Circuits and architectures for broadband spectrum channelizers with sub-band gain control

Broadband receiver architectures for full-band or concurrent multi-band reception of signals are required in several applications. One approach to implementing such receivers is a spectrum channelizer that employs a frequency-folded analog-to-digital converter (FF-ADC). The design downconverts and channelizes a broadband input signal into multiple sub-bands at baseband by employing the harmonics of non-overlapping rectangular clocks. The downconverted and aliased baseband signal in each path is digitized by a baseband ADC, referred to as a sub-ADC below, that operates with a sampling rate that is lower than the Nyquist sampling rate set by the full bandwidth of the input signal. Sub-band separation is performed through digital harmonic rejection (HR) and image rejection (IR). The design operates similar to a time-interleaved ADC, except that it significantly reduces the bandwidth requirement of the samplers. If rectangular pulse waveforms are used in the FF-ADC down-converter, all sub-bands experience nearly equal gain during frequency down-conversion. Since all sub-bands are aliased to baseband before they are separated in the digital domain, a sub-band with large relative power can reduce the sub-ADC dynamic range that is available for other sub-bands, in addition to appearing as a blocker for other sub-bands. The research presented in this dissertation addresses approaches to overcome this issue, by embedding sub-band gain control within an FF-ADC. Chapter 2 proposes an approach that employs pulse-width-modulated local oscillator (PWM-LO) waveforms in the polyphase paths of an FF-ADC for scaling individual sub-band signal levels at baseband before digitization. The PWM-LO waveforms, which directly drive switches in each path, can be used to vary the gain in each sub-band by varying the level of harmonics in the waveforms. This is achieved by controlling the pulse-widths of the PWM-LO waveforms. This design avoids the requirement for N ×N switch matrices and variable transconductance cells in prior demonstrated approaches. The proposed architecture makes it possible to vary the spectral response of the FF-ADC with low signal-path complexity. Prediction of pulse widths for the desired harmonic, and hence the gain profile across all sub-bands, is performed using an off-chip supervised learning approach employing a neural network. Chapter 3 presents the implementation of a spectrum channelizer employing the PWM-LO-based sub-band amplitude control. The design allows for scaling the relative gain of the sub-bands over a 20-dB range. This relaxes the compression performance of the channelizer baseband and the sub-ADC dynamic range in the presence of sub-bands with significantly higher signal levels. Gain control on individual sub-bands is performed by employing customized PWM-LO waveforms,where the PWM-LO pulses are generated using delay-locked loops (DLLs). The off-chip neural-network based learning technique for estimating the PWM symbol pulse widths required for setting the desired LO harmonic levels is described. A 1.6 GS/s spectrum channelizer IC is implemented in a 65-nm CMOS process to verify the architecture. The measured channelizer gain is 51.6-56.5 dB without gain scaling and provides a range of 37-59 dB with PWM-LO gain control. Gain-scaling at a specific harmonic improves blocker compression in an unattenuated sub-band from -34 dBm to -16 dBm. The in-band gain compression with gain-scaling also increases from -32 dBm to -17 dBm. Chapter 4 describes a spectrum channelizer that uses voltage-mode downconversion. The approach requires a single voltage-mode input amplifier to drive the downconversion switches. Frequency-folding and sub-band gain control are achieved in a single signal path. This contrasts with the current-mode approach that requires a main FF-ADC path and a separate auxiliary path for sub-band gain control. By avoiding the requirement for an auxiliary input path, the approach presented here significantly simplifies the signal chain with identical gain-scaling capability. The contributions of this research and scope for future related work are summarized in Chapter 5.Electrical and Computer Engineerin

Isotropic three-dimensional gap in the iron-arsenide superconductor LiFeAs from directional heat transport measurements

The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~ 18K) was measured in single crystals at temperatures down to T~50mK and in magnetic fields up to H=17T, very close to the upper critical field Hc2~18T. For both directions of the heat current, parallel and perpendicular to the tetragonal c-axis, a negligible residual linear term k/T is found as T ->0, revealing that there are no zero-energy quasiparticles in the superconducting state. The increase in k with magnetic field is the same for both current directions and it follows closely the dependence expected for an isotropic superconducting gap. There is no evidence of multi-band character, whereby the gap would be different on different Fermi-surface sheets. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal Tc).Comment: 4 pages, 3 figure

Multiphase oil transport at complex micro geometry

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 137-139).Dynamic sealing systems, such as mechanical face seals and piston rings in internal combustion engines are critical components of modern machines. These sealing systems present a unique challenge in controlling the lubricant supply and flow in the contact areas in order to minimize leakage, friction, and wear. Due to the sealing requirement, the minimum oil film thickness is in the order of surface roughness, which is one critical design parameter for the sealing systems. On the other hand, the wavelength of the surface geometrical features ranges from the size of the asperities of the surface roughness, which is in the order of a few microns to the size of the sealing components, which is in the order of millimeters to hundreds of millimeters. It is helpful for engineers to have a good understanding of lubricant transport across a large range of length scales. The aim of this thesis is to establish efficient and robust hydrodynamic lubrication models that are able to handle arbitrary complex geometries, flexible boundary conditions, and penetration of foreign gases. In this thesis, first a general oil transport model was developed. The model considers the variation of oil volume occupation fraction and establishes the dynamic mass flow balance in all locations. Instead of using inefficient small relaxation coefficients to assure convergence, we adjust the local linearization scheme according to local full film or partial film status. The model also applies quick contour detection algorithm to avoid the problems caused by equation's singularity around contact points and slow convergence caused by complex contact patterns. Furthermore, the models can be easily adapted to different scales. With the strong link between the numerical scheme and critical physical processes, the model eases the analysis of complicated results. This model has served as fundamental block of applications that predict and optimize the performance of metal face seals and piston ring pack liner system. Based on the success of single specie two phase oil transport model, a new multi phase oil transport model have been developed with reasonable assumptions about oil/gas mixture coexistence pattern and oil contact pattern. This multi phase model expands our analytical capability to some important but formerly not reachable areas like the starving oil supply boundary condition, pressurized gas boundary condition and the influence of gas penetration to oil film between mechanical components. Some preliminary investigations about the influence of gas penetration to the lubrication of the piston rings in internal combustion engines has been carried out. The results demonstrate that this model is able to preserve the oil mass conservation while capturing the gas penetration, gas pressure variation, and its interaction with the liquid oil. Furthermore, the results show that the liner finish effects becomes more and more prominent when the ring face profile becomes flatter and flatter with either fully-flooded or starved oil supply boundary condition.by Yong Li.Ph.D

Polymers containing calix[4]arenes and triptycenes : new syntheses, properties and applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2008.Includes bibliographical references.This thesis details the development of new synthetic methods for making polymers with complex molecular structures. Calix[4]arenes and triptycenes are the two major molecular building blocks for constructing the functional polymers. Unusual properties, with the emphasis on mechanical and sensory properties, are demonstrated. In Chapter 2, early investigations on the synthesis of main-chain calix[4]arene polymers are introduced. High molecular-weight main-chain calixarene homopolymers via linkages on lower rims are synthesized by the polymerization based on acetylenic coupling reaction. The conformation of calixarene monomers dictates the ability to produce the new polymers: the calix[4]arene monomers with a partial cone conformation produce high molecular-weight polymers while the calix[4]arene monomers with a cone conformation only afford oligomers. Other polymerization methods, including Sonogashira coupling and Cu(I)catalyzed Huisgen 1,3-dipolar cycloaddition, are also investigated but found to be inferior to the acetylenic coupling polymerization. In Chapter 3, main-chain calix[4]arene elastomers are achieved by metathesis reactions. Alkene-bridged calix[4]arene monomers are synthesized by ring-closing metathesis (RCM). All the three possible conformers (cone, partial cone, and 1,3-alternate) are used as comonomers with cyclooctene and norbornene in ring-opening metathesis polymerization (ROMP). The resultant polymers are high molecular-weight, transparent and stretchable materials with high calixarene incorporation (up to 25 mol% or 70 wt%) and low glass transition temperatures. The conformational properties of the calixarene moieties play a crucial role in determining the mechanical properties of the polymers.(cont.) In Chapter 4, the single-walled carbon nanotube (SWCNT)/polythiophene (PT)/calixarene polymer (CP) three-component hybrid system is demonstrated to be a simple chemiresistive sensory platform by utilizing the host-guest chemistry between calixarenes and analytes. To improve the sensing performance, a new polythiophene with pedant calixarenes (CalixPT) is then synthesized by a precursor polymer route. The new CalixPT is able to disperse SWCNTs. The stable and homogeneous dispersions can be easily processed to chemiresistive sensors for selective detection of xylene isomers. The sensing performance of the simplified SWCNT/CalixPT two-component system is greatly enhanced. In Chapter 5, high molecular-weight 1,4-linked triptycene polycarbonates (PCs) are synthesized by an optimized melt polymerization process. The triptycene-PCs exhibit improvements in modulus by over 20% and improvements in compressive strengths by nearly 50% at both low and high strain rates without any apparent sacrifice to ductility and transparency, in comparison to commercial bisphenol A PC. This is a further evidence of previously proposed molecular threading and interlocking mechanisms derived from the unique internal free volume (IFV) of the triptycene units. In Chapter 6, a modified ring-opening insertion metathesis polymerization (ROIMP) in ionic liquids (ILs), which employs the use of simple acyclic dienes as the comonomers and takes advantage of ILs' high boiling point to allow the application of high reaction temperature and low pressure, is developed. The new polymerization method is applied to making triptycene elastomers (both 1,4-linked and 2,6-linked) with high molecular weights and triptycene incorporations.(cont.) The 1,4-linked triptycene elastomers are for fundamental studies of the IFV mechanism. The 2,6-linked triptycene elastomers are proposed to exhibit negative Poisson's ratio at the molecular level.by Yong Yang.Ph.D

Neoclassical polarization

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006."June 2006."Includes bibliographical references (p. 127-129).Sheared zonal flow is known to be the predominant saturation mechanism of plasma turbulence. Rosenbluth and Hinton(R-H) have shown that the zonal flow level is inversely proportional to the plasma radial polarizability due to magnetic drift departure from a flux surface. In another calculation, Hinton and Rosenbluth (H-R) considered the weakly collisional case in the banana regime and calculated the neoclassical polarization and associated zonal flow damping in the high frequency and low frequency limits. The work presented here extends R-H's calculation in several aspects. We calculate the neoclassical polarization for arbitrary radial wavelength zonal flows so that finite ion banana width and ion gyroradius are retained. We also add plasma shape effects into the R-H collisionless calculation and find the influence of elongation and triangularity on neoclassical polarization and zonal flow damping. In addition, we extend the H-R collisional calculation using an exact eigenfunction expansion of the collision operator to calculate neoclassical polarization for the entire range of frequencies. A semi-analytical fit of the exact results is obtained that gives the polarization to within 15% and allows the collisional zonal flow damping rate to be evaluated for arbitrary collisionality.by Yong Xiao.Ph.D

Phenomenological study of Au and Pt nanowires grown in porous alumina scaffolds

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, February 2011."February 2011." Cataloged from PDF version of thesis.Includes bibliographical references (p. 76-81).Porous anodic aluminum oxide, commonly known as AAO, has been widely used as a scaffold to synthesize nanowires and nanotubes. The porous alumina structure can be obtained from a simple electrochemical oxidation process, applying a positive voltage to an aluminum film placed in an electrolyte, and resulting in the formation of periodically arranged arrays of pores. It is possible to tune pore diameters and pore spacing by adjusting parameters such as the type of electrolyte, the pH, and the applied voltage. Once the barrier oxide is removed from the bottom of the pores, porous alumina that has been formed on conducting substrates can be used for growth of metal nanowires using electrodeposition. We synthesized Au and Pt nanowire arrays on Au or Pt substrates. During electrodeposition, Au nanowires that grew out of the pores developed a pyramidlike faceted shape. This was not observed for overgrown Pt nanowires. To understand this phenomenon, the microstructure and crystallographic characteristics of the overgrown Au and Pt nanowires were studied using SEM, TEM and XRD. It was found that the overgrown Au caps were single crystalline with (111) facets and textured along the [100] direction, while the Au nanowires in the pores were poly-crystalline with a [11 11] texture. Pt nanowires grown in pores were also polycrystalline and had a [111] texture, but the grain size was much smaller than that of the Au. In contrast with Au, no change of texture or microstructure was observed when Pt grew out of pores. The structure change observed for Au involves nucleation of a new crystal with a (100) texture. This is thought to be related to the changes in the overpotential that occur when the Au emerges from the pores.by Yong Cheol Shin.S.M

Exploring the mechanisms critical to the operation of metal face seals through modeling and experiments

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2008.Cataloged from PDF version of thesis.Includes bibliographical references (p. 133-135).This thesis aims to explore operation mechanisms of a special type of mechanical face seals: the flexible metal-to-metal face seal (FMMFS). Unique features of the FMMFS include much more flexibility in the circumferential than in the radial direction, identical rotating and stationary seal rings, and a loading mechanism using elastomeric O-rings. Two versions of the numerical models have been developed to evaluate seal performance under various operating conditions. Both models consider interactions among surface deformations due to thermo-mechanical twists, unsteady lubrication in the sealing band, and heat transfer in the seal pair simultaneously. Outputs include contact pressures, oil film thickness, cavitation zone, partial film density, friction coefficients, dynamic oil transport, and seal temperature distributions. In the meantime, experimental efforts have been made to measure the friction coefficients and seal temperatures during different operations. The model predictions were then compared with the experiment results through the two above-mentioned quantities. The comparisons show that the numerical simulations consistently overestimate the friction by 15%-20%. However, overall trend of friction variation with speed and even some details of the friction can be captured, indicating that the current models are able to properly predict some underlying physics of seal operations. The numerical models were then used to evaluate scoring and leakage failures of the FMMFS through three important variables: surface temperature, contact wetness, and oil exchange. Some surface geometric features, which contribute to differences of scoring and leakage behaviors, are identified. In order to achieve higher scoring resistance and minimum leakage, the sealing surface should have the following features: (1) random or dispersed asperity distributions, (2) relatively large surface roughness, and (3) combination of concave and half-concave- half-convex radial profiles.by Yong Wang.Ph.D

A high-resolution spectroscopic search for the remaining donor for Tycho'S supernova

In this paper, we report on our analysis using Hubble Space Telescope astrometry and Keck-I HIRES spectroscopy of the central six stars of Tycho's supernova remnant (SN 1572). With these data, we measured the proper motions, radial velocities, rotationa

Exact Kohn-Sham exchange kernel for insulators and its long-wavelength behavior

We present an exact expression for the frequency-dependent Kohn-Sham exact-exchange (EXX) kernel for periodic insulators, which can be employed for the calculation of electronic response properties within time-dependent (TD) density-functional theory. It is shown that the EXX kernel has a long-wavelength divergence behavior of the exact full exchange-correlation kernel and thus rectifies one serious shortcoming of the adiabatic local-density approximation and generalized-gradient approximations kernels. A comparison between the TDEXX and the GW-approximation-Bethe-Salpeter-equation approach is also made.Comment: two column format 6 pages + 1 figure, to be publisehd in Physical Review