Oscillators are ubiquitous to radio frequency circuits, where frequency translations and channel selection play a central role in the analogue communications channel. Oscillators also form part of digital systems as a time reference. Typical heterodyne receivers require an intermediate frequency channel. The associated oscillators and variable filters can only be centred perfectly at a single frequency, and degrade performance at the boundaries of the channel. These circuits also require image-rejecting filters and phase-locked loops in order to enable down-conversion. The penalties for these components are increased circuit area and power consumption. A direct down-conversion circuit will reduce the number of components in the system. A requirement added by the structural change is a passive sub-harmonic mixer. Quadrature oscillators may be achieved by cross-coupling two nominally identical LC differential voltage-controlled oscillators. Because of the widespread use of voltage-controlled oscillators in wireless communication systems, the development of comprehensive nonlinear analysis is pertinent in theory and applications. A key characteristic that defines the performance of an oscillator is the phase noise measurement. The voltage-controlled oscillator is also a key component in phase-locked loops, as it contributes to most of the out-of-band phase noise, as well as a significant portion of in-band noise. Current state-of-the-art modulation techniques, implemented at 60 GHz, such as quadrature amplitude modulation, and orthogonal frequency domain multiplexing, require phase noise specifications superior to 90 dBc/Hz at a 1 MHz offset. It has been shown that owing to the timing of the current injection, the Colpitts oscillator tends to outperform other oscillator structures in terms of phase noise performance. The Colpitts oscillator has a major flaw in that the start-up gain must be relatively high in comparison to the cross-coupled oscillator. The oscillation amplitude cannot be extended as in the cross-coupled case. The oscillator’s bias current generally limits the oscillation amplitude. The phase noise is defined by a stochastic differential equation, which can be used to predict the system’s phase noise performance. The characteristics of the oscillator can then be defined using the trajectory. The model projects the noise components of the oscillator onto the trajectory, and then translates the noise into the resulting phase and amplitude shift. The phase noise performance of an oscillator may be improved by altering the shape of the trajectory. The trajectory of the oscillator is separated into slow and fast transients. Improving the shape of the oscillator’s slow manifold may improve its phase noise performance, and improving the loaded quality factor of the tank circuit may be shown to directly improve upon close-in phase noise. The approach followed describes oscillator behaviour from a circuit-level analysis. The derived equations do not have a closed form solution, but are reformulated using harmonic balance techniques to yield approximate solutions. The results from this closed form approximation are very close to both the numerical solutions of the differential equations, as well as the Simulation Program with Integrated Circuit Emphasis solutions for the same circuits. The derived equations are able to predict the amplitude and frequency in the single-phase example accurately, and are extended to provide a numerical platform for defining the amplitude and frequency of a multiphase oscillator. The analysis identifies various circuit components that influence the oscillator’s phase noise performance. A circuit-level modification is then identified, enabling the decoupling of some of the factors and their interactions. This study demonstrates that the phase noise performance of a Colpitts oscillator may be significantly improved by making the proposed changes to the oscillator. The oscillator’s figure of merit is improved even further. When a given oscillator is set at its optimum phase noise level, the collector current will account for approximately 85% of the phase noise; with the approach in this work, the average collector current is reduced and phase noise performance is improved. The key focus of the work was to identify circuit level changes to an oscillator’s structure that could be improved or changed to achieve better phase noise performance. The objective was not to improve passive components, but rather to identify how the noise-to-phase noise transfer function could be improved. The work successfully determines what can be altered in an oscillator that will yield improved phase noise performance by altering the phase noise transfer function. / The concept is introduced on a differential oscillator and then extended to the multiphase oscillator. The impulse sensitivity function of the modified multiphase oscillator is improved by altering the typical feedback structure of the oscillator. The multiphase oscillator in this work is improved from -106 dBc/Hz to -113 dBc/Hz when considering the phase noise contribution from the tank circuits’ bias current alone. This is achieved by uniquely altering the feedback method of the oscillator. This change alters the noise-to-phase noise properties of the oscillator, reducing phase noise. The improvement in the phase noise does not account for further improvements the modification would incorporate in the oscillator’s limit cycle. For a given tank circuit, supply current and voltage, compared to an optimised Colpitts oscillator, the modifications to the feedback structure proposed in this work would further improve the figure of merit by 9 dB. This is not considering the change in the power consumption, which would yield a further improvement in the figure of merit by 7 dB. This is achieved by relaxing the required start-up current of the oscillator and effecting an improvement in the impulse sensitivity function. Future research could include further modelling of the phase shift in the feedback network, including the transmission lines in the feedback networks using the harmonic balance technique in a numerical form. The feedback technique can also be modified to be applicable to single and differential oscillators. / Dissertation (MEng)--University of Pretoria, 2017. / National Research Foundation / The Department of Science and Technology, South Africa / GEW Technologies (Pty) Ltd / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

In 1997 the Federal Communications Commission (FCC) released 300 MHz of spectrum between 5-6 GHz designated the unlicensed national information infrastructure (U-NII) band. The intention of the FCC was to provide an unlicensed band of frequencies that would enable high-speed wireless local area networks (WLANs) and facilitate wireless access to the national information infrastructure with a minimum interference to other devices. Currently, there is a lack of cost-effective technologies for developing U-NII band components. With the commercial market placing emphasis on low cost, low power, and highly integrated implementations of RF circuitry, alternatives to the large and expensive distributed element components historically used at these frequencies are needed. Silicon Germanium (SiGe) BiCMOS technology represents one possible solution to this problem. The SiGe BiCMOS process has the potential for low cost since it leverages mature Si process technologies and can use existing Si fabrication infrastructure. In addition, SiGe BiCMOS processes offer excellent high frequency performance through the use of SiGe heterojunction bipolar transistors (HBTs), while coexisting Si CMOS offers compatibility with digital circuitry for high level 'system-on-a-chip' integration. The work presented in this thesis focuses on the development of a SiGe RFIC front-end for operation in the U-NII bands. Specifically, three variants of a packaged low noise amplifier (LNA) and a packaged active x2 sub-harmonic mixer (SHM) have been designed, simulated and measured. The fabrication of the Rifts was through the IBM SiGe foundry; the packaging was performed by RF Micro devices. The mixer and LNA designs were fabricated on separate die, packaged individually, and on-chip matched to a 50 ohm system so they could be fully characterized. Measurements were facilitated in a coaxial system using standard FR4 printed circuit boards. The LNA designs use a single stage, cascoded topology. The input ports are impedance matched using inductive emitter degeneration through bondwires to ground. One version of the LNA uses an shunt inductor/series capacitor output match while the other two variation use a series inductor output match. Gain, isolation, match, linearity and noise figure (NF) were used to characterize the performance of the LNAs in the 5 - 6 GHz frequency band. The best LNA design has a maximum gain of 9 dB, an input VSWR between 1.6:1 and 2:1, an output match between 1.7:1 and 3.6:1, a NF better than 3.9 dB and an input intercept point (IIP3) greater than 5.4 dBm. The LNA operates from a 3.3 V supply voltage and consumes 4 mA of current. The SHM is an active, double-balance mixer that achieves x2 sub-harmonic mixing through two quadrature (I/Q) driven, stacked Gilbert-cell switching stages. Single-ended-to-differential conversion, buffering and I/Q phase separation of the LO signal are integrated on-chip. Measurements were performed to find the optimal operating range for the mixer, and the mixer was characterized under these sets of conditions. It was found that the optimal performance of the mixer occurs at an IF of 250-450 MHz and an LO power of -5 dBm. Under these conditions, the mixer has a measured conversion gain of 9.3 dB, a P_1-dB of -15.7 dBm and an 2LO/RF isolation greater than 35 dB at 5.25 GHz. At 5.775 GHz, the conversion gain is 7.7 dB, the P_1-dB is -15.0 dBm, and the isolation is greater than 35 dB. The mixer core consumes 9.5 mA from a 5.0 V supply voltage. This work is sponsored by RF Microdevices (RFMD)through the CWT affiliate program.The author was supported under a Bradley Foundation fellowship. / Master of Science

No description available.

The advent of high-fT silicon CMOS/BiCMOS technologies has led to a dramatic upsurge in the research and development of radio and microwave frequency integrated circuits (ICs) in silicon. The integration of silicon-germanium heterojunction bipolar transistors (SiGe HBTs) into established "digital" CMOS processes has provided analog performance in silicon that is not only competitive with III-V compound-semiconductor technologies, but is also potentially lower in cost. Combined with improvements in silicon on-chip passives, such as high-Q metal-insulator-metal (MIM) capacitors and monolithic spiral inductors, these advanced RF CMOS and SiGe BiCMOS technologies have enabled complete silicon-based RF integrated circuit (RFIC) solutions for emerging wireless communication standards; indeed, both the analog and digital functionalities of an entire wireless system can now be combined in a single IC, also known as a wireless "system-on-a-chip" (SoC). This approach offers a number of potential benefits over multi-chip solutions, such as reductions of parasitics, size, power consumption, and bill-of-materials; however, a number of critical challenges must be considered in the integration of such SoC solutions. The focus of this research is the application of SiGe BiCMOS technology to on-going challenges in the development of receiver components for high speed wireless data networks. The research seeks to drive SoC integration by investigating circuit topologies that eliminate the need for off-chip components and are amenable to complete on-chip integration. The first part of this dissertation presents the design, fabrication, and measurement of a 5--6GHz sub-harmonic direct-conversion-receiver (DCR) front-end, implemented in the IBM 0.5um 5HP SiGe BiCMOS process. The design consists of a fully-differential low-noise amplifier (LNA), a set of quadrature (I and Q)x~2 sub-harmonic mixers, and an LO conditioning chain. The front-end design provides a means to address performance limitations of the DCR architecture (such as DC-offsets, second-order distortion, and quadrature phase and amplitude imbalances) while enabling the investigation of high-frequency IC design complications, such as package parasitics and limited on-chip isolation. The receiver front-end has a measured conversion gain of ~18dB, an input second-order intercept point of +17.5dBm, and a noise figure of 7.2dB. The quadrature phase balance at the sub-harmonic mixer IF outputs was measured in the presence of digital switching noise; 90<degree> balance was achieved, over a specific range of LO power levels, with a square wave noise signal injected onto the mixer DC supply rails. The susceptibility of receiver I/Q balance to mixed-signal effects in a SoC environment motivates the second part of this dissertation --- the design of a phase and amplitude tunable, quadrature voltage-controlled oscillator (QVCO) for the on-chip synthesis of quadrature signals. The QVCO design, implemented in the Freescale (formerly Motorola) 0.18um SiGe:C RFBiCMOS process, uses two identical, differential LC-tank VCOs connected such that the two oscillator outputs lock in quadrature to the same frequency. The QVCO designs proposed in this work provide the additional feature of phase-tunability, i.e. the relative phase balance between the quadrature outputs can be adjusted dynamically, offering a simulated tuning range of ~90<degree>+/-10â ¹degree> in addition, a variable-gain buffer/amplifier circuit that provides amplitude tunability is introduced. One potential application of the QVCO is in a self-correcting RF receiver architecture, which, using the phase and amplitude tunability of the QVCO, could dynamically adjust the IF output quadrature phase and amplitude balance, in near real-time, in the analog-domain. The need for high-quality inductors in both the DCR and QVCO designs motivates the third aspect of this dissertation --- the characterization and modeling of on-chip spiral inductors with patterned ground shields, which are placed between the inductor coil and the underlying substrate in order to improve the inductor quality factor (Q). The shield prevents the coupling of energy away from the inductor spiral to the typically lossy Si substrate, while the patterning disrupts the flow of induced image currents within the shield. The experimental effort includes the fabrication and testing of a range of inductors with different values, and different types of patterned ground shields in different materials. Two-port measurements show a ~50% improvement in peak-Q and a ~20% degradation in self-resonant frequency for inductors with shields. From the measured results, a scalable lumped element model is developed for the rapid simulation of spiral inductors with and without patterned ground shields. The knowledge gained from this work can be combined and applied to a range of future RF/wireless SoC applications. The designs developed in this dissertation can be ported to other technologies (e.g. RF CMOS) and scaled to other frequency ranges (e.g. 24GHz ISM band) to provide solutions for emerging applications that require low-cost, low-power RF/microwave circuit implementations. / Ph. D.

The inverter dead-time is integral to the safety of a voltage source inverter (VSI). Dead-time is introduced between the complementary gating signals of the top and bottom switches in each VSI leg to prevent shoot-through fault. This thesis reports and investigates dead-time induced sub-harmonic oscillations in open-loop induction motor drives of different power levels, under light-load conditions. The thesis develops mathematical models that help understand and predict the oscillatory behaviour of such motor drives due to dead-time act. Models are also developed to study the impact of under-compensation and over-compensation of dead-time act on stability. The various models are validated through extensive simulations and experimental results. The thesis also proposes and validates active damping schemes for mitigation of such sub-harmonic oscillations. The thesis reports high-amplitude sub-harmonic oscillations in the stator current, torque and speed of a 100-kW open-loop induction motor drive in the laboratory, operating under no-load. Experimental studies, carried out on 22-kW, 11-kW, 7.5-kW and 3.7-kW open-loop induction motor drives, establish the prevalence of dead-time induced sub-harmonic oscillations in open-loop motor drives of different power levels. An experimental procedure is established for systematic study of this phenomenon in industrial drives. This procedure yields the operating region, if any, where the motor drive is oscillatory. As a first step towards understanding the oscillatory behaviour of the motor drive, a mathematical model of the VSI is derived in a synchronously revolving reference frame (SRF), incorporating the of dead-time on the inverter output voltage. This leads to a modified dynamic model of the inverter-fed induction motor in the SRF, inclusive of the dead-time act. While the rotor dynamic equations are already non-linear, dead-time is found to introduce nonlinearities in the stator dynamic equations as well. The nonlinearities in the modified dynamic model make even the steady solution non-trivial. Under steady conditions, the dead-time can be modelled as the drop across an equivalent resistance (Req0) in the stator circuit. A precise method to evaluate the equivalent resistance Req0 and a simple method to arrive at the steady solution are proposed and validated. For the purpose of stability analysis, a small-signal model of the drive is then derived by linearizing the non-linear dynamic equations of the motor drive, about a steady-state operating point. The proposed small-signal model shows that dead-time contributes to different values of equivalent resistances along the q-axis and d-axis and also to equivalent cross-coupling reactance’s that appear in series with the stator windings. Stability analysis performed using the proposed model brings out the region of oscillatory behaviour (or region of small-signal instability) of the 100-kW motor drive on the voltage versus frequency (V- f) plane, considering no-load. The oscillatory region predicted by the small-signal analysis is in good agreement with simulations and practical observations for the 100-kW motor drive. The small-signal analysis is also able to predict the region of oscillatory behaviour of an 11-kW motor drive, which is con consumed by simulations and experiments. The analysis also predicts the frequencies of sub-harmonic oscillations at different operating points quite well for both the drives. Having the validity of the small-signal analysis at different power levels, this analytical procedure is used to predict the regions of oscillatory behaviour of 2-pole, 4-pole, 6-pole and 8-pole induction motors rated 55 kW and 110 kW. The impact of dead-time on inverter output voltage has been studied widely in literature. This thesis studies the influence of dead-time on the inverter input current as well. Based on this study, the dynamic model of the inverter fed induction motor is extended to include the dc-link dynamics as well. Simulation results based on this extended model tally well with the experimentally measured dc-link voltage and stator current waveforms in the 100-kW drive. Dead-time compensation may be employed to mitigate the dead-time and oscillatory behaviour of the drive. However, accurate dead-time compensation is challenging to achieve due to various factors such as delays in gate drivers, device switching characteristics, etc. Effects of under-compensation and over-compensation of dead time are investigated in this thesis. Under-compensation is shown to result in the same kind of oscillatory behaviour as observed with dead-time, but the fundamental frequency range over which such oscillations occur is reduced. On the other hand, over-compensation of dead-time effect is shown to result in a different kind of oscillatory behaviour. These two types of oscillatory behaviour due to under- and over-compensation, respectively, are distinguished and demonstrated by analyses, simulations and experiments on the 100-kW drive. To mitigate the oscillatory behaviour of the drive, an active damping scheme is proposed. This scheme emulates the effect of an external inductor in series with the stator winding. A small-signal model is proposed for an induction motor drive with the proposed active damping scheme. Simulations and experiments on the 100-kW drive demonstrate effective mitigation of light-load instability with this active damping scheme. In the above inductance emulation scheme, the emulated inductance is seen by the sub-harmonic components, fundamental component as well as low-order harmonic components of the motor current. Since the emulated inductance is also seen by the fundamental component, there is a fundamental voltage drop across the emulated inductance, leading to reduced co-operation of the induction motor. Hence, an improved active damping scheme is proposed wherein the emulated inductance is seen only by the sub-harmonic and low-order harmonic components. This is achieved through appropriate altering in the synchronously revolving domain. The proposed improved active damping scheme is shown to mitigate the sub-harmonic oscillation effectively without any reduction in flux.

Les ondes gravito-inertielles jouent un rôle majeur dans les échanges d'énergie globaux sur la planète. Si la génération des ondes est bien connue dans l'atmosphère et l'océan, le devenir de ces ondes au cours de leur propagation n'est pas complètement défini aujourd'hui. Ces ondes peuvent interagir de façon non-linéaire avec elles-mêmes et créer des structures de plus petite échelle qui vont se dissiper plus facilement. Ainsi, le phénomène d'instabilité paramétrique sous-harmonique (PSI), a été étudié de façon expérimentale. Nous avons effectué la première mise en évidence expérimentale de l'interaction de trois ondes planes inertielles bi-dimensionnelles, sous la forme d'une triade résonnante. Cette étude améliore en outre la compréhension de la turbulence en rotation. Les ondes internes peuvent aussi créer, ou interagir avec des écoulements lents de grande échellequi peuvent modifier la biodiversité au fond des océans. Nous avons mis en évidence une situation expérimentale à l'origine d’un tel écoulement moyen induit par les ondes et, à l'aide d'un modèle théorique simplifié, nous avons expliqué la formation de ces écoulements. Enfin, on étudie également des tourbillons en fluide stratifié pour permettre de futures études sur l'interaction d'ondes gravito-inertielles avec des tourbillons. / Inertia-gravity waves play a major role in the global transfer of energy on Earth. Even if wave generation is well understood in the atmosphere and in the ocean, their subsequent evolution is not completely understood. These waves can interact nonlinearly with themselves and create small-scales structures that dissipate more rapidly. Motivated by this, the phenomenon of parametric subharmonic instability (PSI), was studied experimentally. We conducted the first laboratory demonstration of the interaction of three two-dimensional inertial plane waves, as a resonant triad. Inertia-gravity waves can also interact with, and create, mean flows of large scale that can modify the transport of energy, chemical and biological compounds, and thereby have an impact on biodiversity in the ocean. We therefore also demonstrated an experimental situation that gives rise to such a flow field and using a simplified theoretical model, we explained the formation of this flow. Finally, we performed some studies of vortices in stratified fluid, to assist future studies of the interaction of inertia-gravity waves with vortices.

L’amélioration du contraste des images échographiques occupe une place importante en imagerie ultrasonore médicale puisque la qualité des images impacte directement le diagnostic médical. Les imageries de contraste sous et ultra-harmoniques sont deux techniques permettant de produire des images à fort contraste. Le développement de techniques de filtrage, permettant de mieux extraire les composantes sous et ultra-harmoniques présentes dans les signaux renvoyés par les produits de contraste ultrasonore, a pour conséquence d’améliorer encore plus le contraste des images. Les modèles non linéaires dits « boîtes noires » du type Volterra, Hammerstein, et Wiener, sont souvent utilisés pour identifier et modéliser les systèmes non linéaires tels que les produits de contraste ultrasonore. Ces modèles, bien que très bien adaptés pour modéliser les composantes harmoniques entières, ne le sont plus en présence de sous et ultra-harmoniques. Dans les années 2000, une méthode permettant la modélisation de sous et ultra-harmoniques en utilisant un modèle de Volterra à plusieurs entrées (Multiple Input Single Output : MISO) a été développée. Pourtant, bien que cette méthode soit complètement originale dans le domaine considéré, les points clés qui la sous-tendent ne sont pas clairement identifiés. C’est en identifiant clairement les points clés de la méthode existante que nous pouvons la généraliser et proposer plusieurs nouveaux paradigmes pour la modélisation et l’extraction de sous et ultra-harmoniques. Nous proposons alors des alternatives beaucoup plus simples du type mono-entrée mono-sortie (Single Input Single Output : SISO). En modulant en fréquence l’entrée du système, nous avons pu ainsi identifier les réponses impulsionnelles des différents canaux du modèle de Hammerstein généralisé. À partir de ce cadre général, nous avons aussi proposé de nouvelles approches pour réduire la complexité du modèle de Volterra. / Contrast enhancement of echographic images occupies an important place in medical ultrasound imaging, since the quality of images has a direct impact on medical diagnosis. Sub and ultraharmonic contrast imaging are two imaging techniques that provide high contrast images. Developing filtering techniques, that allow better extraction of sub and ultra-harmonic components present in the echo backscattered by the ultrasound contrast agents, results in additional improvement of the contrast. Non linear models known as “black box” models, like Volterra model, Hammerstein model, and Wiener model are usually used to identify and model non linear systems such ultrasound contrast agents. Although these models are well suited to model integer harmonic components, they are not suitable in the presence of sub and ultra-harmonic components. In the 2000s, A method that allows the modeling of sub and ultra-harmonic components, using a multi inputs single output Volterra model (MISO) has been developed. Although this method was original in the considered domain, the key points underlying this method are not clearly identified. By clearly identifying the key points of the existing method, we can generalize it and propose new paradigms for modeling and extraction of sub and ultra-harmonics. We propose much simpler alternatives based on single input single output (SISO) models. By modulating the input frequency of the system, we can also identify the different impulse responses of the different channels of the generalized Hammerstein model. From this general framework, we also propose new approaches to reduce the complexity of Volterra model.