Input impedance matching of acoustic transducers operating at off-resonant frequencies

The input impedance matching technique of acoustic transducers at off-resonant frequencies is reported. It uses an inherent impedance property of transducers and thus does not need an external electric matching circuit or extra acoustic matching section. The input electrical equivalent circuit includes a radiation component and a dielectric capacitor. The radiation component consists of a radiation resistance and a radiation reactance. The total reactance is the sum of the radiation reactance and the dielectric capacitive reactance. This reactance becomes zero at two frequencies where the impedance is real. The transducer size can be properly chosen so that the impedance at one of the zero-crossing frequencies is close to 50 Ω, the output impedance of signal generators. At this off-resonant operating frequency, the reflection coefficient of the transducer is minimized without using any matching circuit. Other than the size, the impedance can also be fine tuned by adjusting the thickness of material that bonds the transducer plate to the substrates. The acoustic impedance of the substrate and that of the bonding material can also be used as design elements in the transducer structure to achieve better transducer matching. Lead titanate piezoelectric plates were bonded on Lucite, liquid crystal polymer (LCP), and bismuth (Bi) substrates to produce various transducer structures. Their input impedance was simulated using a transducer model and compared with measured values to illustrate the matching principle.     

Characterisation of alumina scales on Fecralloy using impedance spectroscopy

Impedance measurements of alumina scales on Fecralloy substrates were made at temperature from 400 °C to 650 °C. The alumina scales were also examined using scanning electron microscopy, transmission electron microscopy, coupled with energy dispersive X-ray analysis. Impedance spectra of the alumina scale in modulus format indicate that the thickness of the alumina can be measured based on impedance measurements whereas the change of impurity level in the alumina can be monitored according to variation of activation energy for electrical conduction.     

On the application of Biot's theory to acoustic wave propagation in snow

A fluid-saturated, elastic, porous media model is used to describe acoustic wave propagation in snow. This model predicts the existence of two dilatational waves and a shear wave. In homogeneous, isotropic snow the two dilatational waves are uncoupled from one another but involve coupled motion between the interstitial air and ice skeleton. Dilatational waves of the first kind and shear waves are slightly dispersive and attenuated with distance. Dilatational waves of the second kind are strongly dispersive and highly attenuated. The model also predicts that the wave impedance for snow is close to that of air and that snow strongly absorbs acoustic wave energy.Available experimental phase velocity, impedance and attenuation data support the calculated results. Phase velocity measurements indicate three identifiable categories: fast dilatational waves (phase velocity ? 500 m/s), slow dilatational waves (phase velocity < 500 m/s) and shear waves. Wave impedance and attenuation measurements illustrate the low impedance, highly absorbing characteristics of snow. Additional impedance, attenuation and phase velocity data are required to further test and improve the model.     

Fabrication of anticorrosive multilayer onto magnesium alloy substrates via spin-assisted layer-by-layer technique

To improve the corrosion resistance of magnesium alloy, we reported a novel approach for the fabrication of anticorrosive multilayers onto AZ91D substrates. The multilayers were composed of poly(ethylene imine) (PEI), poly(styrene sulfonate) (PSS) and 8-hydroxyquinoline (8HQ). They were deposited onto AZ91D substrates via a spin-assisted layer-by-layer (LbL) technique. The multilayered structure was stabilized with glutaraldehyde (GA) as crossing linker. It was confirmed by Fourier transform infrared spectroscopy (FT-IR). Surface morphologies and elemental compositions of the formed anticorrosive multilayers were characterized with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The corrosion performance of the multilayer coated AZ91D substrates was characterized by hydrogen evolution. The results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements suggested that the multilayered coating improved the corrosion resistance of AZ91D substrates. In vitro study revealed that the multilayered coating was cytocompatible. The study provides a potential alternative for the fabrication of corrosion resistant magnesium alloy-based implants.     

Input impedance of grid-connected converters with programmable harmonic resistance

Power factor correction converters with a programmable harmonic input impedance can be used to reduce the harmonic distortion in the utility grid. Whereas previously proposed controllers tried to obtain a resistive behaviour of the converter with a constant input impedance for all frequencies, including the fundamental, another recently proposed control strategy allows the setting of a harmonic input resistance independent of the fundamental input impedance. Although previous papers showed the usefulness of this approach, the analysis and verification of the input impedance of the converter in a broad frequency spectrum is focussed here. The characteristics of the current controller will determine the input impedance of the converter in the frequency domain. The implementation of the control strategy with a programmable harmonic resistance on a digitally controlled full-bridge bidirectional converter is described. Experimental tests on a 1-kW prototype show that with the proposed implementation good concordance with the theoretical input impedance curves is obtained     

Study of the variation of the input impedance of induction machines with frequen

The results of a study of the variation of three-phase induction machines' input impedance with frequency are proposed. A range of motors were analysed, both two-pole and four-pole, and the magnitude and phase of the input impedance were obtained over a wide frequency range of 20 Hz-1 MHz. For test results that would be useful in the prediction of the performance of induction machines during typical use, a test procedure was developed to represent closely typical three-phase stator coil connections when the induction machine is driven by a three-phase inverter. In addition, tests were performed with the motor's cases both grounded and not grounded. The results of the study show that all induction machines of the type considered exhibit a multiresonant impedance profile, where the input impedance reaches at least one maximum as the input frequency is increased. Furthermore, the test results show that the grounding of the motor's case has a significant effect on the impedance profile. Methods to exploit the input impedance profile of an induction machine to optimise machine and inverter systems are also discussed.     

余弦形负载超声变幅杆输入阻抗特性的研究

研究余弦形负载超声变幅杆输入阻抗特性。求解了加负载时四种超声变幅杆的输入阻抗,得到加负载时输入阻抗和M"obius变换参数的统一算式。当变幅杆的输入抗分量为零时,计算了余弦形负载变幅杆的纵向振动共振频率方程和放大系数。由输入阻抗的表达式讨论了此类变幅杆的工作稳定性条件和相对阻抗相等点,对余弦形变幅杆的实际应用有一定的参考意义。     

A functional for the input impedance of thin electric vibrator

A functional for the input impedance of electric vibrator is derived for the variation of the distribution of vibrator current. Using the approximation of sinusoidal current distribution, a simple approximate formula for the input impedance of a symmetric electric vibrator is derived. The active and reactive components of impedance are calculated versus the ratio of the length of vibrator arm to wavelength for three values of the vibrator diameter.     

回热器的热声指标

在热声谐振管中,回热管的结构尺寸对于热声谐振管的性能起着关键的作用。本文通过分析得出了采用板叠结构的回热器的有源网络模型,并在此基础上提出了评价回热器性能的指标;输出端放空时的输入阻抗,输出端封闭时的输入阻抗,输出端放空时的正向流放大倍数,以此为基础可以进行热声制冷机的整机优化匹配设计。     

Characterization of cathodically deposited carbonaceous films on a silicon substrate

Carbon-based deposits were electrochemically formed on silicon substrates in ethanol at room temperature. This work was based on the work reported by Namba, who described the electrochemical deposition of diamond from organic solutions. The deposits were analysed using a scanning electron microscope, energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and electrochemical impedance spectroscopy. Scanning electron micrographs showed some crystalline deposits on the silicon. EDS was unable to identify carbon in the film, but did reveal impurities such as sodium, potassium, calcium and zinc. It was later established that the impurities most likely came from impurities in the graphite used for a counter electrode. XPS showed the presence of carbon species, and subsequently Raman spectroscopy was used to classify further the carbon deposits. Raman spectroscopy showed the presence of amorphous carbon in some films, but no diamond peak was observed for any of the films. EIS revealed that the impedance of the deposited films was nearly identical to that of the uncoated silicon, and did not resemble the impedance of diamond. Thus, in this work, carbon–based films were formed electrochemically, but these films were not diamond.     

Frequency response of top-gated carbon nanotube field-effect transistors

The ac performance of carbon nanotube field-effect transistors (CNFETs) has been characterized using two approaches involving: 1) time- and 2) frequency-domain measurements. A high input impedance measurement system was used to demonstrate time-domain switching of CNFETs at frequencies up to 100 kHz. The low level of signal crosstalk in CNFETs fabricated on quartz substrates enabled frequency-domain measurements of the ac response of CNFETs in the megahertz range, over five orders of magnitude higher in frequency than previously reported ac measurements of CNFET devices.     

Superconductivity and phase stability of Nb3Ge

A method is described for calculating the surface impedance of thin superconducting films using the theory of Mattis and Bardeen of the surface impedance of superconductors. An approximate extension of the theory to the case where the superconducting order parameter inside the film varies slowly in space is presented. Calculations using the theory are compared with experimental measurements of the surface impedance at 3 GHz of thin tin films on bulk lead substrates. Agreement between theory and experiment is good.     

Flush-mounted cylindrical-rectangular microstrip antennas

The analysis of flush-mounted cylindrical microstrip antennas is presented. The analysis is performed using method of moments in spectral domain, and results are obtained for radiation pattern and input impedance. The results are compared with those obtained for similar geometries, where the dielectric layers extend to infinity. The results show that the housing has not significantly altered the radiation pattern, but the input impedance is affected for small cavity sizes. It is also shown that the resonant frequency diminishes as the superstrate thickness increases.     

Method of determining feed positions in coupled half-wavelength stepped impedance resonator bandpass filter

In this study, the input impedance of coupled half wavelength stepped impedance resonator bandpass filter with directly tapped feed structure is deduced, and the method of determining the feed positions in the filter is proposed by using the concept of impedance matching. Two microstrip bandpass filter examples are described for demonstrating the procedure. The simulated and measured results indicate that the proposed method is an effective way for designing similar filters.     

Calculation of self- and mutual impedances in planar sandwichinductors

High-frequency planar magnetic components, employing thin film and thick film technology, have become important components in applications, such as filters and switching converters, due to their ease of manufacture and reliability. In a previous paper, the authors established a frequency dependent impedance formula for planar coils on a magnetic substrate that is infinitely thick. In this paper, two new impedance models are described: the first is for planar coils on a magnetic substrate of finite thickness, and the second represents a planar coil sandwiched between two substrates. The models include the electrical conductivity of the magnetic material so that the effects of eddy currents, particularly at high frequencies, are taken into account. The eddy currents reduce the inductance and increase the losses associated with the device. The new impedance formulas are derived from Maxwell's equations. Simulations were carried out on a typical device, using finite element analysis, and the results validate the new formulas. This paper establishes the frequency limitations of lossy magnetic substrates     

A Novel Indirect Method to Determine the Radiation Impedance of a Handheld Antenna Structure

A novel indirect measurement technique is developed in which the input impedance of an unknown antenna structure is determined by measuring the change in the measured input impedance of a second probe antenna, while the unknown antenna is loaded at its input by a set of known impedances. The feasibility of the approach is explored using method-of-moment boundary element simulations of the interaction between two dipole antennas in free space. Examples of the measurement of the helical antenna of a terrestrial trunked radio (TETRA) handheld radio and a small portable device with an electrically small antenna are presented. The significant advantage of the technique in the accurate measurement of electrically small antennas is confirmed.      

损耗地面上线天线的一种近似计算方法

在普遍镜像法分析地面上垂直天线特性的基础上，提出了一种修正的镜像法，该方法能提高计算的速度．通过仿真计算以及对测量结果的比较，发现VLF天线输入阻抗随着地面电导率的改变而明显变化，而且大地的形状和地质在一定程度上影响VLF天线的发射效率。     

用于大功率超声的电缆传输特征的实验研究

大功率超声电信号通过特种电缆传输时,其电流损耗比较大。为了解决该电信号的长距离传输问题,用4294A阻抗分析仪对单芯同轴电缆进行了有针对性的测试。通过改变电缆的负载电阻,获得了一系列电缆的输入阻抗随频率的变化曲线。当负载电阻比较小时,在一些特定的频率处,电缆的输入阻抗很大。这时,其电功率传输效率很低。当负载电阻比较大时,电缆的输入阻抗取极大值的频率位置发生改变。借助于电磁波在电缆负载端的反射和透射特征,分析了这些实验结果,探讨了这些实验结果对大功率超声电功率传输特征的影响,最后结合实验结果给出了大功率超声传输电缆的设计原则。     

Measurement of the input resistance of a rf biased SQUID

Measurements of the real part of the input impedance of a RF biased SQUID are described. The experimental data are in good agreement with the predictions of the recently published theory. In particular the dependence of the input resistance on the detuning of the rf bias frequency from tank circuit resonance frequency is confirmed.     

Effects of Mg-contents and substrate parameters on structure and optical properties of Mg-doped ZnO nanorods fabricated by hydrothermal method

Mg-doped ZnO nanorods with different contents have been fabricated on various substrates by hydrothermal method. The effects of Mg-contents and different substrates on structural and optical properties are analyzed by scanning electron microscopy, X-ray diffraction, photoluminescence (PL) spectra, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results reveal that the Mg-doped ZnO nanorods possess good crystalline quality and morphology when the molar ratio of Mg/Zn is 1. The PL spectra show that the UV emissions have an obvious blue shift with the increase of Mg-content. The results of investigation for the samples grown on different substrates show that the crystal quality and morphology of the samples grown on ZnO layer are perfect, and the UV emission also occurs blue shift owing to the effects of different substrates.