Electronic Engineering

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The Design of high-voltage planar transistors with specific reference to the collector region.
(1984) Smithies, Stafford Alun.; Verster, T. C.
The thesis represents a major contribution to the understanding of the design and fabrication of high-voltage planar silicon bipolar transistors, and reports on the original research carried out and the special methods evolved leading to the successful design, development and industrialization of two highly specialized transistors. The development of these transistors, destined for high-reliability applications in subscriber telephone systems, was funded by the South African Department of Posts and Telecommunications. The first device developed was a discrete transistor meeting the requirements of a singularly difficult specification that included the following. An accurately controlled upper limit to quasi-saturation operation, so that above a collector-emitter voltage of 4 V at 60 mA, the device characteristics should be extremely linear. An extremely small range of acceptable gains, with lower and upper limits of 80 and 180 respectively. Both accurately reproducible and high breakdown-voltages exceeding 200 V. The ability to withstand 100 W pulses of 10ps duration at a case temperature of 95 °c and a collector-emitter voltage of 130 V. The second device represents a design and development breakthrough resulting in a unique high-voltage integrated Darlington transistor incorporating the following design features. The standard discrete high-voltage transistors used initially in the Darlington application were found to fail frequently due to an external breakdown mechanism under lightning surge conditions, which are common in South Africa. To overcome this weakness, the integrated Darlington incorporates a special clamping circuit to absorb the surge energy non-destructively within the bulk of the device and thereby prevent external breakdown. To act as an electrostatic shielding system a new 'inverted metallization structure' was developed and incorporated in the Darlington transistor design. With this structure it was possible to realize transistors with a combination of extremely high gains, approaching 105 , and very low collector-emitter leakage currents, often lower than 1 nA at an applied 240 V, and no device with comparable properties has been reported on elsewhere. During the development of the integrated Darlington it was recognized that there was a necessity for a simple yet accurate method of predicting quasi-saturation operation. This consideration led to the development of a totally new, user-orientated, graphical model for predicting the gain of a transistor when operating in the quasi-saturation mode a model involving the use of entirely new yet easily measured parameters. The model was successfully applied to the verification of the Darlington design and the optimization of processing parameters for the device. Although undertaken in a research environment, the projects were handled under pressures normally associated with industrial conditions. Time schedules were constrained, and this influenced design strategy. As a consequence, however, the need arose to develop fast and efficient design aids since much of the theoretical design was implemented for production without recourse to long-term experimental verification in the laboratory. Whilst the author viewed this approach as less than ideal, the successful production of almost two million of these highly specialized devices, including both types, has lent authority to the design techniques developed. In spite of the industry-like pressures imposed during the course of the work, many aspects of the development programmes were further investigated and refined by research that would have been omitted had the author accepted the realization of a working device as the only goal. This research has not only contributed to the production of devices of exceptionally high quality, but has also produced a wealth of new information valuable to future designers. These aids include a new and highly accurate correction for the parasitic collector resistance of a transistor; design data for the specification of epitaxial layers for transistors with collector-emitter breakdown voltages ranging between 5 V and 800 V; information on Gate Associated Transistor (GAT) structures; and the entirely new graphical method, mentioned above, for modelling saturation effects in bipolar transistors. Process development was successfully carried out within the strict confines of compatibility with available equipment, and the pre-requisite that the existing production of low-voltage bipolar integrated circuits should in no way be compromised. Successful transfer of the technology, followed by industrialization, has demonstrated the effectiveness of a method developed by the author for the rapid communication and dissemination of appropriate information in a system without precedents for such procedures. Listed below are other examples showing that useful information was gathered and new techniques developed. Emitter-region defects associated with the metallization process were identified. Test data were used to monitor project performance and in the development of data management techniques. Interaction with the author resulted in the establishment of the first Quality Assurance and Audit function for microelectronics activities by the Department of Posts and Telecommunications in the Republic of South Africa. The group formed had the authority to handle the certification of semiconductor capabilities and the qualification for service of semiconductor components. An entirely new continuous failure analysis programme was introduced covering both the products manufactured and similar types from other sources: a programme that has brought to light the major failure mechanisms in the high-voltage transistors. On the basis of the knowledge gained during the research and development programmes it has been possible to make recommendations, substantiated by preliminary investigations for further original research work on a new type of negative-resistance high-voltage device. This would initially be destined for use in subscriber telephones to improve their immunity to surges, and it would form the basis of the development of a totally new type of interface circuit with in-built protection against surges, for application at the subscriber line interface in electronic exchanges.
Travelling-wave frequency conversion.
(1985) Ham, Ronald Edgar.; Nattrass, Henry Lee.
Travelling-wave distributed amplifiers are providing gain over broad frequency ranges for microwave applications. Similar concepts are applicable to distributed mixers and, with the use of controlled feedback, to a multifunction component simultaneously emulating a mixer, amplifier and an oscillator. The concept of this new travelling-wave frequency converter is introduced and data for a discrete component test circuit is presented. To facilitate the converter operation a new three-port travelling-wave mixer is introduced and characterized. Four-port scattering and wave scattering transformations are derived as a method of analysis of the four-port distributed structure. This enables sequential circuit analysis on a small computer. Practical applications unique to the advanced automatic network analyser, including time domain measurements, are presented to characterize test circuits as well as to develop ancillary equipment such as a transistor test fixture. Automated error corrected transistor measurements and de-embedding are also discussed. A piecewise linear quantum mechanical method of modelling the conduction channel of a short gate field effect transistor is given to aid the extrapolation of the distributed frequency converter concept to submicron and heterojunction structures.
An Analytic model for high electron mobility transistors.
(1986) Hill, Adrian John.; Nattrass, Henry Lee.
The last six years has seen the emergence and rapid development of a new type of field effect transistor, the High Electron Mobility Transistor (HEMT), which offers improved performance in both digital and analogue circuits compared with circuits incorporating either MEtal Semiconductor (MES) or Metal Oxide Semiconductor (MOS) FETs. A new physically-based analytic model for HEMTs, which predicts the DC and RF electrical performance from the material and structural parameters of the device, is presented. The efficacy of the model is demonstrated with comparisons between simulated and measured device characteristics, at DC and microwave frequencies. The good agreement with experiment obtained with the model indicates that velocity overshoot effects are considerably less important in HEMTs than has been widely assumed, and that the electron transit velocity in submicron devices is approximately 10 cm/s, rather than around 2x10 cm/s. The Inverted HEMT, one of the major HEMT structural variants, is emphasized throughout this work because of its potential advantages over other variants, and practical results from 0.5 micron gate length Inverted HEMTs are presented.
An adaptive protocol for use over meteor scatter channels.
(1987) Spann, Michael Dwight.; Broadhurst, Anthony D.
Modem technology has revived interest in the once popular area of meteor scatter communications. Meteor scatter systems offer reliable communications in the 500 to 2000 km range all day, every day. Recent advances in microprocessor technology have made meteor scatter communications a viable and cost effective method of providing modest data rate communications. A return to the basic fundamentals has revealed characteristics of meteor scatter propagation that can be used to optimize the protocols for a meteor scatter link. The duration of an underdense trail is bounded when its initial amplitude is known. The upper bound of the duration is determined by maximizing the classical underdense model. The lower bound is determined by considering the volume of sky utilized. The duration distribution between these bounds is computed and compared to measured values. The duration distribution is then used to specify a fixed data rate, frame adaptive protocol which more efficaciously utilizes underdense trails, in the half duplex environment, than a non-adaptive protocol. The performance of these protocols is verified by modeling.
An intelligent multi-terminal interface.
(1987) Peplow, Roger Charles Samuel.; Nattrass, Henry Lee.
The document describes the development of a micro-processor based terminal multiplexer to connect four terminals to a standard Hewlett Packard series 1000 mini-computer. The project was required to fulfill the dual roll of both increasing the number of terminals that the HPI000 could support and of reducing the peripheral load on the host CPU. The final product occupied a standard 200mm square HP size interface card and used an 8085 micro-processor and several 8085 family peripheral chips to provide four full duplex serial channels and a high speed data link with the host. A multi-tasking executive was written to control the multiplexer software which was finally implemented as 15 independent tasks occupying 8 kilo-bytes of eprom. The software was written to perform all terminal interaction and editing in order to reduce the host CPU involvement to a single interrupt per record. The resultant interface proved capable of handling an aggregate throughput in excess of 4000 characters per second which was sufficient to cope with all four terminals running at 9600 bits per second, even when all four were transferring in burst mode. The interface also proved to be between five and eighteen times less demanding on the host than the two standard Hewlett Packard interfaces then available. When compared to the low cost HP12531 interface, the multiplexer increased the 9600b/s terminal handling capability of the host from 3 terminals to 52.
Modelling and testing microwave magnetrons.
(1988) Schumann, Erwin Wilhelm.; Nattrass, Henry Lee.
Though declining in popularity over the last decade, the magnetron still has applications where portable high power is needed. This study examines the predicted performance of cylindrical microwave magnetrons using analytic lumped-spoke models based on the energy conservation principle. The analytic approach is still favoured when small computer systems are used and the overall performance of the tube is to be predicted. The magnetron elements are examined and the role they play in the overall device performance analysed. Simplified representations of these elements are used to construct a complete magnetron model. The Hartree threshold condition is reexamined and a new, more accurate analytic formulation proposed. This formulation is based on electric field strengths at the base of the magnetron spoke. The effect of the space charge on the threshold condition is included. Spoke current has been evaluated at the edge of the Brillouin hub. The resulting anode- cathode voltage performance predictions are consistent with measured results. A computer program has been written to analyse the performance predicted by this model. Models proposed by other authors are examined, and compared to this model. The resulting model has been tested by comparing predicted results to the measured performance of four slot-and-hole magnetrons. To facilitate accurate magnetron testing, a new automated triple-stub high power microwave load has been developed. The load operates at a peak power of IMW from 2,7-3,OGHZ, and allows the change of the VSWR to any value along any path within the VSWR=I, 5: I circle. The development of the triple tuner and termination is discussed in detail. A new waterload configuration which has the advantage of simple construction yet good matching characteristics is presented. Automated measurement of pulling figure and construction of Rieke diagrams is discussed. The accuracy of the complete load is compared to conventional loads currently in service in the tube industry.
Using a terminal switching exchange for computer peripheral sharing and other purposes.
(1988) Barrett, Alan Peter.; Levy, David C.; Peplow, Roger Charles Samuel.
Several computers and several tens of terminals at the University of Natal are linked to a terminal switching exchange network known as NUNET, which is in fact part of a countrywide network known as NllNET. This thesis describes software that was written to enhance the usefulness of the network, particularly in the Department of Electronic Engineering. The enhancements include a simple facility to provide help to users of the network, and programs that allow various computers in the Department of Electronic Engineering to create and close network connections. The programs that create and close network connections are used to provide access to printers and plotters that are connected to the network and shared by several computers. Access to peripherals through the network has been integrated into existing printer and plotter driver programs on some of the computers, thus allowing the network to be completely transparent to the user. The same network access programs also allow connections to be made between computers for the purpose of file transfers.
A CAD tool for the prediction of VLSI interconnect reliability.
(1988) Frost, David Frank.; Poole, Kelvin F.
This thesis proposes a new approach to the design of reliable VLSI interconnects, based on predictive failure models embedded in a software tool for reliability analysis. A method for predicting the failure rate of complex integrated circuit interconnects subject to electromigration, is presented. This method is based on the principle of fracturing an interconnect pattern into a number of statistically independent conductor segments. Five commonly-occurring segment types are identified: straight runs, steps resulting from a discontinuity in the wafer surface, contact windows, vias and bonding pads. The relationship between median time-to-failure (Mtf) of each segment and physical dimensions, temperature and current density are determined. This model includes the effect of time-varying current density. The standard deviation of lifetime is also determined as a function of dimensions. A· minimum order statistical method is used to compute the failure rate of the interconnect system. This method, which is applicable to current densities below 106 AI cm2 , combines mask layout and simulation data from the design data base with process data to calculate failure rates. A suite of software tools called Reliant (RELIability Analyzer for iNTerconnects) which implements the algorithms described above, is presented. Reliant fractures a conductor pattern into segments and extracts electrical equivalent circuits for each segment. The equivalent circuits are used in conjunction with a modified version of the SPICE circuit simulator to determine the currents in all segments and to compute reliability. An interface to a data base query system provides the capability to access reliability data interactively. The performance of Reliant is evaluated, based on two CMOS standard cell layouts. Test structures for the calibration of the reliability models are provided. Reliant is suitable for the analysis of leaf cells containing a few hundred transistors. For MOS VLSI circuits, an alternative approach based on the use of an event-driven switch-level simulator is presented.
A proportional timing generator for measuring intermodulation product distortion on television transposers.
(1989) Bouwer, Paul Frederick.; Broadhurst, Anthony D.
Broadcasting authorities presently measure intermodulation distortion by applying a three tone simulation of a composite video and sound signal to the transposer and then measuring the relative amplitude of the major in-band intermodulation product (nominally at vision carrier frequency plus 1,57 MHz in the 625 line I/PAL System) on a spectrum analyser. This method is slow and requires a skilful operator to achieve repeatable results. Furthermore it tests the common RF amplification equipment at one luminance level and one chrominance level and therefore does not subject the transposer equipment to operation over its full range. A new sampling measurement technique has been proposed which overcomes all these problems by selectively mixing, while transmitting a colour bar test pattern, the demodulated output video signal of the frequency transposer with a pulse train coinciding with a particular colour. This thesis describes the design of a very stable proportional timing generator and its application to the measurement of intermodulation distortion on frequency transposers. The timing generator, which locks automatically onto the video signal and produces narrow sampling pulses which coincide accurately with a particular section of each line over a 50°C temperature range, is applicable to all PAL and NTSC TV Systems.
A comparative study of various speech recognition techniques.
(1990) Pitchers, Richard Charles.; Broadhurst, Anthony D.
Speech recognition systems fall into four categories, depending on whether they are speaker-dependent or independent of speaker population and on whether they are capable of recognizing continuous speech or only isolated words. A study was made of most methods used in speech recognition to date. Four speech recognition techniques for speaker-dependent isolated word applications were then implemented in software on an IBM PC with a minimum of interfacing hardware. These techniques made use of short-time energy and zero-crossing rates, autocorrelation coefficients, linear predictor coefficients and cepstral coefficients. A comparison of their relative performances was made using four test vocabularies that were 10, 30, 60 and 120 words in size. These consisted of 10 digits, 30 and 60 computer terms and lastly 120 airline reservation terms. The performance of any speech recognition system is affected by a number of parameters. The effects of frame length, pre-emphasis, window functions, dynamic time warping and the filter order were also studied experimentally.
Predicted and measured arrival rates of meteors over forward-scatter links.
(1991) Mawrey, Robert Stanley.; Broadhurst, Anthony D.
Investigations into currently accepted methods of modelling variations in the arrival rate of meteors over forward-scatter meteor links have revealed some shortcomings. In these investigations, particular emphasis is placed on the work of Rudie due to its current acceptance in the literature. The non-uniform radiant distribution of meteors measured by Davies and modelled by Rudie, is critically examined and predictions using these models are compared with measured results taken over two forward-scatter links in the Southern Hemisphere. A new, alternative method of including the effect of non-uniform radiant distributions on the predicted arrival rate of meteors is given. The method used by Rudie to model Davies' measured radiant distribution is shown to be unnecessarily complicated and a simpler alternative is given. Furthermore, Rudie's distribution is shown not to be derived from a particular set of Davies' results as implied by Rudie. Other non-uniform distributions of meteors are also investigated. Comparisons between monthly-averaged daily cycles of measured and predicted arrival rates of meteors for a midpath and an endpath meteor link are used to reveal the validity and limitations of the published distributions. A new graphical method is used to aid in determining the validity and limitations of the non-uniform distributions. Discrepancies in the published predicted and measured annual variations in the arrival rate of meteors are investigated. Contrary to recently published information, predicted annual variations in the arrival rate of meteors for meteor radiants close to the ecliptic are shown to be comparable to measured results.
A packet radio system for an industrial data network.
(1992) Sewnath, Gajadhar.; Levy, David C.
This project was undertaken for a commercial electronics company, CONTROL LOGIC (CONLOG) which is involved in the research, design, development and manufacture of data acquisition, control, energy management and automotive equipment. Currently CONLOG uses an inhouse token passing local area network CONET for industrial data communications.The need had arisen to provide a means of data communication amongst widely geographically distributed remote terminal units (RTUs) generating demands at a very low duty cycle. A need for communications between RTUs and a centralised controller was also required. In addition to this, multihop communications between the RTUs was required. Packet switching using a broadcast radio network provides an efficient means of achieving this. An investigation into to the various media access control protocols and contention techniques using packet radio was carried out. The various media access techniques were compared with respect to throughput and normalised delay. This led to the selection of a media access scheme for the packet radio network using RTUs. A protocol specification control is centralised or Interconnect Organisation.The switching protocol (OSI) for the packet radio network, in which distributed, was done. The architechure of specified adheres to the Open Systems model of the International Standards.An experimental packet switching radio network was implemented using the protocol specification defined above. The packet radio network (PACNET) uses existing off the shelf radios and purpose built hardware for the remote terminal units. The thesis describes methods of data communications suitable for widely dispersed industrial data communications, the selection of the packet switching media access methods and control protocols, and the design and implementation of the prototype system.
Optimized digital signal processing algorithms applied to radio communications.
(1992) Carter, Alan James Auchmuty.; Broadhurst, Anthony D.
The application of digital signal processing to radio communications has come of age with the advent of low power, high speed microprocessors and over the past five years, various transceiver architectures, utilizing this new technology have been extensively researched. Due to the flexible nature of a software based transceiver, a myriad of possible applications exist and currently the emphasis is on the development of suitable algorithms. The principal aim of this research is the derivation of optimized digital signal processing algorithms applicable to three separate areas of radio communications. Optimized, as used by the author within this dissertation, implies a reasonable compromise between performance, complexity and numerical processing efficiency. This compromise is necessary since the algorithms are applied to a portable transceiver where power consumption, size and weight are limited. The digital signal processing algorithms described by this research is as follows:- 1. The derivation and assessment of a multirate speech amplitude modulation demodulator which exhibits low distortion (typically less than 2%) for a wide range of modulation indices, carrier frequency offsets and deviations. The demodulator is processing efficient and requires only five multiplications and five decisions for every output sample. 2. The derivation and assessment of a low sampling rate speech frequency modulation demodulator for signals whose bandwidth exceed quarter the sampling frequency. The demodulator exhibits low distortion (typically less than 2%) and is processing efficient requiring eighteen multiplications and three decisions for every output sample. 3. The derivation and assessment of a multirate single-sideband suppressed carrier automatic frequency control system which is a combination of a simple second order adaptive line enhancer and a digital phase-locked loop. The processing efficient automatic frequency control system is suited for low signal to noise power conditions, in both stationary and mobile communication channels.
Computer-aided design of RF MOSFET power amplifiers.
(1992) Hoile, Gary Alec.; Reader, H. C.
The process of designing high power RF amplifiers has in the past relied heavily on measurements, in conjunction with simple linear theory. With the advent of the harmonic balance method and increasingly faster computers, CAD techniques can be of great value in designing these nonlinear circuits. Relatively little work has been done in modelling RF power MOSFETs. The methods described in numerous papers for the nonlinear modelling of microwave GaAsFETs cannot be applied easily to these high power devices. This thesis describes a modelling procedure applicable to RF MOSFETs rated at over 100 W. This is achieved by the use of cold S parameters and pulsed drain current measurements taken at controlled temperatures. A method of determining the required device thermal impedance is given. A complete nonlinear equivalent circuit model is extracted for an MRF136 MOSFET, a 28 V, 15 W device. This includes two nonlinear capacitors. An equation is developed to describe accurately the drain current as a function of the internal gate and drain voltages. The model parameters are found by computer optimisation with measured data. Techniques for modelling the passive components in RF power amplifiers are given. These include resistors, inductors, capacitors, and ferrite transformers. Although linear ferrite transformer models are used, nonlinear forms are also investigated. The accuracy of the MOSFET model is verified by comparison to large signal measurements in a 50 0 system. A complete power amplifier using the MRF136, operating from 118 MHz to 175 MHz is built and analysed. The accuracy of predictions is generally within 10 % for output power and DC supply current, and around 30 % for input impedance. An amplifier is designed using the CAD package, and then built, requiring only a small final adjustment of the input matching circuit. The computer based methods described lead quickly to a near-optimal design and reduce the need for extensive high power measurements. The use of nonlinear analysis programs is thus established as a valuable design tool for engineers working with RF power amplifiers.
A reconfigurable distributed process control environment for a network of PC's using Ada and NetBIOS.
(1992) Randelhoff, Mark Charles.; Levy, David C.
No abstract
Neural networks and early fast Doppler for prediction in meteor-burst communications systems.
(1994) Fraser, David Douglas.; Broadhurst, Anthony D.
In meteor-burst communications systems, the channel is bursty with a continuously fluctuating signal-to-noise ratio. Adaptive data rate systems attempt to use the channel more optimally by varying the bit rate. Current adaptive rate systems use a method of closed-loop decision-feedback to control the transmitted data rate. It is proposed that an open-loop adaptive data rate system without a decision feedback path may be possible using implicit channel information carried in the first few milliseconds of the link establishment probe signal. The system would have primary application in low-cost half-duplex telemetry systems. It is shown that the key elements in such a system would be channel predictors. The development of these predictors is the focus of this research. Two novel methods of predicting channel parameters are developed. The first utilises early fast Doppler information that precedes many long duration, large signal-to-noise-ratio overdense trails. The presence of early fast Doppler at the trail commencement is used as a toggle to operate at a higher data rate. Factors influencing the use of early fast Doppler for this purpose are also presented. The second method uses artificial neural networks. Data measured during trail formation is processed and presented to the neural networks for prediction of trail parameters. Several successful neural networks are presented which predict trail type, underdense or overdense, and peak trail amplitude from the first 50ms of the trail's lifetime. This method allows better estimation of the developing trail. This fact can be used to implement a multi-rate open-loop adaptive data rate system.
The development of an experimental, low profile, broadband travelling wave tube amplifier.
(1994) Vassilopoulos, Nicolaos.; Nattrass, Henry Lee.
An experimental, field replaceable travelling wave tube (TWT) with low profile, TEM transitions has been developed with the aim of combining the cost advantages of field replaceable TWTs with the bandwidth advantages of TEM transitions. Field replaceable TWTs are cheaper because their periodic permanent magnet (PPM) stacks (which are expensive) are easier to produce and can be reused. Standard TEM transitions have a high profile and prevent TWTs from being plugged into PPM stacks. The stacks of packaged TWTs are therefore built onto the tubes, using split ring magnets, before the tubes are potted. Upon failure, such tubes are discarded together with their focusing, coupling and cooling structures. Two types of low profile, TEM transitions were investigated: a ring and a plug transition. In the former, the helix was joined to a metal ring that was sandwiched between two ceramic rings in the vacuum envelope. Due to its high shunt capacitance, this transition was narrowband. In the plug transition, the helix was joined to a small metal plug in the wall of a ceramic tube. Plug transitions with a return loss of better than 10 dB over an octave were made but some technological difficulties were encountered. Metal/ceramic joining with active solder is therefore discussed. The development of graphite attenuators was attempted but graphite of high enough resistivity to achieve the attenuation and matching required was not available. Thin film attenuators were therefore used instead. Some good attenuators were made but the molybdenum films were sensitive to processing conditions. An empirical procedure was developed to magnetize PPM stacks to a wanted profile. The stacks that were designed and built resulted in good beam focusing. TWT tests were made using an automated test station that was built. Maximum RF gains and output powers of 20 dB and 43 dBm respectively were measured. The TWT design, construction and testing was a combination of materials science and microwave work. Some simulations were used to solve electromagnetic problems but much of the work was practical, involving trial and error and many measurements.
The analysis, simulation and testing of an experimental travelling- wave tube.
(1994) Reynolds, Christopher Garth.; Nattrass, Henry Lee.
As a design and analysis aid for the development of an experimental TWT, a computer program is written which allows the small-signal gain to be computed for various operating conditions, such as various conditions of tube bias (beam voltage and current) and frequency. In order to arrive at a value for the gain, a number of parameters need first to be defined or calculated. Using the method (Approach II) of Jain and Basu [17] which is applicable to a helix with a free-space gap between it and circular dielectric support rods surrounded by a metal shell, the dielectric loading factor (DLF) for the structure is found and the dispersion relation then solved to obtain the radial propagation constant y and axial propagation constant B. The method is tested for a helix with measured data and found to be acceptably accurate. Helix losses are calculated for the low-loss input and output sections of the helix, using the procedures developed by Gilmour et al [14,18], from which values are found for the helix loss parameter d. Another value for d, obviously much larger, is also found for the lossy attenuator section of the helix. Here measured data for the attenuator is used as a basis for a polynomial which models the attenuator loss as a function of frequency. The Pierce gain parameter C is found using the well-known equations of Pierce [21,22,26], and then the space-charge parameter Q. Here knowledge of the space-charge reduction factor F is required to find Q, and a simple non-iterative method is presented for its calculation, with some results. From the other parameters already calculated the velocity parameter, b, is then found. since sufficient information is now available, the electronic equations are solved. These equations are in a modified form, better accounting for the effects of space-charge than the well-known standard forms. Results are compared and slight differences found to exist in the computed gain. Now that the x's and y's (respectively the real and imaginary parts of the complex propagation constants for the slow and fast space-charge waves) are known the launching loss can be calculated. Launching losses are found for the three space-charge waves, not just for the gaining wave. The gain of the TWT is not found from the asymptotic gain equation but from a model which includes the effects of internal feedback due to reflections at the ports and attenuator. Values of reflection coefficients are modelled on the results of time-domain measurements (attenuator) and found by calculation (ports). This model permits the unstable behaviour of the tube to be predicted for various conditions of beam current and voltage and anticipates the frequencies at which instability would be likely. Results from simulations are compared with experimental observations. The need to pulse the experimental tube under controlled conditions led to the development of a high-voltage solid state pulse modulator providing regulated output pulses of up to 5000V and 200mA directly, without the use of transformers. The pulse modulator design embodies two unusual features a) its operation is bipolar, delivering positive or negative output pulses, depending only on the polarity of the rectifier input, and b) the use of multiple regulating loops and stacked pass elements to achieve high-voltage operation. Some results are presented.
Towards automatic face recognition using discrete cosine transforms and neural networks.
(1998) Debipersad, Sanjeev Chundurduth.; Broadhurst, Anthony D.
Abstract available in PDF.
Protocols for voice/data integration in a CDMA packet radio network.
(1999) Judge, Garth.; Takawira, Fambirai.
Wireless cellular communications is witnessing a rapid growth in, and demand for, improved technology and range of information types and services. Future third generation cellular networks are expected to provide mobile users with ubiquitous wireless access to a global backbone architecture that carries a wide variety of electronic services. This thesis examines the topic of multiple access protocols and models suitable for modem third-generation wireless networks. The major part of this thesis is based on a proposed Medium Access Control (MAC) protocol for a Code Division Multiple Access (CDMA) data packet radio network, as CDMA technology is proving to be a promising and attractive approach for spectrally efficient, economical and high quality digital communications wireless networks. The proposed MAC policy considers a novel dual CDMA threshold model based on the Multiple Access Interference (MAl) capacity of the system. This protocol is then extended to accommodate a mixed voice/data traffic network in which variable length data messages share a common CDMA channel with voice users, and where the voice activity factor of human speech is exploited to improve the data network performance. For the protocol evaluation, the expected voice call blocking probability, expected data throughput and expected data message delay are considered, for both a perfect channel and a correlated Rayleigh fading channel. In particular, it is shown that a significant performance enhancement can be made over existing admission policies through the implementation of a novel, dynamic, load-dependent blocking threshold in conjunction with a fixed CDMA multiple access threshold that is based on the maximum acceptable level of MAl.

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