Concept Of Impedance Research Articles

Modeling of photochemical reactions in a focused laser beam

Fluorescent materials play a prominent role in the qualitative and quantitative measurement of scientific phenomena of importance in biotechnology and biomedical applications. Photodegradation of fluorophores is a process that determines the accuracy and sensitivity of such measurements. This is the motivation for developing methods for accurately measuring fluorophore photodegradation rates. Recently, illumination consisting of short pulses has been used to examine the decay of photochemical reaction products. However, the time resolved measurements are difficult to interpret since the photodegradation process usually involves multiple time scales. The frequency domain measurement technique discussed here looks at the frequency response of a fluorescent sample to a frequency modulated illuminating light. The photodegradation rate is obtained by interpreting the frequency domain measurements in terms of traditional impedance concepts. In the measurements described in this paper, a focused laser beam is used to illuminate a sample of slowly flowing fluorescent solution. The laser beam is assumed to have a Gaussian power distribution hence illumination is spatially non-uniform in the region of interest. The photochemical reaction rates depend on power, so they will also vary with the position in the beam. However in the case of photodegradation of fluorophores, the measurement of the resulting decrease in fluorescence is given in terms of the radiation emitted from the entire illuminated region. In this work we present a mathematical description of the time evolution of the fluorescence response integrated over a non-uniformly illuminated domain. As a result of our analysis, an experimentally accessible and tractable mathematical model Eq. (19) and Eq. (30) is obtained from a more fundamental description given by Eq. (4) and Eq. (5). The model is used to create a functional form for fitting experimental measurements from a lock-in amplifier.

Cold Cathode Fluorescent Lamps Driven by Piezoelectric Transformers: Stability Conditions and Thermal Effect

The envelope impedance (EI) concept was used to develop a systematic and simple approach for studying the conditions for stability of piezoelectric transformers (PT) based ballasts for cold cathode fluorescent lamps (CCFL) systems and the implications of the thermal operating conditions on such systems. The envelope analysis of the CCFL, which was verified experimentally, revealed that the magnitude of the negative EI decreases with temperature. This might cause the system to become unstable at low temperatures due to the fact that the loop gain of the PT-CCFL system encircled the (-1, 0) point in the Nyquist plane. In such cases, the PT-CCFL system may enter a quasi-stable oscillatory operation mode in which the current is AM modulated by a low frequency parasitic signal. This phenomenon is analyzed and explained by considering the temperature effect on the EI of the CCFL. The results of this work provide an insight into the stability issue of the PT-CCFL system, and could help in finding remedies to the instability

Microwave measurements Part I: Linear Measurements

By convention, radio frequency (RF) and microwave frequencies range between 30 MHz and 300 GHz. Conversely, this means their wavelengths range between 10 m and 1 mm. Intense research in radar development during World War II extended the RF spectrum beyond the usual applications in radio communications. The use of shorter wavelengths resulted in laboratory equipment with proportionally smaller dimensions to generate, convey, transmit, and detect higher-frequency signals. Wavelengths shorter than 1 mm require equipment too small to be realized. Voltage, current, and impedance concepts lose their conventional meanings when the operating wavelength is approximately equal to the dimensions of the structures under test. The behavior of propagating electromagnetic waves must then be analyzed in terms of electric and magnetic field. Unfortunately, no simple and direct way exists to measure these quantities, so we must resort to indirect methods. In the following sections, we briefly review the concept and need for measurements of scattering parameters. We also describe the two key instruments for microwave measurements: VNAs and spectrum analyzers.

Dual composite right/left‐handed (D‐CRLH) leaky‐wave antenna with low beam squinting and tunable group velocity

AbstractThe propagation and radiation properties of dual composite right/left‐handed (D‐CRLH) transmission line metamaterials are presented, and validated experimentally with a 1D microstrip prototype. In its fast‐wave regime, this structure operates as a leaky‐wave antenna capable of backfire‐to‐endfire radiation scanning with beam squinting smaller than that of conventional CRLH (C‐CRLH) antennas. It is shown that this D‐CRLH beam squinting can be further reduced by increasing the group velocity but that this can occur, close (and possibly beyond) to luminal velocities, only at the expense of losses produced by parasitic effects, a fundamental limitation which is demonstrated via the Bloch impedance concept. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

PSpice for Circuit Theory and Electronic Devices

PSpice for Circuit Theory and Electronic Devices is one of a series of five PSpice books and introduces the latest Cadence Orcad PSpice version 10.5 by simulating a range of DC and AC exercises. It is aimed primarily at those wishing to get up to speed with this version but will be of use to high school students, undergraduate students, and of course, lecturers. Circuit theorems are applied to a range of circuits and the calculations by hand after analysis are then compared to the simulated results. The Laplace transform and the s-plane are used to analyze CR and LR circuits where transient signals are involved. Here, the Probe output graphs demonstrate what a great learning tool PSpice is by providing the reader with a visual verification of any theoretical calculations. Series and parallel-tuned resonant circuits are investigated where the difficult concepts of dynamic impedance and selectivity are best understood by sweeping different circuit parameters through a range of values. Obtaining semiconductor device characteristics as a laboratory exercise has fallen out of favour of late, but nevertheless, is still a useful exercise for understanding or modelling semiconductor devices. Inverting and non-inverting operational amplifiers characteristics such as gain-bandwidth are investigated and we will see the dependency of bandwidth on the gain using the performance analysis facility. Power amplifiers are examined where PSpice/Probe demonstrates very nicely the problems of cross-over distortion and other problems associated with power transistors. We examine power supplies and the problems of regulation, ground bounce, and power factor correction. Lastly, we look at MOSFET device characteristics and show how these devices are used to form basic CMOS logic gates such as NAND and NOR gates.

Lightning-induced currents in buried coaxial cables: A frequency-domain approach and its validation using rocket-triggered lightning

The aim of this paper is to present an experimental validation of a frequency-domain approach to the solution of the lightning electromagnetic field-to-buried cable coupling equations. The coupling to the inner conductor is evaluated using the concept of cable transfer impedance. The theoretical model and relevant computer code are tested using experimental data on lightning-induced currents in buried cables carried out in 2002 and 2003 at the International Center for Lightning Research & Testing (ICLRT) at Camp Blanding, Florida where currents induced by triggered lightning events were measured at the ends of a buried coaxial cable, both in the shield and in the inner cable conductors. Reasonably good agreement has been found between numerical simulations and recorded waveforms. In particular, the early-time response of the cable and the peak value of the induced currents were generally well reproduced by the simulations.

Direct visual tracking control of remote cellular robots

This paper presents the design of a stable non-linear control system for the remote visual tracking of cellular robots. The robots are controlled through visual feedback based on the processing of the image captured by a fixed video camera observing the workspace. The control algorithm is based only on measurements on the image plane of the visual camera–direct visual control–thus avoiding the problems related to camera calibration. In addition, the camera plane may have any (unknown) orientation with respect to the robot workspace. The controller uses an on-line estimation of the image Jacobians. Considering the Jacobians’ estimation errors, the control system is capable of tracking a reference point moving on the image plane–defining the reference trajectory–with an ultimately bounded error. An obstacle avoidance strategy is also developed in the same context, based on the visual impedance concept. Experimental results show the performance of the overall control system.

Accurate modeling, wave mechanisms, and design considerations of a substrate integrated waveguide

A new method of analysis is presented in this paper for the determination of complex propagation constants in substrate integrated waveguides (SIWs). This method makes use of the concept of surface impedance to model the rows of conducting cylinders, and the proposed model is then solved by combining a method of moments and a transverse resonance procedure. The proposed method is further applied to extract results in terms of parametric curves and graphs which demonstrate fundamental and interesting wave guidance and leakage properties of this type of periodic waveguide. Useful design rules are extracted from this analysis, suggesting that appropriate design parameters and regions should be carefully selected for practical applications. In addition, comprehensive review and comparisons with published results are also presented to show the performance and accuracy of the proposed modeling technique. Practical measurements of fabricated samples with different levels of loss have confirmed the accuracy of this new method and validity of design rules.

Electrical impedance for an electrolytic cell

We analyze in which experimental conditions the concept of electrical impedance is useful for an electrolytic cell. The analysis is performed by solving numerically the differential equations governing the phenomenon of the redistribution of the ions in the presence of an external electric field and comparing the results with the ones obtained by solving the linear approximation of these equations. The control parameter in our study is the amplitude of the applied voltage, assumed a simple harmonic function of the time. We show that the bulk distribution of ions close to the electrodes differs from the one obtained by means of the linear analysis already for small amplitudes of the applied voltage. Nevertheless, the concept of electrical impedance remains valid. For larger amplitudes, the current in the circuit is no longer harmonic at the same frequency of the applied voltage. Therefore the concept of electrical impedance is no longer meaningful.

Optical impedance of metallic nano-structures

Impedance matching refers to the suppression of reflected radiation from an interface and is a concept that applies right across the electromagnetic spectrum. In particular it has come to prominence in relation to the propagation of light in metallic structures and associated meta-materials. Whilst established for microwaves and electrical circuits, this concept has only very recently been observed in the optical domain, yet is not well defined or understood. We present a framework to elucidate the concept of optical impedance. We describe using a scattering matrix approach the characteristic, iterative, image and wave impedances of an optical system. With a numerical model, we explore each form of impedance matching in metal-dielectric structures. Thin gold layers may extend the concept of Brewster's angle to normal incidence and s polarization. Optical impedance for recently realized metallic gold nanopillars which has shown negative permeability is also explored and we show that current measurements are inconclusive to robustly state its characteristic impedance is matched to the vacuum.

Significance of small voltage in impedance spectroscopy measurements on electrolytic cells

We investigate, theoretically, for what amplitude of the applied voltage to an electrolytic cell the concept of impedance is meaningful. The analysis is performed by means of a continuum model, by assuming the electrodes perfectly blocking. We show that, in the low-frequency range, the electrolytic cell behaves as a linear system only if the amplitude of the measurement voltage is small with respect to the thermal voltage VT=kBT∕q, where kBT is the thermal energy, and q is the modulus of the electrical charge of the ions, assumed identical except for the sign of the charge. On the contrary, for large frequency, we prove that the amplitude of the applied signal has to be small with respect to a critical voltage that is frequency dependent. The same kind of analysis is presented for the case in which the diffusion coefficients of the positive ions is different from that for negative ions, and for the case where surface adsorption takes place.

Suppression of Line Voltage Related Distortion in Current Controlled Grid Connected Inverters

The influence of selected control strategies on the level of low-order current harmonic distortion generated by an inverter connected to a distorted grid is investigated through a combination of theoretical and experimental studies. A detailed theoretical analysis, based on the concept of harmonic impedance, establishes the suitability of inductor current feedback versus output current feedback with respect to inverter power quality. Experimental results, obtained from a purpose-built 500-W, three-level, half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an output current total harmonic distortion (THD) some 29% lower than that achieved using output current feedback. A feed-forward grid voltage disturbance rejection scheme is proposed as a means to further reduce the level of low-order current harmonic distortion. Results obtained from an inverter with inductor current feedback and optimized feed-forward disturbance rejection show a THD of just 3% at full-load, representing an improvement of some 53% on the same inverter with output current feedback and no feed-forward compensation. Significant improvements in THD were also achieved across the entire load range. It is concluded that the use of inductor current feedback and feed-forward voltage disturbance rejection represent cost-effect mechanisms for achieving improved output current quality.

Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides

We investigate the performance of bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides. We show that bends and splitters with no additional loss over a very wide frequency range can be designed for metal-dielectric-metal waveguides with center layer thickness small compared to the wavelength. We also introduce the concept of characteristic impedance for such systems to account for their behavior.

Reflections on the history of electrochemical impedance spectroscopy

The history of electrochemical impedance spectroscopy (EIS) is briefly reviewed, starting with the foundations laid by Heaviside in the late 19th century in the form of Linear Systems Theory (LST). Warburg apparently was the first to extend the concept of impedance to electrochemical systems at the turn of the 19th century, when he derived the impedance function for a diffusional process that still bears his name. Impedance spectroscopy was next employed extensively using reactive bridges to measure the capacitance of ideally polarizable electrodes (mostly mercury), leading to the development of models for the electrified interface. However, it was the invention of the potentiostat in the 1940s and the development of frequency response analyzers in the 1970s that led to the use of EIS in exploring electrochemical and corrosion mechanisms, primarily because of their ability to probe electrochemical systems at very low frequencies. These inventions have led to an explosion in the use of EIS for exploring a wide range of systems and processes, ranging from conduction in the solid and liquid states, ionic and electronic conduction in polymers, heterogeneous reaction mechanisms, and the important phenomenon of passivity. It is evident that the use of EIS in identifying reaction mechanisms makes use of pattern recognition, currently through inspection. It is argued that, in the future development of EIS, reaction mechanism analysis (RMA) would be most efficiently done by using artificial neural networks operating in the pattern recognition mode. This strategy would require the creation of libraries of reaction mechanisms for which the theoretical impedance functions are known.

Time domain surface impedance concept for low frequency electromagnetic problems—Part II: Application to transient skin and proximity effect problems in cylindrical conductors

A time domain boundary element formulation employing the surface impedance boundary conditions (SIBCs) is developed for the 3-dimensional transient eddy current problem of cylindrical conductors. SIBCs of different orders of approximation are implemented using the perturbation technique in the small parameter proportional to the ratio of the skin depth and characteristic size of the conductor cross-section. The formulation consists of a set of time domain surface integral equations that have identical left-hand sides and can be solved using the same program procedure. The number of equations is determined by the order of approximation of the SIBC, namely: solutions in the perfect electrical conductor (PEC) limit (lowest order) and in the so-called Rytov approximation (highest order) are given by one and four equations, respectively. It is demonstrated that each equation admits separation of variables into space and time components, a property that significantly reduces computational costs compared with traditional time domain formulations that require the integral equations to be solved at each time step. For the purpose of validation, a test problem is solved by the proposed formulation and by the ‘original’ BEM based on the time-dependent fundamental solution. Conditions of applicability are discussed and the effect of such factors as the shape of the incident current pulse and proximity effect are considered.

Time domain surface impedance concept for low frequency electromagnetic problems—Part I: Derivation of high order surface impedance boundary conditions in the time domain

The surface impedance boundary conditions (SIBCs) for the tangential component of the electric field and normal component of the magnetic field on the smooth curved surface of a homogeneous non-magnetic conductor are derived in time- and frequency-domain. Scale factors for the basic variables are introduced in such a way that, a small parameter, equal to the ratio of the penetration depth and the body's characteristic size, appears in the dimensionless Maxwell's equations for the conducting region. The perturbation method is then used to represent the SIBCs in the form of power series in this small parameter and the first four terms of the expansions are derived. The zero-order, first-order, second-order and third-order terms of the expansions are the solution of the problem in the perfect electrical conductor limit, the Leontovich approximation, the Mitzner approximation and in the high order approximation (referred to as Rytov's approximation), respectively. Therefore, the accuracy of the proposed conditions exceeds the accuracy of the SIBC for planar surfaces (Leontovich's approximation) that are usually used in the time-domain analysis, by two orders of magnitude. In Part II of this paper, the formulation of the SIBCs developed here in conjunction with a boundary element method is demonstrated and applied to the problem of transient skin and proximity effect problems in cylindrical conductors.

Considerations on the Field Balancing of the Overhung Rigid Rotors

Any unbalance distribution in a rigid rotor assembly can be completely balanced by fixing at a determined radius two correction weights (equal and opposite to the equivalent unbalance weights), each one located in any two planes perpendicular to the rotation axis. This paper presents a tutorial that develops, in a simple, clear, concise, and precise form, the general equations that relate the unbalance weights in the correction planes with the vibrations in the supporting bearings, based on the concept of the mechanical impedance, for the particular case of an overhung rigid rotor. Important insights are provided to understand thoroughly the inherent additional cross-effect that is produced by the application of each of the existing exact methods of influence coefficients to the balancing of the overhung rigid rotors. Also discussed is a recent advance that consists of a new solution for the field balancing of the overhung rigid rotors. This alternative solution again combines the least cross-effect with the minimum number for the trial weights, and gives the correction weights directly for both balancing planes. The advantage of this new technique, with respect to the previous variation of the method of the influence coefficients, is that its implementation in any industrial balancer would require only minor changes in the programming. An explanation of the conversion that is necessary for different phase conventions within a computer program and examples of calculations with experimental data are included for completeness.

The Evolution of Aural Acoustic-Immittance Measurements

The Evolution of Aural Acoustic-Immittance Measurements

A Controllable Reactor of Transformer Type

This paper clarifies the operating principles of a controllable reactor of transformer type (CRT). In nature, a CRT is equivalent to a multi-winding transformer for which the low-voltage windings (control windings) operate at short-circuited states in turn. This kind of reactor possesses such advantages as smooth power regulation, low current harmonic content, and fast response. Taking account of the coupling of the control windings, this paper examined the current harmonic content for the work winding by Fourier series decomposition, and thus derived the current harmonic coefficients. Based on the solution of the branch current equation of a multi-winding transformer, and introduction of the concept of equivalent winding impedance, a polygon-type equivalent circuit is proposed for a multi-winding transformer. Subsequently, formulae for the branch impedances of the equivalent circuit are presented. According to the harmonic content constraints, the number of control steps is determined. Then, by combining the MMF and branch current equations, an expanded equation about the winding currents and current-limiting reactances is obtained. According to the feature that the control windings are short-circuited in turn, the current-limiting reactances are calculated for each control winding step by step. At last, based on the proposed equivalent circuit for a multi-winding transformer, a simulation model for a CRT is fabricated with MATLAB/SIMULINK/PSB. The simulation results confirmed the presented work, which underpins the further analysis and design of a CRT.

Load-Impedance and Bias-Network Dependence of Power Amplifier With Second Harmonic Injection

For the power amplifier (PA) used in code-division multiple-access cellular phones, the supply voltage is switched from around 1.5 to 3.5 V at the boundary transmission power of several decibels higher than 10 dBm using a DC-DC converter to improve operation efficiency. Recently, however, this boundary transmission power is being raised. We applied the second harmonic injection to reduce distortion and maximize the boundary transmission power under low supply voltage. This paper presents the design concept of the PA's load impedance and bias network with second harmonic injection using Volterra-series analysis. We also present a design of the final stage of the PA under low supply voltage and show that the final stage has a drain efficiency of 53% at an output power of 21 dBm.