Luis B. Oliveira

Analysis and Design of Quadrature Oscillators

The following are some features of Analysis and Design of Quadrature Oscillators make it different from the existing literature on electronic oscillators: (1) focus on quadrature oscillators with accurate quadrature and low phase-noise, required by modern communication systems; (2) a detailed comparative study of quadrature LC and RC oscillators, including cross-coupled LC quasi-sinusoidal oscillators, cross-coupled RC relaxation oscillators, a quadrature RC oscillator-mixer, and two-integrator oscillators; (3) a thorough investigation of the effect of mismatches on the phase-error and the phase-noise; (4) the conclusion that quadrature RC oscillators can be a practical alternative to LC oscillators when area and cost should be minimized (in cross-coupled RC oscillators both the quadrature-error and phase-noise are reduced, whereas in LC oscillators the coupling increases the phase-noise.

Insights into the molecular mechanism of protein native-like aggregation upon glycation

Several human neurodegenerative diseases such as Alzheimers disease, Parkinsons disease and Familial Amyloidotic Polyneuropathy, have long been associated with, structural and functional changes in disease related proteins leading to aggregation into amyloid fibrils. Such changes can be triggered by post-translational modifications. Methylglyoxal modifications have been shown to induce the formation of small and stable native-like aggregates in the case of the amyloidogenic proteins insulin and α-synuclein. However, the fundamental biophysical mechanism underlying such methylglyoxal-induced protein aggregation is not yet fully understood. In this work cytochrome c (Cyt c) was used as a model protein for the characterization of specific glycation targets and to study their impact on protein structure, stability, and ability to form native-like aggregates. Our results show that methylglyoxal covalently modifies Cyt c at a single residue and induces early conformational changes that lead to the formation of native-like aggregates. Furthermore, partially unfolded species are formed, but do not seem to be implicated in the aggregation process. This shows a clear difference from the amyloid fibril mechanisms which involve partially or totally unfolded intermediates. Equilibrium-unfolding experiments show that glycation strongly decreases Cyt c conformational stability, which is balanced with an increase of conformational stability upon aggregation. Data collected from analytical and spectroscopic techniques, along with kinetic analysis based on least-squares parameter fitting and statistical model discrimination are used to help to understand the driving force underlying glycation-induced native-like aggregation, and enable the proposal of a comprehensive thermodynamic and kinetic model for native-like aggregation of methylglyoxal glycated Cyt c.

Noise Performance of a Regulated Cascode Transimpedance Amplifier for Radiation Detectors

A low noise transimpedance amplifier (TIA) is used in radiation detectors to transform the current pulse produced by a photo-sensitive device into an output voltage pulse with a specified amplitude and shape. We consider here the specifications of a PET (positron emission tomography) system. We review the traditional approach, feedback TIA, using an operational amplifier with feedback, and we investigate two alternative circuits: the common-gate TIA, and the regulated cascode TIA. We derive the transimpedance function (the poles of which determine the pulse shaping); we identify the transistor in each circuit that has the dominant noise source, and we obtain closed-form equations for the rms output noise voltage. We find that the common-gate TIA has high noise, but the regulated cascode TIA has the same dominant noise contribution as the feedback TIA, if the same maximum transconductance value is considered. A circuit prototype of a regulated cascode TIA is designed in a 0.35 μm CMOS technology, to validate the theoretical results by simulation and by measurement.

A Pulse Generator for UWB-IR Based on a Relaxation Oscillator

We propose the adaptation of a well-known relaxation oscillator to produce a modulated Gaussian pulse suitable for ultra-wide-band (UWB) impulse radio (IR). The proposed circuit has low area and low power consumption, can easily be modulated by a digital signal, and requires no special technology options; these features make it suitable for UWB-IR low-cost applications. The circuit behavior is confirmed by simulation and by experimental results on a prototype designed in austriamicrosystems 0.35-mum SiGe-BiCMOS technology.

The Effect of Mismatches and Delay on the Quadrature Error of a Cross-Coupled Relaxation Oscillator

Cross-coupled relaxation oscillators can produce two highly accurate quadrature output signals (Verhoeven, 1992). We present a high-level model of these oscillators in terms of circuit parameters, from which we obtain explicit equations for duty-cycle, oscillation frequency, and quadrature error. They show the influence on the oscillator performance of component mismatches and other nonideal effects, such as delays. The results provide useful guidelines for the design of high performance oscillators. The theoretical results are confirmed by simulation and by measurements on a test chip.

Regulated Common-Gate Transimpedance Amplifier Designed to Operate With a Silicon Photo-Multiplier at the Input

A transimpedance amplifier (TIA) in the front-end of a radiation detector is required to convert the current pulse produced by a light-detector to a voltage pulse with amplitude and shape suitable for the subsequent processing. We consider in this paper the specifications of a positron emission tomography (PET) scanner for medical imaging. The conventional approach is to use an avalanche photo-diode (APD) as the light-detector and a feedback TIA. We point out here that, when the APD is replaced by the more recent silicon photomultiplier (SiPM), a feedback TIA is not suitable, and we propose the use of a regulated common-gate (RC-G) TIA. We derive the transimpedance function of the RC-G TIA considering the parasitic capacitances that have a dominant effect on the pulse shaping. We use the result obtained to establish TIA design guidelines, and we show that these should be different with an APD and with a SiPM at the input. We identify the dominant noise source in the RC-G TIA, and we derive a closed form equation for the output noise rms voltage. A prototype TIA was designed for UMC 130 nm CMOS technology. We present simulation and experimental results that confirm that the RC-G circuit is suitable for implementation of the TIAs in the front-end of a PET scanner using SiPMs at the input.

Experimental Evaluation of Phase-Noise and Quadrature Error in a CMOS 2.4 GHz Relaxation Oscillator

Contrary to what happens with LC oscillators, the increase of coupling in cross-coupled relaxation RC-oscillators leads to a lower quadrature error and lower phase-noise. We present a 2.4 GHz CMOS quadrature relaxation oscillator, and show the measurements confirming this property. Increasing the coupling block gain, the oscillator phase-noise is reduced from -97 dBc/Hz to -104 dBc/Hz @ 1 MHz. The quadrature error is reduced from 4.3deg to 0.8deg. These results show that RC quadrature oscillators may be a practical alternative for RF transceiver applications.

Balun LNA with continuously controllable gain and with noise and distortion cancellation

In this paper we present a balun LNA with gain adjustable continuously by a voltage. The LNA is based on the combination of a common-gate and a common-source stage to cancel the noise and distortion of the common-gate stage. To obtain higher gain with the same DC voltage drop we replace resistors by PMOS transistors. This also allows continuous gain control and we show that by proper design, the effect on IIP3 and IIP2 can be neglected. With this approach, we avoid the use of switches, and the input impedance and the noise figure are not affected. Simulation results with a 130 nm CMOS technology show that the balun LNA has gain continuously tunable between 12 and 20 dB. The NF is below than 3.2 dB and the best IIP3 is higher than 0 dBm and the maximum IIP2 is 14 dBm. The total power dissipation is only 4.8 mW for a bandwidth of 5 GHz.

On phase noise in quadrature cross-coupled oscillators

A cross-coupled relaxation oscillator has two outputs of the same frequency, which are in quadrature. In this paper a comparative study of the phase noise in a cross-coupled relaxation oscillator is given. We present a high level (system) study of the oscillator and analyse the oscillator stability. The derivation proves analytically that cross-coupling reduces the influence of a perturbation (noise) on the oscillator output. Both high-level and circuit simulations at 5GHz are presented to confirm the theoretical results. Close to 10dB improvement in phase noise performance due to coupling was found, and the phase noise performance compared with that of a coupled LC oscillator was achieved.

An inductorless CMOS quadrature oscillator continuously tuneable from 3.1 to 10.6 GHz

Modern RF front-ends require wide tuning-range oscillators with quadrature outputs. In this paper we present a two-integrator quadrature oscillator, which covers the whole bandwidth of UWB applications. A circuit prototype in a 130 nm CMOS technology is continuously tuneable from 3.1 to 10.6 GHz. The circuit die area is less than 0.013mm2, leading to a figure-of-merit FOMA of −176.7dBc/Hz at the upper frequency. The supply voltage is 1.2 V, and the power consumption is 7 mW at the lower frequency and 13 mW at the upper frequency. Copyright