SciCombinator

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Journal: IEEE transactions on ultrasonics, ferroelectrics, and frequency control

28

Understanding amplifier phase noise is a critical issue in many fields of engineering and physics, such as oscillators, frequency synthesis, telecommunication, radar, and spectroscopy; in the emerging domain of microwave photonics; and in exotic fields, such as radio astronomy, particle accelerators, etc. Focusing on the two main types of base noise in amplifiers, white and flicker, the power spectral density of the random phase ��(t) is S��( f ) = b(0) + b(���1)/f. White phase noise results from adding white noise to the RF spectrum in the carrier region. For a given RF noise level, b(0) is proportional to the reciprocal of the carrier power P(0). By contrast, flicker results from a near-dc 1/f noise-present in all electronic devices-which modulates the carrier through some parametric effect in the semiconductor. Thus, b(-1) is a parameter of the amplifier, constant in a wide range of P(0). The consequences are the following: Connecting m equal amplifiers in parallel, b(-1) is 1/m times that of one device. Cascading m equal amplifiers, b(-1) is m times that of one amplifier. Recirculating the signal in an amplifier so that the gain increases by a power of m (a factor of m in decibels) as a result of positive feedback (regeneration), we find that b(���1) is m(2) times that of the amplifier alone. The feedforward amplifier exhibits extremely low b(-1) because the carrier is ideally nulled at the input of its internal error amplifier. Starting with an extensive review of the literature, this article introduces a system-oriented model which describes the phase flickering. Several amplifier architectures (cascaded, parallel, etc.) are analyzed systematically, deriving the phase noise from the general model. There follow numerous measurements of amplifiers using different technologies, including some old samples, and in a wide frequency range (HF to microwaves), which validate the theory. In turn, theory and results provide design guidelines and give suggestions for CAD and simulation. To conclude, this article is intended as a tutorial, a review, and a systematic treatise on the subject, supported by extensive experiments.

Concepts: Amplifier, Decibel, Spectrum, Gain, Colors of noise, Oscillators, Flicker noise, Noise

28

Beamforming of plane-wave ultrasound echo signals in the Fourier domain provides fast and accurate image reconstruction. Conventional implementations perform a k-space interpolation from the uniform sampled grid to a nonuniform acoustic dispersion grid. In this paper, we demonstrate that this step can be replaced by a nonuniform Fourier transform. We study the performance of the nonuniform fast Fourier transform (NUFFT) in terms of signal-to-noise ratio and computational cost, and show that the NUFFT offers an advantage in the trade-off between speed and accuracy, compared with other frequency-domain beamforming strategies.

Concepts: Convolution theorem, Phase, Convolution, Frequency domain, Fourier series, Discrete Fourier transform, Digital signal processing, Fourier transform

28

The free vibrations of a two-layered C-axis inclined zig-zag ZnO thin-film bulk acoustic wave resonator (FBAR) connected to external impedance are analyzed. The frequency equation and mode shape for this resonator are derived based on the linear piezoelectric theory. The impedance characteristics of the FBAR are derived and compared with previous experimental results.

Concepts: Titanium dioxide, Mode shape, Zinc oxide, Resonance, Standing wave, Wave, Acoustics, Vibration

27

Airborne ultrasonic ranging is used in a variety of different engineering applications for which other positional metrology techniques cannot be used, for example in closed-cell locations, when optical line of sight is limited, and when multipath effects preclude electromagnetic-based wireless systems. Although subject to fundamental physical limitations, e.g., because of the temperature dependence of acoustic velocity in air, these acoustic techniques often provide a cost-effective solution for applications in mobile robotics, structural inspection, and biomedical imaging. In this article, the different techniques and limitations of a range of airborne ultrasonic ranging approaches are reviewed, with an emphasis on the accuracy and repeatability of the measurements. Simple timedomain approaches are compared with their frequency-domain equivalents, and the use of hybrid models and biologically inspired approaches are discussed.

Concepts: Wireless, Time-of-flight, Units of measurement, Test method, Debut albums, Psychometrics, Metrology, Measurement

27

The application of microbubbles and ultrasound to deliver nanoparticle carriers for drug and gene delivery is an area that has expanded greatly in recent years. Under ultrasound exposure, microbubbles can enhance nanoparticle delivery by increasing cellular and vascular permeability. In this review, the underlying mechanisms of enhanced nanoparticle delivery with ultrasound and microbubbles and various proposed delivery techniques are discussed. Additionally, types of nanoparticles currently being investigated in preclinical studies, as well as the general limitations and benefits of a microbubble- based approach to nanoparticle delivery, are reviewed.

Concepts: Review, Nanoparticle

27

In diagnostic medicine, microbubbles are used as contrast agents to image blood flow and perfusion in large and small vessels. The small vessels (the capillaries) have diameters from a few hundred micrometers down to less than 10 μ m. The effect of such microvessels surrounding the oscillating microbubbles is currently unknown, and is important for increased sensitivity in contrast diagnostics and manipulation of microbubbles for localized drug release. Here, oscillations of microbubbles in tubes with inner diameters of 25 μm and 160 ¿m are investigated using an ultra-high-speed camera at frame rates of ~12 million frames/s. A reduction of up to 50% in the amplitude of oscillation was observed for microbubbles in the smaller 25-μm tube, compared with those in a 160-μm tube. In the 25-μm tube, at 50 kPa, a 48% increase of microbubbles that did not oscillate above the noise level of the system was observed, indicating increased oscillation damping. No difference was observed between the resonance frequency curves calculated for microbubbles in 25-μm and 160-μm tubes. Although previous investigators have shown the effect of microvessels on microbubble oscillation at high ultrasound pressures, the present study provides the first optical images of low-amplitude microbubble oscillations in small tubes.

Concepts: Simple harmonic motion, Oscillation, Beat, Wave, Feedback, Harmonic oscillator, Resonator

27

A novel square-plate piezoelectric ultrasonic linear motor operated in two orthogonal first bending vibration modes (B(1)) is proposed. The piezoelectric vibrator of the linear motor is simply made of a single PZT ceramic plate (sizes: 15 x 15 x 2 mm) and poled in its thickness direction. The top surface electrode of the square ceramic plate was divided into four active areas along its two diagonal lines for exciting two orthogonal B(1) modes. The achieved driving force and speed from the linear motor are 1.8 N and 230 mm/s, respectively, under one pair orthogonal voltage drive of 150 V(p-p) at the resonance frequency of 92 kHz. The proposed linear motor has advantages over conventional ultrasonic linear motors, such as relatively larger driving force, very simple working mode and structure, and low fabrication cost.

Concepts: Damping, Resonance, Frequency, Mechanical resonance, Wave, Sound, Piezoelectricity, Vibration

27

The main global navigation satellite systems (GNSS) technique currently used for accurate time and frequency transfer is based on an analysis of the ionosphere-free combinations of dual-frequency code and carrier phase measurements in a precise point positioning (PPP) mode. This technique analyses the observations of one GNSS station using external products for satellite clocks and orbits to determine the position and clock synchronization errors of this station. The frequency stability of this time transfer is limited by the noise and multipath of the Global Positioning System (GPS) and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) codes. In the near future, Galileo will offer a broadband signal E5, with low noise in the centimeter range and with the lowest multipath error ever observed. This paper investigates new analysis procedures based on the E5 codeplus- carrier (CPC) combination for time transfer. The CPC combination with E5 provides a noise level 10 times lower than the ionosphere-free combination of Galileo E1 and E5, which is very promising for improving GNSS time transfer performances. From some tests with simulated Galileo data, it is shown here that the use of the CPC combination with E5 does not improve, at present, the medium- and long-term stability of time transfer with respect to the ionosphere-free combination of Galileo E1 and E5 codes, because of the need for a second frequency signal to correct for the ionospheric delays and ambiguities.

Concepts: Positioning system, European Union, Atomic clock, Navigation, GPS, Satellite navigation systems, Global navigation satellite system, Global Positioning System

27

Accurate and precise displacement estimation has been a hallmark of clinical ultrasound. Displacement estimation accuracy has largely been considered to be limited by the Cramer¿Rao lower bound (CRLB). However, the CRLB only describes the minimum variance obtainable from unbiased estimators. Unbiased estimators are generally implemented using Bayes¿ theorem, which requires a likelihood function. The classic likelihood function for the displacement estimation problem is not discriminative and is difficult to implement for clinically relevant ultrasound with diffuse scattering. Because the classic likelihood function is not effective, a perturbation is proposed. The proposed likelihood function was evaluated and compared against the classic likelihood function by converting both to posterior probability density functions (PDFs) using a noninformative prior. Example results are reported for bulk motion simulations using a 6¿ tracking kernel and 30 dB SNR for 1000 data realizations. The canonical likelihood function assigned the true displacement a mean probability of only 0.070 ± 0.020, whereas the new likelihood function assigned the true displacement a much higher probability of 0.22 ± 0.16. The new likelihood function shows improvements at least for bulk motion, acoustic radiation force induced motion, and compressive motion, and at least for SNRs greater than 10 dB and kernel lengths between 1.5 and 12λ.

Concepts: Probability theory, Accuracy and precision, Bayesian statistics, Likelihood function, Estimation theory, Bayes' theorem, Conditional probability, Probability density function

27

Ultrasonic displacement estimates have numerous clinical uses, including blood flow estimation, elastography, therapeutic guidance, and acoustic radiation force imaging (ARFI). These clinical tasks could be improved with better ultrasonic displacement estimates. Traditional ultrasonic displacement estimates are limited by the Cramer¿Rao lower bound (CRLB). The CRLB can be surpassed using biased estimates. In this paper, a framework for biased estimation using Bayes¿ theorem is described. The Bayesian displacement estimation method is tested against simulations of several common types of motion: bulk, step, compression, and acoustic-radiation-force-induced motion. Bayesian estimation is also applied to in vivo ARFI of cardiac ablation lesions. The Bayesian estimators are compared with the unbiased estimator, normalized cross-correlation. As an example, the peak displacement of the simulated acoustic radiation force response is reported because this position results in the noisiest estimates. Estimates were made with a 1.5-λ kernel and 20 dB SNR on 100 data realizations. Estimates using normalized cross-correlation and the Bayes¿ estimator had mean-square errors of 17 and 7.6 μm2, respectively, and contextualized by the true displacement magnitude, 10.9 μm. Biases for normalized cross-correlation and the Bayes¿ estimator are -0.12 and -0.28 ¿m, respectively. In vivo results show qualitative improvements. The results show that with small amounts of additional information, significantly improved performance can be realized.

Concepts: Scientific method, BIAS Peak, Ultrasound, Bias, Bias of an estimator, Estimation theory, Estimator