A space communications study Final report, 15 Sep. 1966 - 15 Sep. 1967

Investigation of signal to noise ratios and signal transmission efficiency for space communication system

Control of a Modular Multilevel Flying Capacitor Based STATCOM for Distribution Systems

Voltage fluctuation and power losses in the distribution line are problems in distribution networks. One method to mitigate these problems is by injecting reactive power into the network using a Static Synchronous Compensator (STATCOM). This can be used both for regulating the voltage and reducing the losses. A STATCOM is critically dependent on a grid synchronisation scheme that can accurately track the changes occurring in the grid phase and frequency. The Modular Multilevel Converter (MMC) is a promising topology for STATCOM applications because of its simple modular circuit structure that allows for higher voltage ratings, and conventionally uses a stack of sub-modules which are either two-level half or H-bridge converters. As a novel alternative, the thesis investigates the practicality of a STATCOM based on a three-level flying capacitor (FC) converter. Two variants of this topology are presented; the FC Half-bridge and FC H-bridge. A comprehensive study is undertaken to compare these with the Half and H-bridge sub-module under STATCOM operation. Most importantly, an FC H-bridge-based STATCOM is investigated for reactive power compensation. The challenges of multilevel, multi-module PWM control schemes achieving good waveforms at low switching frequency, whilst maintaining module capacitor voltage balance, are thoroughly addressed. Simulation results validate the operation for both line voltage regulation and power factor correction. An experimental power system with an FC-based STATCOM rig is designed and built, and validates the simulation results for power factor correction. It demonstrates correct operation of a control scheme that includes a system for maintaining capacitor voltage balance. Another new contribution is the investigation of a phase locking technique based on the Energy Operator (EO). The method, combining two different EO computations, is shown to achieve fast and accurate detection of frequency and phase angle when combined with an appropriate filter, and crucially operates well under unbalanced voltage conditions. The technique is compared with two other well-known phase locked loop (PLL) schemes, showing that it outperforms the others in terms of speed and accuracy. A hardware implementation of the EO-PLL validates the principle, showing the simplicity of the metho

Design and numerical simulation of the real-time particle charge and size analyser

The electrostatic charge and size distribution of aerosol particles play a very important role in many industrial applications. Due to the complexity and the probabilistic nature of the different charging mechanisms often acting simultaneously, it is difficult to theoretically predict the charge distribution of aerosol particles or even estimate the relative effect of the different mechanisms. Therefore, it is necessary to measure the size and also the bipolar charge distribution on aerosol particles. The main aim of this research project was to design, implement and simulate a signal processing system for novel, fully functional measurement instrument capable of simultaneously measuring in real time the bipolar charge and size distribution of medical aerosols. The Particle Size and Charge Analyser (PSCA), investigated in this thesis, uses Phase Doppler Anemometry (PDA) technique. The PDA system was used to track the motion of charged particles in the presence of an electric field. By solving the equation of particle motion in a viscous medium combined with the simultaneous measurement of its size and velocity, the magnitude as well as the polarity of the particle charge can be obtained. Different signal processing systems in different excitation fields have been designed and implemented. These systems include: velocity estimation system using spectral analysis in DC excitation field, velocity estimation system based on Phase Locked Loop (PLL) technique working in DC as well as sine-wave excitation fields, velocity estimation system based on Quadrature Demodulation (QD) technique under sine-wave excitation method, velocity estimation system using spectral analysis in square-wave excitation field and phase shift estimation based on Hilbert transformation and correlation technique in both sine-wave and square-wave excitation fields. The performances of these systems were evaluated using Monte Carlo (MC) simulations obtained from the synthesized Doppler burst signals generated from the mathematical models implemented in MATLAB. The synthesized Doppler Burst Signal (DBS) was subsequently corrupted with the added Gaussian noise. Cross validation of the results was performed using hardware signal processing system employing Arbitrary Waveform Generator and also NASA simulator to further confirm the validity of the estimation

RAPID CLOCK RECOVERY ALGORITHMS FOR DIGITAL MAGNETIC RECORDING AND DATA COMMUNICATIONS

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN024293 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Demodulator techniques in satellite communication systems for direct broadcast systems

This thesis is concerned with the FM demodulator techniques used in terrestrial TV receiver designs for Direct Broadcast Systems (DBS) from satellites. The various MAC/Packet schemes intended for DBS applications are described and the international standards that apply to them considered, with particular emphasis on the D2-MAC system. Noise in FM systems is discussed and a suitable threshold noise model is chosen for use in DBS TV demodulator systems. The characteristics of the various types of noise effects are considered in terms of their effect upon the TV picture. The threshold performance of a conventional FM demodulator for differing types of modulation is reviewed and it is shown how the threshold characteristic depends upon the nature of the modulation. The literature review carried out represents a significant component of the thesis and combines material from patent literature with more conventional source materials from professional journals, conferences, textbooks, etc. Some ten existing demodulator concepts that exhibit threshold extension characteristics are examined, and where relevant their potential performance in D2-MAC format systems is assessed. The demodulator characteristics that limit their performance in TV systems are identified. It is concluded that designing a threshold extension demodulator, with reliable operation, for all picture contents and for a wide range of input carrier-to-noise ratios, is a formidable task using existing design techniques. On the basis of this examination an adaptive threshold extension demodulator concept is proposed, that utilises information contained within the signal structure to achieve an improved performance over a wide range of input carrier-to-noise ratios and picture content. It is shown how the relevant signal structures may be derived from conventional (PAL, SECAM and NTSC), MAC format and all-digital television systems. Illustrations are given that show how the adaptive demodulator concept can be applied to certain existing threshold extension demodulators, enhancing their performance for television picture reception. Future trends in all-digital DBS TV systems intended ultimately for DBS applications are briefly discussed together with their demodlilation requirements

Digital transmission systems operating over high frequency radio channels

Imperial Users onl

Enhanced coding, clock recovery and detection for a magnetic credit card

Merged with duplicate record 10026.1/2299 on 03.04.2017 by CS (TIS)This thesis describes the background, investigation and construction of a system for storing data on the magnetic stripe of a standard three-inch plastic credit in: inch card. Investigation shows that the information storage limit within a 3.375 in by 0.11 in rectangle of the stripe is bounded to about 20 kBytes. Practical issues limit the data storage to around 300 Bytes with a low raw error rate: a four-fold density increase over the standard. Removal of the timing jitter (that is prob-' ably caused by the magnetic medium particle size) would increase the limit to 1500 Bytes with no other system changes. This is enough capacity for either a small digital passport photograph or a digitized signature: making it possible to remove printed versions from the surface of the card. To achieve even these modest gains has required the development of a new variable rate code that is more resilient to timing errors than other codes in its efficiency class. The tabulation of the effects of timing errors required the construction of a new code metric and self-recovering decoders. In addition, a new method of timing recovery, based on the signal 'snatches' has been invented to increase the rapidity with which a Bayesian decoder can track the changing velocity of a hand-swiped card. The timing recovery and Bayesian detector have been integrated into one computation (software) unit that is self-contained and can decode a general class of (d, k) constrained codes. Additionally, the unit has a signal truncation mechanism to alleviate some of the effects of non-linear distortion that are present when a magnetic card is read with a magneto-resistive magnetic sensor that has been driven beyond its bias magnetization. While the storage density is low and the total storage capacity is meagre in comparison with contemporary storage devices, the high density card may still have a niche role to play in society. Nevertheless, in the face of the Smart card its long term outlook is uncertain. However, several areas of coding and detection under short-duration extreme conditions have brought new decoding methods to light. The scope of these methods is not limited just to the credit card

Combined-channel instantaneous frequency analysis for audio source separation based on comodulation

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2008.Includes bibliographical references (p. 295-303).Normal human listeners have a remarkable ability to focus on a single sound or speaker of interest and to block out competing sound sources. Individuals with hearing impairments, on the other hand, often experience great difficulty in noisy environments. The goal of our research is to develop novel signal processing methods inspired by neural auditory processing that can improve current speech separation systems. These could potentially be of use as assistive devices for the hearing impaired, and in many other communications applications. Our focus is the monaural case where spatial information is not available. Much perceptual evidence indicates that detecting common amplitude and frequency variation in acoustic signals plays an important role in the separation process. The physical mechanisms of sound generation in many sources cause common onsets/offsets and correlated increases/decreases in both amplitude and frequency among the spectral components of an individual source, which can potentially serve as a distinct signature. However, harnessing these common modulation patterns is difficult because when spectral components of competing sources overlap within the bandwidth of a single auditory filter, the modulation envelope of the resultant waveform resembles that of neither source. To overcome this, for the coherent, constant-frequency AM case, we derive a set of matrix equations which describes the mixture, and we prove that there exists a unique factorization under certain constraints. These constraints provide insight into the importance of onset cues in source separation. We develop algorithms for solving the system in those cases in which a unique solution exists. This work has direct bearing on the general theory of non-negative matrix factorization which has recently been applied to various problems in biology and learning. For the general, incoherent, AM and FM case, the situation is far more complex because constructive and destructive interference between sources causes amplitude fluctuations within channels that obscures the modulation patterns of individual sources.(cont.) Motivated by the importance of temporal processing in the auditory system, and specifically, the use of extrema, we explore novel methods for estimating instantaneous amplitude, frequency, and phase of mixtures of sinusoids by comparing the location of local maxima of waveforms from various frequency channels. By using an overlapping exponential filter bank model with properties resembling the cochlea, and combining information from multiple frequency bands, we are able to achieve extremely high frequency and time resolution. This allows us to isolate and track the behavior of individual spectral components which can be compared and grouped with others of like type. Our work includes both computational and analytic approaches to the general problem. Two suites of tests were performed. The first were comparative evaluations of three filter-bank-based algorithms on sets of harmonic-like signals with constant frequencies. One of these algorithms was selected for further performance tests on more complex waveforms, including AM and FM signals of various types, harmonic sets in noise, and actual recordings of male and female speakers, both individual and mixed. For the frequency-varying case, initial results of signal analysis with our methods appear to resolve individual sidebands of single harmonics on short time scales, and raise interesting conceptual questions on how to define, use and interpret the concept of instantaneous frequency. Based on our results, we revisit a number of questions in current auditory research, including the need for both rate and place coding, the asymmetrical shapes of auditory filters, and a possible explanation for the deficit of the hearing impaired in noise.by Barry David Jacobson.Ph.D