses can be computed. The results are illustrated using several discrete- and continuous-time non-linear systems.