Impulse Response Time Research Articles

Si/SiGe heterostructures grown on SOI substrates by MBE for integrated optoelectronics

We have investigated the structural, electrical, and optical quality of epitaxial Si and SiGe films grown by MBE on SIMOX (separation by implanted oxygen) silicon substrates. Epitaxial films grown on these SOI substrates have been characterized using planar and cross-sectional TEM, SIMS, and Seeco chemical etching to delineate defects. We have fabricated the first Si/SiGe integrated waveguide-photodetector for long wavelength applications. Low reverse leakage current densities were seen in these device structures. The detector exhibited a responsivity of 0.43 A/W at 1.1 μm with an impulse response time of 200 ps.

Polarization correction and extension of the Kennaugh-Cosgriff target-ramp response equation to the bistatic case and applications to electromagnetic inverse scattering

An analytical time-domain expression derived by Kennaugh (1967) for the early time impulse response for smooth, convex, perfectly conducting scatterers under the physical optics approximation for the bistatic case is reinterpreted. The physical optics bistatic early time impulse responses can be interpreted as cross-sectional areas of the scatterer. A crude polarization correction to the leading edge of the physical optics impulse response is obtained for the bistatic case, leading to a simple asymptotic relation between the specular principal curvature difference and certain co-polarized phase terms in the bistatic scattering matrix. Applications to direct scattering are discussed. Profile reconstruction from bistatic data with a priori knowledge of the validity range of physical optics in the time domain is proposed and tested with the sphere.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dependence of the response of the magnetosphere–ionosphere current systems on the preconditioning of the auroral oval and on the level of the solar–terrestrial interaction

It is now well accepted that the impulse response time of the magnetosphere to sudden changes in the interplanetary medium is of the order of 2 h with the shape of the impulse response function approximating a Rayleigh function with a peak near 50 min. In a recent study, Bargatze et al. (J. Geophys. Res. 90, 6387 (1985)) examined the response of the magnetosphere for varying activity levels and found that the impulse response function has two well-defined peaks for moderate activity and a single broad peak for low and high activity levels. They explain the two peaks in the response function as the sequential contributions of the directly driven process and the unloading of stored magnetotail energy. In this paper, we ascribe to the magnetosphere–ionosphere system the bulk properties of self-inductance, capacitance, and resistance. We then proceed to construct an equivalent current system for the magnetosphere–ionosphere coupling process and study its response to changes in the cross polar cap potential drop. In particular, we permit the bulk electrical parameters to change in the manner expected as the input of energy from the solar wind modifies the magnetosphere–ionosphere system. We find that the double peak in the impulse response function identified by Bargatze et al. can be understood purely in terms of changes in the directly driven system without the need to introduce the effects of the unloading of stored energy in the magnetotail.

Impulse response and reverberation time measurements using a periodic pseudorandom sequence

The cross‐correlation method using a periodic pseudorandom sequence or M sequence for impulse response and reverberation time measurements proposed by Schroeder and his colleagues [H. Alrutz and M. R. Schroeder, Proc. 11th Congr. Acoust., Paris (1983)] represents a very significant contribution to the fields of Architectural Acoustics and Noise Control. With this technique, it is not only possible to make measurements in auditoria or offices without disturbing the audience or the occupants, but also in factories or construction sites without closing down their operations. Implementation of this method requires special signal processing techniques which will be reviewed in this presentation. Examples of measurements in simulated noisy environments will also be presented.

Flaw detection close to the interface using ultrasonic echography

The detection of flaws by ultrasonic echography becomes difficult when the flaw is at a shallow depth. The time delay between the interface and flaw echos is less than the width of the impulse-response time of the transducer, and the flaw echo generally has an amplitude much smaller than the dominating interface echo. When the surface is not too rough, the interface echo can be assumed to be known, and its comparison with the signal interface and flaw echo permits the detection of the flaw. In this article it is shown that signal processing techniques allow small flaws to be detected very close to the surface (∼0.5 mm). Two methods are proposed, the first one is based on the minimization of the mean-square error, and the second on the spectral substraction of the two echos.

Transient fields radiated by curved surfaces—Application to focusing

A theoretical model is presented for computing the transient radiated field (potential or pressure) resulting from a curved surface having an arbitrary velocity distribution. The method is a generalization of the angular spectrum theory giving the time impulse response for a given shaped surface. The technique leads to a systems theory interpretation of the radiation and diffraction effects. General expressions for arbitrary surfaces are given but important simplifications occur for radially symmetric geometries. For this case, simple expressions for the wave location may be obtained without requiring solution of the wave. Numerical simulations for common focused waves are given using computationally efficient FFT algorithms.

Elastic wave scattering from irregular voids

Results are presented for elastic wave scattering from irregular voids embedded in Ti alloy by the diffusion bonding process. The defects examined are: two overlapping spherical voids of unequal radii, two overlapping voids consisting of a sphere and a prolate spheroid, and a spherical void with an encircling crack. Representative plots are given for the raw waveforms, magnitude of the deconvolved Fourier transform, and in some cases the time impulse response function. The data are compared to and analyzed in terms of two current theoretical approaches. While good quantitative agreement was observed over certain ranges, the comparisons point to definite (in some cases not unexpected) limitations in either the pertaining theory or experiment or both. The results are discussed with an eye toward applications to nondestructive evaluation.

Subpicosecond coherent analysis of the impulse response of optical fibers

Deals with a new optical technique which analyses the amplitude transfer function of optical frequencies passing through fibers and modulus and phase of the associated time impulse response. The dispersion of a short monomode fiber was measured around lambda =5900 AA with 0.03 ps time resolution.

Narrow-band AP latencies in normal and recruiting human ears.

Derived narrow-band action potential latencies increase monotonically with decreasing central frequency, and can be interpreted as reflecting the traveling wave delay in the cochlea. It was found that, for recruiting human ears with average flat hearing losses around 40 dB, this accumulating latency increase was smaller than for normal ears. A comparison of 15 normal ears and 37 recruiting ears showed, however, that in only half of the recruiting ears this difference was significant. These recruiting ears were therefore divided in two groups based on the waveform of the narrow-band action potential AP, which correlated well with the subdivision according to latency. The findings have been explained on the basis that latency of the narrow-band APs is not determined solely by the mechanical traveling-wave delay, but also by the response time of the (second?) cochlear filter. When this filter broadens, one expects a decrease in its impulse response time. Since this impulse response time. Since this impulse response depends on the sum of the high- and low-frequency slope values of the cochlear filter, one expects only a latency decrease when the steep high-frequency slope also becomes more shallow. A support for the influence of the response times of the cochlear filter is found in the narrow-band AP latencies for restricted cochlear losses (e.g., in a 4-kHz noise dip). It appears that the latency in that area actually is shorter than for the higher central frequencies, a fact which cannot be explained solely on the basis of a traveling wave phenomenon.

Polychromatic speckle technique for three dimensional non-destructive photoelasticimetry

Isoclinic and isochromatic fringes of slices optically isolated in birefringent models are visualized using spectroscopic analysis of speckle patterns in polychromatic radiation. Concepts of time impulse response and spectral transfer function of random birefringent linear optical pupils are involved in this work.

Spatial and temporal impulse response of optical diffracting apertures

The optical diffracting apertures are characterised by means of two related functions: the space impulse response and the time impulse response of the pupil. The functions are derived from the Rayleigh-Sommerfeld diffraction formulation, and the time impulse response concept discussed.

Time impulse response and time frequency response of optical pupils

We define the time impulse response of optical pupils and relate this function to the optical characteristics of the pupils. Then we link the time impulse response to the spatial impulse response of the pupil.

Time impulse response and time frequency response of optical pupils.:Experimental confirmations and applications

Time impulse response and time frequency response of optical pupils.:Experimental confirmations and applications

The Time-Dependent Force and Radiation Impedance on a Piston in a Rigid Infinite Planar Baffle

An approach is presented to compute the time-dependent force acting on a piston in a rigid infinite planar baffle as a result of the specified velocity of the piston. The approach to computing the force is applicable to both sinusoidal and nonsinusoidal velocity pulses and is valid for all piston shapes. The approach, which is based on a Green's-function solution to the time-dependent boundary value problem, utilizes a transformation of coordinates to simplify the evaluation of the double surface integrals. An impulse response function is defined such that the time-dependent force can be obtained by differentiating the convolution of the impulse response and piston velocity time functions. A closed-form expression for the impulse response of a circular piston is derived and discussed. Numerical results for the impulse response and the forces on large square pistons resulting from sinusoidal piston velocities are then presented and discussed to compare the transient and steady-state behavior of the forces. Finally, an approach is presented to compute the radiation impedance as a function of normalized frequency from the impulse response data, and the approach is used to obtain the normalized radiation resistance and reactance for square pistons.

On: “Three-Dimensional Filtering with an Array of Seismometers” by John P. Burg (GEOPHYSICS, October 1964, p. 693–713)

In Section IV of the above paper, Burg writes the three-dimensional impulse response of a seismometer as [Formula: see text] where [Formula: see text] is the time-impulse response of a seismometer, and [Formula: see text] is the location of the seismometer in an [Formula: see text]coordinate system. Although at first glance this is intuitively appealing, it is not correct.

Quadratic programming in the statistical design of sampled-data control systems

Quadratic programming is applied to the statistical design of sampled-data feedback control systems with finite impulse response time. Given a fixed plant, quadratic programming is used to determine the compensator which minimizes the mean-square sampled error subject to a set of constraints. The system inputs are described in terms of sampled values from autocorrelation functions. A variety of constraints can be employed, including constraints which characterize the closed-loop response of the system to deterministic inputs.

Rate of Adaptation in Control Systems

Adaptive control systems are capable of giving near opt imum performance in the face of changing input command and noise characteristics, changing dynamics of the controlled processes, and changing goal or mission requirements. The principles of feedback control are used by the adaptation mechanism to control the structure of systems. The quality of adaptation is given by the system , the ratio of the mean increase in mean square error (from adjustments based on finite statistical data) divided by the minimum mean square error. A control system can adapt to a major change in process statistics in about 10 times the impulse response time of the system itself, with a misadjustment of only 20%. Faster adaptation is possible with patternrecognizing adaptive control systems tha t use longer term experiences. Pattern-recognizing niters may be composed of adaptive Adaline neurons . A new electric circuit element called the memistor (a resistor with memory) has been devised to facilitate the realization of the Adaline neuron. It is a compact rugged electrochemical element whose resistance can be controlled reversibly by electroplating. The experiences of the neuron are stored in resistance values in a simple and directly usable form.