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No description available.

HF-waves incident on an overdense (HF-frequency < penetration frequency) ionosphere are known to produce large scale electron density irregularities. It is predicted that similar irregularities are formed during underdense HF-modification. The propagation of UHF radio waves originating from radio stars will be affected by such irregularities in the ionosphere. The interest in a scintillation experiment is twofold. One may obtain information on the electron density irregularies and one may learn about the propagation of radio waves through such a perturbed medium. A thin screen (diffractive) theory is derived which allows to draw conclusions on the electron density irregularities from the intensity fluctuations measured on the ground if the phase perturbations are much less than one radian. Since radio stars suitable for scintillation measurements at UHF are very faint an antenna with a large collection area is required. The observations reported in this dissertation were performed with the 300m diameter spherical reflector of the Arecibo Observatory. Successful observations were performed at 430 MHz and at 1400 MHz. Intensity fluctuations at such high frequencies measured with a large antenna suffer severe filtering in the thin phase screen regime. The theory presented in this dissertation includes these filtering effects. Many observations agree with the predictions of that theory. Some observations indicate that refraction effects have to be included to explain the data. HF-induced electron density irregularities were only observed during overdense heating. Several attempts to detect irregularities at an observing frequency of 430 MHz during underdense heating (HF-frequencies 1.1 to 2 times penetration frequency) failed. All the measurements reported in this dissertation are therefore during overdense heating except for the measurement on the 22 September 1980 whichs shows a transition from underdense to overdense HF-modification. Attempts were made to measure electron density irregularities at an observing frequency of 2380 MHz due to overdense heating but the fluctuations were too weak to be discriminated against instrumental uncertainties. The formation time for the irregularities after the HF-power was turned on was (TURN)25 seconds. Their lifetime after the HF-power was turned off was on the order of hours. The electron density perturbations appear to exceed 1% at times. One observation on 11/12 September 1980 reveals that the density perturbation actually corresponds to a depletion.

Several new algorithms are presented for the optimal approximation and design of various classes of digital filters. An iterative algorithm is developed for the efficient design of unconstrained and constrained infinite impulse response (IIR) digital filters. Both in the unconstrained and constrained cases, the numerator and denominator of the filter transfer function are designed iteratively by recourse to the Remez algorithm and to appropriate design parameters and criteria, at each iteration. This makes it possible for the algorithm to be implemented by means of a short main program which uses (at each iteration) the linear phase FIR filter design algorithm of McClellan et al. as a subroutine. The approach taken also permits the filter to be designed with a desired ripple ratio. Also, the algorithm determines automatically the minimum passband ripple corresponding to the prescribed orders and band edges of the filter. The filter is designed directly without guessing the passband ripple or stopband ripple. Another algorithm, based on similar principles, is developed for the design of a nonlinear phase finite impulse response (FIR) filter, whose transfer function optimally approximates a desired magnitude response, there being no constraints imposed on the phase response. A similar algorithm is presented for the design of two new classes of FIR digital filters, one linear phase and the other nonlinear phase. A filter of either class has significantly reduced number of multiplications compared to the one obtained by its conventional counterpart, with respect to a given frequency response. In the case of linear phase, by introducing the new class of digital filters into the design of multistage decimators and interpolators for narrow-band filter implementation, it is found that an efficient narrow-band filter requiring considerably lower multiplication rate than the conventional linear phase FIR design can be obtained. The amount of data storage required by the new class of nonlinear phase FIR filters is significantly less than its linear phase counterpart. Finally, the design of a (finite-impulse-response) FIR digital filter with some of the coefficients constrained to zero is formulated as a linear programming (LP) problem and the LP technique is then used to design this class of constrained FIR digital filters. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI

Polysilicon is believed to be a key element for continued evolution of silicon integrated circuits. Recent advances in radiation processing and passivation techniques have enhanced the possibility of realizing acceptable active devices in polysilicon thin films. Of late, MOS devices fabricated in polysilicon do hold up a distinct possibility of achieving 3-D integration for higher packing density. P-n junction diode constitutes an essential element of any device. It is therefore imperative to have a quantitative model of p-n junction diodes in polysilicon. In this thesis, a model for the polysilicon p-n junction diodes is developed. The present model is based on incorporating the effective minority carrier lifetime operative in crystalline grain and amorphous conducting boundary. The bulk resistance effect especially at lower doping levels is accounted for. In addition, in the present model, the grain boundaries parallel to the current flow in the junction depletion depth are shown to serve as ohmic conduction channels. This additional amorphous channel can account for the unusually high current level observed at small applied voltages. The role of grain boundary in affecting minority carrier diffusion processes is illustrated by considering the presence of one grain boundary in the analysis of continuity equation operative in minority carrier diffusion region.

Four methods for designing digital filters optimal in the Chebyshev sense are developed. The properties of these filters are investigated and compared. An analytic method for designing narrow-band FIR filters using Zolotarev polynomials, which are extensions of Chebyshev polynomials, is proposed. Bandpass and bandstop narrow-band filters as well as lowpass and highpass filters can be designed by this method. The design procedure, related formulae and examples are presented. An improved method of designing optimal minimum phase FIR filters by directly finding zeros is proposed. The zeros off the unit circle are found by an efficient special purpose root-finding algorithm without deflation. The proposed algorithm utilizes the passband minimum ripple frequencies to establish the initial points, and employs a modified Newton's iteration to find the accurate initial points for a standard Newton's iteration. The proposed algorithm can be used to design very long filters (L = 325) with very high stopband attenuations. The design of FIR digital filters in the complex domain is investigated. The complex approximation problem is converted into a near equivalent real approximation problem. A standard linear programming algorithm is used to solve the real approximation problem. Additional constraints are introduced which allow weighting of the phase and/or group delay of the approximation. Digital filters are designed which have nearly constant group delay in the passbands. The desired constant group delay which gives the minimum Chebyshev error is found to be smaller than that of a linear phase filter of the same length. These filters, in addition to having a smaller, approximately constant group delay, have better magnitude characteristics than exactly linear phase filters with the same length. The filters have nearly equiripple magnitude and group delay. The problem of IIR digital filter design in the complex domain is formulated such that the existence of best approximation is guaranteed. An efficient and numerically stable algorithm for the design is proposed. The methods to establish a good initial point are investigated. Digital filters are designed which have nearly constant group delay in the passbands. The magnitudes of the filter poles near the passband edge are larger than of those far from the passband edge. A delay overshooting may occur in the transition band (don't care region), and it can be reduced by decreasing the maximum allowed pole magnitude of the design problem at the expense of increasing the approximation error.

Indium Phosphide epitactic layers grown by MOCVD and lightly doped with Zn and Mg (p-type) have been studied by Hall effect measurements and secondary ion mass spectroscopy (SIMS). The Zn-doped samples showed a uniform distribution of the acceptor concentration as a function of depth. Mg doped samples showed strong Mg concentration gradients, with substantial diffusion into the substrate. An electrical transport model has been developed for each of the previous cases. These models provided excellent agreement with experiment. The uniformly Zn-doped samples have been used for a detailed study of hopping conduction. Excellent quantitative agreement can be reached between theory and experiment both in the variable range hopping regime and in the nearest neighbor hopping regime. The saturation of the latter regime has been observed, and a model has been found in qualitative agreement with it. An interpretation of the low temperature data on the basis of the presence of a Coulomb gap in the impurity band density of states is not appropriate.

A gas-solid reaction analyzer was built in order to study the kinetics of various interesting gas-solid reactions, including the interstitial modification kinetics of carbonitride magnets. The system performs an analysis of the pressure and temperature of the carbonitride during the interstitial modification, as well as performing atomic mass analysis of the gases present throughout the gas-solid reaction. / All the hardware, software, and electronics that make up the system were constructed as part of the requirements for the M.Sc. thesis, with the exception of the Quadrupole Mass Spectrometer (QMS) and the Transpector electronics unit which shipped with the mass spectrometer.

The design and construction of a capacitor-discharge pulsed magnet is described. The magnet is capable of generating peak fields up to 22 T in a multi-turn solenoid coil with a 3/4" (19 mm) bore. The coil design, calculation of peak field, and an analysis of the eddy currents in the metal surrounding the coil are discussed in detail. The pulsed magnet includes a magnetometer and a data acquisition system which measure the coil field and sample magnetization. A series of magnetization curves have been obtained for a sample of Nd$ rm{ sb2 Fe sb{14} B}$.

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