Showing papers on "Frequency response published in 1998"

Range dependence of the response of a spherical head model

Abstract: The head-related transfer function (HRTF) varies with range as well as with azimuth and elevation. To better understand its close-range behavior, a theoretical and experimental investigation of the HRTF for an ideal rigid sphere was performed. An algorithm was developed for computing the variation in sound pressure at the surface of the sphere as a function of direction and range to the sound source. The impulse response was also measured experimentally. The results may be summarized as follows. First, the experimental measurements were in close agreement with the theoretical solution. Second, the variation of low-frequency interaural level difference with range is significant for ranges smaller than about five times the sphere radius. Third, the impulse response reveals the source of the ripples observed in the magnitude response, and provides direct evidence that the interaural time difference is not a strong function of range. Fourth, the time delay is well approximated by well-known ray-tracing formula due to Woodworth and Schlosberg. Finally, except for this time delay, the HRTF for the ideal sphere appears to be minimum-phase, permitting exact recovery of the impulse response from the magnitude response in the frequency domain.

Simulation of transmission line transients using vector fitting and modal decomposition

Abstract: This paper introduces a fast and robust method for rational fitting of frequency domain responses, well suited for both scalar and vector transfer functions. Application of the new method results in increased computational efficiency for transmission line models using modal decomposition with frequency dependent transformation matrices. This is due to the fact that the method allows the fitted elements of each eigenvector to share the same set of poles, and that accurate fitting can be achieved with a relatively low number of poles.

Optimized frequency-shaping circuit topologies for LDOs

Abstract: Typical low drop-out (LDO) regulator architectures suffer from an inherent load regulation performance limitation. This limitation manifests itself through limited DC open-loop gain, and results from stringent closed-loop bandwidth requirements. The frequency response of the system is highly sensitive to the loading conditions, thereby making proper compensation a laborious endeavor. This paper discusses and addresses the limitation on regulating performance imposed by frequency compensation. Several LDO circuit topologies are subsequently developed to this end. They enhance load regulation performance by relaxing the DC open-loop gain restrictions. The circuit structures essentially alter the frequency response of the system via the error amplifier. A low drop-out regulator adopting an embodiment of the proposed technique was fabricated in the MOSIS 2-/spl mu/m process technology. The system, designed for an output capacitor of 4.7 /spl mu/F, was stable with an equivalent series resistance (ESR) ranging from 0 to 12 /spl Omega/, bypass capacitors ranging from 0 to 2.2 /spl mu/F, and a load current ranging from 0 to 50 mA.

Control of a voltage-source converter connected to the grid through an LCL-filter-application to active filtering

Abstract: In this paper, a control principle for a voltage source converter connected to the grid through an LCL-filter is presented. The dynamic performance is compared with the performance obtained with an L-filter and a dead-beat vector control system. Measured frequency responses and an active filtering operation verify the control principle. By using an LCL-filter, high attenuation of harmonics caused by the PWM and high dynamic performance can be obtained simultaneously. Different methods for active filtering are compared. It is advantageous to use a Fourier-method since it allows for compensation for time delays occurring in the control system in the frequency domain. By using the LCL-filter along with compensation for time delays in the frequency domain, active filtering can be performed at moderate switching frequencies. This is a major advantage in systems, such as a rectifier with an active filtering option.

FDTD analysis of wave propagation in nonlinear absorbing and gain media

Abstract: An explicit finite-difference time-domain (FDTD) scheme for wave propagation in certain kinds of nonlinear media such as saturable absorbers and gain layers in lasers is proposed here. This scheme is an extension of the auxiliary differential equation FDTD approach and incorporates rate equations that govern the time-domain dynamics of the atomic populations in the medium. For small signal intensities and slowly varying pulses, this method gives the same results as frequency-domain methods using the linear susceptibility function. Population dynamics for large signal intensities and the transient response for rapidly varying pulses in two-level (absorber) and four-level (gain) atomic media are calculated to demonstrate the advantages of this approach.

On the multiple microphone method for measuring in-duct acoustic properties in the presence of mean flow

Abstract: Nowadays, the two-microphone method is accepted as the standard as specified in ASTM E1050-90 for measuring in-duct acoustic properties. However, research results on using the least square method with multiple measurement points and broadband excitation have been reported for enhancing the frequency response of the two-microphone method. In this paper, the effects of varying the relative measurement positions on errors in the estimation of the acoustic quantities is studied for the multiple microphone method. Both the theoretical and experimental results show that, among possible sensor positioning configurations, the equidistant positioning of sensors yields the smallest error within the effective measurement frequency range. In addition, it is noted that the measurement accuracy can be increased and the effective frequency range can be widened by increasing the number of equidistant sensors. Measurement examples are shown and the results support the findings.

Disk drive capable of autonomously evaluating and adapting the frequency response of its servo control system

Abstract: The invention is a method of characterizing the frequency response of the servo control system in a disk drive having a sampled servo system having a sampling rate and a nominal bandwidth, wherein the sampled servo system comprises a plant and a servo controller that controls the plant using a compensator and a gain element with a nominal open loop gain. The invention is, in more detail, a method for adaptively modifying the servo controller to compensate for plant variations which are incompatible with the nominal gain and bandwidth, including the steps of implementing a self-generated bode plot to determine a gain margin and a phase margin, and if the gain margin is not greater than a predetermined minimum, adjusting the open loop gain of the servo controller to provide a gain margin which is greater than the predetermined minimum at a bandwidth which is different than the nominal bandwidth; and adjusting the compensator to provide a phase margin which is greater than a predetermined minimum.

Transmission characteristics of dichroic filters measured by THz time-domain spectroscopy

Abstract: The transmission characteristics of perforated flat plates which act as dichroic filters or frequency selective surfaces were measured in the far-infrared (FIR) region by Terahertz Time-Domain Spectroscopy (THz-TDS) The me- chanically made dichroic filters were designed for the cutoff frequencies of 155, 586 ,a nd773 GHz Due to their sharp cut- off characteristics, dichroic filters are used in quasi-optical systems as compact, high-pass filters throughout the sub- millimeter wave and FIR region Knowledge of the frequency response of such filters is important for various applications in frequency mixing and frequency multiplying experiments in the microwave region The spectrum of a freely propa- gating THz pulse with and without filter was measured by electro-optic sampling in a ZnTe crystal The experimental results for the frequency dependence of the transmittance of the dichroic filters are in good agreement with calculations based on the theory given by Chen (1)

Three-dimensional organization of otolith-ocular reflexes in rhesus monkeys. III. Responses To translation

Abstract: The three-dimensional (3-D) properties of the translational vestibulo-ocular reflexes (translational VORs) during lateral and fore-aft oscillations in complete darkness were studied in rhesus monkeys at frequencies between 0.16 and 25 Hz. In addition, constant velocity off-vertical axis rotations extended the frequency range to 0.02 Hz. During lateral motion, horizontal responses were in phase with linear velocity in the frequency range of 2-10 Hz. At both lower and higher frequencies, phase lags were introduced. Torsional response phase changed more than 180 degrees in the tested frequency range such that torsional eye movements, which could be regarded as compensatory to "an apparent roll tilt" at the lowest frequencies, became anticompensatory at all frequencies above approximately 1 Hz. These results suggest two functionally different frequency bandwidths for the translational VORs. In the low-frequency spectrum ( 90 degrees at frequencies <0.2 Hz). No consistent dependence on static head orientation was observed for the vertical response components during up-down motion and the horizontal and torsional response components during lateral translation. The frequency response characteristics of the translational VORs were fitted by "periphery/brain stem" functions that related the linear acceleration input, transduced by primary otolith afferents, to the velocity signals providing the input to the velocity-to-position neural integrator and the oculomotor plant. The lowest-order, best-fit periphery/brain stem model that approximated the frequency dependence of the data consisted of a second order transfer function with two alternating poles (at 0.4 and 7.2 Hz) and zeros (at 0.035 and 3.4 Hz). In addition to clearly differentiator dynamics at low frequencies (less than approximately 0.5 Hz), there was no frequency bandwidth where the periphery/brain stem function could be approximated by an integrator, as previously suggested. In this scheme, the oculomotor plant dynamics are assumed to perform the necessary high-frequency integration as required by the reflex. The detailed frequency dependence of the data could only be precisely described by higher order functions with nonminimum phase characteristics that preclude simple filtering of afferent inputs and might be suggestive of distributed spatiotemporal processing of otolith signals in the translational VORs.

Rule-based autotuning based on frequency domain identification

Abstract: A frequency domain procedure is proposed for the automatic tuning of proportional integral derivative (PID) controllers. The nonparametric open-loop frequency response function is estimated online (in closed loop), and simple discriminatory parameters are obtained, which enable appropriate tuning of the controller. For processes with some prior knowledge of the dynamics, the method removes the need for relay tuning or open-loop step response testing. If there is little or no prior knowledge of the process dynamics, relay or step response testing can be used initially, with the method then being used for the subsequent tuning. As part of the paper, a comprehensive comparison of existing rule-based tuning formulas is given using realistic plant examples.

A pulsed GTEM system for UHF sensor calibration

Abstract: A system for measuring the sensitivity of electric field sensors at UHF using a novel gigahertz transverse electromagnetic (GTEM) cell is described. The system was developed to enable calibration of UHF couplers that are used to detect insulation defects in gas insulated substations. The frequency response of a sensor is determined using fast Fourier transform (FFT) processing of its output voltage when it is subjected to a step electric field with a rise time of less than 300 ps. Provided the step response of the sensor is relatively short, the absorbing termination of the GTEM cell can be dispensed with, simplifying its construction. The need for impedance matching at the cell input can be eliminated by using a signal source that absorbs the reflected energy. In addition, the use of a wire septum offers the potential for improving field uniformity. A description of the measurement system is given, and some typical results are presented.

Experimental demonstration of modulation bandwidth enhancement in distributed feedback lasers with external light injection

Abstract: The frequency response of a semiconductor distributed feedback (DFB) laser under optical injection locking has been studied experimentally. It is shown that strong injection locking is very effective at increasing the relaxation oscillation frequency of DFB lasers. Bandwidth enhancement as high as 3.7 times has been achieved for the first time.

Mechanical design and control of a high-bandwidth shape memory alloy tactile display

Abstract: We have constructed a tactile shape display which can be used to convey small scale shape in teleoperation and virtual environments. A line of 10 pins spaced 2 mm on center are each actuated with a shape memory alloy wire. A combination of careful mechanical design and liquid cooling allows a simple proportional controller with constant current feed forward to achieve 40 Hz bandwidth. To quantify the value of increased bandwidth, an experiment involving a prototypical search task has been conducted using the display. A digital filter limited the frequency response of the display to three cutoff frequencies: 1, 5 and 30 Hz. Subjects were able to complete the search more than 6 times as quickly with 30 Hz bandwidth than with 1 Hz.

Implantable hearing system having multiple transducers

Abstract: This invention is an apparatus for improving a frequency response of a piezoelectric input or output transducer (100) in an implantable hearing system. Multiple input or multiple output transducers (140, 145) obtain optimized mechanical to electrical or electrical to mechanical frequency response. Output mechanical coupling is directly to the inner ear (60, 61, 62), or through an ossicular element such as the malleus (40), stapes (50), or incus (45). Input mechanical vibrations are obtained from an auditory element such as the tympanic membrane (30), malleus (40), or incus (45). Substantially nonidentical frequency responses are obtained through use of transducers of different dimensions, different numbers of transducer elements, different material properties, different mounting techniques, or different auditory elements for coupling.

Amplitude response and stimulus presentation frequency response of human primary visual cortex using BOLD EPI at 4 T

Abstract: Detailed measurement of the neural response to flicker frequency using functional MRI (fMRI) were made. The fMRI signal peaks at a flicker frequency of 8 Hz in human V1, in agreement with previous positron emission tomography (PET) and fMRI experiments. The modulation amplitude of the hemodynamic response to varying continuous periods of flicker stimulation was measured. The hemodynamic response was not observed to be modulated by neural modulation for periods shorter than 6.7 s. The resemblance between the BOLD response to the stimulus presentation frequency and the base-line power spectra at the same frequencies suggests that the same underlying mechanism could be responsible for both curves and that the base-line fMRI power spectrum is probably due to base-line electrical activity in the brain. The integrals of the resting base-line power spectrum, the background power spectrum, the respiration component, and the cardiac component were found to be linearly dependent on TE.

Sound generation in gradient coil structures for MRI

Abstract: When supporting plates of plastic material are subjected to alternating transverse Lorentz forces while in a strong magnetic field normal to the plate surface, compressional waves within the solid produce a modulation of the plate surface that launches an acoustic wave in air along the magnetic field axis. We have extended our previous theory describing this process to include a detailed description of the formation of an acoustic interference pattern in air described by Fraunhofer diffraction at a distance from the plate surface. The extended theory predicts that the observed acoustic signal midpoint and normal to the plate surface gives a variation with frequency in approximate agreement with our previous measurements. The acoustic output off axis shows acoustic blazing that produces two main diffraction peaks with a splitting inversely proportional to the velocity of sound in the plate material. The new results could have important ramifications for the minimization of sound output in gradient coil design for MRI. A new arrangement of coils is proposed to ameliorate the acoustic output problem centrally and normal to the plate by extending the frequency response of the supporting plates to much higher frequencies. Also presented are estimates of the compressional wave velocities deduced from frequency response data recorded at the center-point of a number of different plates.

Apparatus and methods for proportional audio compression and frequency shifting

Abstract: Apparatus and methods for audio compression and frequency shifting retain the spectral shape of an audio input signal while compressing and shifting its frequency. The fast Fourier transform of the input signal is generated, to allow processing in the frequency domain. The input audio signal is divided into small time segments, and each is subjected to frequency analysis. Frequency processing includes compression and optional frequency shifting. The inverse fast Fourier transform function is performed on the compressed and frequency shifted spectrum, to compose an output audio signal, equal in duration to the original signal. The output signal is then provided to the listener with appropriate amplification to insure audible speech across the usable frequency range.

Very high-speed metal-semiconductor-metal ultraviolet photodetectors fabricated on GaN

Abstract: We report on the temporal and the frequency response of metal-semiconductor-metal ultraviolet photodetectors fabricated on single-crystal GaN. The best devices show a fast 10%–90% rise time of ∼23 ps implying a bandwidth of >15 GHz. These time domain data have been corroborated by direct measurement of the power spectral content. From this a cutoff frequency, f3 dB, of ∼16 GHz has been obtained. Analysis in terms of reverse bias and geometric scaling indicates that the photodetectors are transit-time limited. Modeling of the temporal behavior indicates that a slow component in the time and frequency response data is a consequence of the hole drift velocity.

A simplified approach to time-domain modeling of avalanche photodiodes

Abstract: A simplified algorithm for calculating time response of avalanche photodiodes (APDs) is presented. The algorithm considers the time course of avalanche processes for the general case of position-dependent double-carrier multiplications including the dead space effect. The algorithm is based on a discrete time setting ideally suited for computer modeling and can be applied to any APD structure. It gives a fast and accurate estimation of the time and frequency response of APDs. As an example, the present method is applied to InP-InGaAs separate absorption, grading, charge, and multiplication (SAGCM) APDs. The variation of multiplication pain with bias voltage and 3-dB electrical bandwidth at different multiplication gain obtained using the new algorithm show good agreement with experimental results. The algorithm can be used to study temperature dependence of APD characteristics and can be easily extended to calculate the excess noise factor.

Dielectric combline resonators and filters

Abstract: By replacing the inner conductor of the conventional combline resonator with a high /spl epsiv//sub r/ dielectric rod, higher unloaded Q of the resonator can be expected. Resonant frequency, unloaded Q and coupling coefficient of the resonator are obtained by rigorous mode matching method. An 8-pole elliptic function dielectric combline resonator filter is designed. Good frequency responses are obtained.

Continuous frequency dynamic range audio compressor

Abstract: An improved multiband audio compressor (10) is well behaved for both wide band and narrow band signals, and shows no undesirable artifacts at filter crossover frequencies The compressor includes a heavily overlapped filter bank (16), which is the heart of the present invention The filter bank filters the input signal (56) into a number of heavily overlapping frequency bands (58) Sufficient overlapping of the frequency bands reduces the ripple in the frequency response, given a slowly swept sine wave input signal, to below about 2 dB, 1 dB, or even 05 dB or less with increasing amount of overlap in the bands Each band is fed into a power estimator (18), which integrates the power of the band and generates a power signal (60) Each power signal is passed to a dynamic range compression gain calculation block (20), which calculates a gain (62) based upon the power signal Each band is multiplied by its respective gain in order to generate scaled bands (64) The scaled bands are then summed to generate an output signal (68)

Seismic wavelet estimation: a frequency domain solution to a geophysical noisy input-output problem

Abstract: In seismic reflection prospecting for oil and gas a key step is the ability to estimate the seismic wavelet (impulse response) traveling through the Earth. Such estimation enables filters to be designed to deblur the recorded seismic time series and allows the integration of "downhole" and surface seismic data for seismic interpretation purposes. An appropriate model for the seismic time series is a noisy-input/noisy-output linear model. The authors tackle the estimation of the impulse response in the frequency domain by estimating its frequency response function. They use a novel approach where multiple coherence analysis is applied to the replicated observed output series to estimate the output signal-to-noise ratio (SNR) at each frequency. This, combined with an estimate of the ordinary coherence between observed input and observed output, and with the spectrum of the observed input and cross-spectrum of the observed input and output, enables estimation of the frequency response function. The methodology is seen to work well on real and synthetic data.

An overview of the PES Pareto Method for decomposing baseline noise sources in hard disk position error signals

Abstract: This paper gives an overview of the PES Pareto Method, a useful tool for identifying and eliminating key contributors to uncertainty in the Position Error Signal (PES) of a magnetic disk drive servo system. Once identified and ranked according to their overall effect on PES, the top-ranking sources can be worked on first, either by finding ways to reduce their magnitude or by altering system components to reduce sensitivity to the contributors. The PES Pareto Method is based on three ideas: (1) an understanding of how Bode's Integral Theorem applies to servo system noise measurements, (2) a measurement methodology that allows for the isolation of individual noise sources, and (3) a system model that allows these sources to be recombined to simulate the drive's Position Error Signal. The method requires the measurement of frequency response functions and output power spectra for each servo system element. Each input noise spectrum can then be inferred and applied to the closed loop model to determine its effect on PES uncertainty. The PES Pareto Method is illustrated by decomposing PES signals that were obtained from a hard disk drive manufactured by Hewlett-Packard Company. In this disk drive, it is discovered that the two most significant contributors to PES baseline noise are the turbulent wind flow generated by the spinning disks ("Windage") and the noise involved in the actual readback of the Position Error Signal ("Position Sensing Noise").

Photodiode DC and microwave nonlinearity at high currents due to carrier recombination nonlinearities

Abstract: We present photodetector compression measurements to demonstrate the consequences of absorption in undepleted regions close to the depletion region of p-i-n photodiodes. This absorption can modify the frequency response of photodetectors operating above a few milliamperes. The frequency response shows power dependence and additional ripple and rolloff.

Locating Structural Damage Using Frequency Response Reference Functions

Abstract: A technique for monitoring vibration measurements to detect structural damage is presented. Frequency response functions from a healthy structure are measured for reference data, then cross-spectral densities between pairs of combined damage and external forces are computed to detect any occurring damage. The excitation forces are not measured, and can be uniform, random, and uncorrelated, or applied at a single point on the structure. The technique bounds the damage location between the closest sensors on the structure, only a small number of sensors are needed, the damage force is approximated, and no model of the structure is used. For bending vibrations, rotations must also be measured which in practice is often difficult. In a finite-element simulation, the method located small damage and approximated the damage force for a beam structure.