Showing papers on "Electrical impedance published in 1992"

Laser surgical unit

Abstract: Apparatus monitors RF return current to maximize the AC signal of impedance at two return electrodes. A transformer with driving and driven windings isolates ESU and patient. At ends of the driving winding are signal and ground terminals joined to the return electrodes with capacitors returning current. An AC coupling capacitor at the signal terminal has a timing circuit in sync to the voltage wave and relative to impedance of the return electrodes. Microprocessing the voltage at the signal terminal of the driving winding watches impedance and determines if the RF return current path is adequate. Voltage detection within the timing circuit has a voltage shaping circuit. A voltage comparator after the voltage detection forms a square wave. A current detection circuit and a coupling capacitor allow AC flow to the driving winding. Current shaping circuit in the current detection circuit has a voltage comparator at the output to form a square wave. Phase detection at the voltage and current detection circuits outputs filters the phase difference that is sampled and held as DC input to a switch, with an output and a few inputs to DC voltages. Phase locking an oscillating voltage source directly and/or through the sample and hold or DC switch tunes oscillation frequency and maximizes the voltage detection circuit output. Monitoring the return current with a signal from the voltage detection circuit connected to an oscillating voltage that is phase locked to the current phase therein shows that no phase difference and maximum signal voltage occur simultaneously.

Apparatus for generating ultrasonic oscillation

Abstract: The apparatus includes an ultrasonic transducer having a hand piece, an ultrasonic vibrating element secured to the hand piece and a probe coupled with the hand piece for propagating the ultrasonic oscillation produced by the ultrasonic vibrating element, a driving circuit for producing a driving signal for the ultrasonic vibrating element, a voltage controlled amplifier for amplifying the driving signal, an impedance matching transformer having a plurality of primary windings connected to the output of the voltage controlled amplifier via a switching circuit and a secondary winding connected to the ultrasonic vibrating element, a probe identification circuit for detecting the probe connected to the hand piece to produce a probe identification signal, a feedback control loop for generating a control voltage which is applied to the voltage controlled amplifier for controlling the amplification factor thereof in accordance with a driving current of the driving signal, an impedance detecting circuit for detecting the impedance of the ultrasonic transducer and controlling, the switching circuit such that a given primary winding is connected to attain the impedance matching between the driving circuit and the ultrasonic transducer, and a voltage limiter arranged in the feedback control loop such that the maximum value of the control voltage is limited in accordance with the probe identification signal.

Method of radiofrequency ablation

Abstract: A cardiac ablation apparatus including a solenoidal antenna, monitoring electrodes, and a coupling network at a distal end of a catheter transmission line, and another coupling network at the proximal end of the catheter transmission line to connect the catheter to the source of radiofrequency (RF) power and to an intracardiac electrogram monitor. Solenoidal antenna design includes single and multiple windings with varying geometrical features. Plated plastic tri-axial design of a transmission line offers unitary fabrication. A catheter with variable impedance electrode and gap coatings has features useful for both ablation and for hyperthermia applications.

Digitally controlled element sizing

Abstract: Effective control of impedance values in integrated circuit applications is achieved with an integrated circuit transistor whose size is digitally controlled. The digitally controlled size is achieved, for example, with a parallel interconnection of MOS transistors. In one application, the digitally controlled transistor serves as a controlled impedance connected to an output terminal of an integrated circuit. In that application, a number of transistors are enabled with control signals, and the collection of enabled transistors is responsive to the input signal that normally is applied to a conventional transistor. In another application, where the digitally controlled transistor serves as a controlled impedance at the input of a circuit, only the control signals that enable transistors and thereby determine the effective developed impedance are employed. In still another application, the digital control of the transistor's size is employed to control the speed or power consumption of the effective transistor. Such control is exercised to erase the manufacturing variability of the integrated circuit. Alternatively, such control is exercised as part of a feedback control of the operational characteristics of the entire circuit. In the feedback control application, the digital signals that control the transistor's size are obtained from an assessment of the circuit's operation. In the manufacturing variability control application, the digital signals that control the transistor's size are obtained from a measure of the integrated circuit's parameters relative to a reference element.

Numerical dosimetry at power-line frequencies using anatomically based models.

Abstract: We have used the finite-difference time-domain (FDTD) method to calculate induced current densities in a 1.31-cm (nominal 1/2 in) resolution anatomically based model of the human body for exposure to purely electric, purely magnetic, and combined electric and magnetic fields at 60 Hz. This model based on anatomic sectional diagrams consists of 45,024 cubic cells of dimension 1.31 cm for which the volume-averaged tissue properties are prescribed. It is recognized that the conductivities of several tissues (skeletal muscle, bone, etc.) are highly anisotropic for power-line frequencies. This has, however, been neglected in the first instance and will be included in future calculations. Because of the quasi-static nature of coupling at the power-line frequencies, a higher quasi-static frequency f' may be used for irradiation of the model, and the internal fields E' thus calculated can be scaled back to the frequency of interest, e.g., 60 Hz. Since in the FDTD method one needs to calculate in the time domain until convergence is obtained (typically 3-4 time periods), this frequency scaling to 5-10 MHz for f' reduces the needed number of iterations by over 5 orders of magnitude. The data calculated for the induced current and its variation as a function ofmore » height are in excellent agreement with the data published in the literature. The average current densities calculated for the various sections of the body for the magnetic field component (H) are considerably smaller (by a factor of 20-50) than those due to the vertically polarized electric field component when the ratio E/H is 377 ohms. We have also used the previously described impedance method to calculate the induced current densities for the anatomically based model of the human body for the various orientations of the time-varying magnetic fields, namely from side to side, front to back, or from top to bottom of the model, respectively. 34 refs.« less

Finite-difference time-domain implementation of surface impedance boundary conditions

Abstract: Surface impedance boundary conditions can be utilized to avoid using small cells, made necessary by shorter wavelengths in conducting media throughout the solution volume. The standard approach is to approximate the surface impedance over a very small bandwidth by its value at the center frequency, and then use that result in the boundary condition. In this paper, two implementations of the surface impedance boundary condition are presented. One implementation is a constant surface impedance boundary condition and the other is a dispersive surface impedance boundary condition that is applicable over a very large frequency bandwidth and over a large range of conductivities. Frequency domain results are presented in one dimension for two conductivity values and are compared with exact results. Scattering width results from an infinite square cylinder are presented as a two dimensional demonstration. >

Method to measure acoustic impedance and reflection coefficient

Abstract: A frequency-domain based system for measuring acoustic impedance and reflection coefficient is described. The calibration procedure uses a least-mean-squares approximation to the Thevenin parameters describing the source and receiver characteristics in which the data measured on closed, cylindrical tubes are matched to a viscothermal tube model. The system is intended for use in acoustical measurement in human ear canals, in which the cross-sectional area of the ear canal at the point of insertion is imprecisely known. This area is acoustically estimated from the impedance data, and the reflection coefficient is calculated in terms of this area and the impedance data. Measurements on a variety of closed tubes show the method is accurate over the frequency range investigated (less than 10.7 kHz). The time-domain reflection function is evaluated by transforming the reflection coefficient from the frequency domain, but the finite bandwidth of the measured data limits the accuracy of time-domain response measurements. The method is well suited for frequency-domain measurements in human ear canals.

A Novel Method for Generating Quantitative Local Electrochemical Impedance Spectroscopy

Abstract: A local electrochemical impedance spectroscopy (LEIS) technique for mapping the ac impedance distribution, as a function of frequency, of an electrode has been developed. In LEIS, as in traditional ac impedance methods, a sinusoidal voltage perturbation between the working and reference electrode is maintained by driving an ac current between the working electrode and a distant counterelectrode with a potentiostat. Local ac impedances are then derived from the ratio of the applied ac voltage and the local ac solution current density. The local ac current density is obtained from potential difference measurements near the electrode surface using a probe consisting of two micro‐electrodes. By measuring the ac potential difference between the micro‐electrodes, and knowing their separation distance and the solution conductivity, the local ac solution current density is derived. The accuracy of the local ac impedance data generated with this technique was established by investigating two model systems. The first provided a homogeneous electrode which allowed LEIS measurements to be compared to traditional EIS, while the second system provided a heterogeneity of known size and location whose components were easily characterized with traditional techniques. It is shown that area‐normalized scanning ac impedance measurements of the homogeneous electrode agreed well with traditional results. In addition, because LEIS maps the impedance properties of an electrode, the defect in the heterogeneous electrode was easily detected, while traditional ac impedance of this electrode gave little indication of its presence.

The use of surface impedance concepts in the finite-difference time-domain method

Abstract: Surface impedance concepts are introduced into the finite-difference time-domain (FDTD) method. Lossy conductors are replaced by surface impedance boundary conditions (SIBC), reducing the solution space and producing significant computational savings. Specifically, a SIBC is developed to replace a lossy dielectric half-space. An efficient implementation of this FDTD-SIBC based on the recursive properties of convolution with exponentials is presented. Finally, three problems are studied to illustrate the accuracy of the FDTD-SIBC formulation: a plane wave incident on a lossy dielectric half-space, a line current over a lossy dielectric half-space, and wave propagation in a parallel-plate waveguide with lossy walls. >

A new impedance spectrometer for the investigation of electrochemical systems

Abstract: The development of a computer‐controlled electrochemical impedance spectrometer, based on a Fourier transform algorithm is described. Together with a fast potentiostat the system can be used in the frequency range 1 mHz to 100 kHz. The perturbation signal is a superposition of sine waves with properly chosen frequencies. The overall measurement time is limited only by the lowest frequency in the spectrum and by the data transfer to the computer, thus, time‐resolved impedance spectra measurements can be performed. The principle of operation and technical details are presented and discussed.

Inherently impedance matched multiple integrated circuit module

Abstract: A multichip module (MCM) is formed with external connections on coaxial pins. This provides an impedance between a ground connection and a signal connection which is substantially equal per unit length. The module may be configured so that the impedances of the connections between the signal connections and integrated circuit may also be optimally impedance matched.

Apparatus and method for measuring, formatting and transmitting combined intracardiac impedance data and electrograms

Abstract: A patient-implantable cardiac stimulation apparatus adapted to measure body impedance, derive at least one physiological parameter from the impedance measurement, sample a time sequence of the physiological parameter and transmit the sequence to an external monitoring device for display and analysis. The apparatus may also sense intracardiac electrograms while it is measuring body impedance and deriving the physiological parameter, in which case it formats and transmits a combined intracardiac electrogram and physiological signal to the monitoring device.

Method and apparatus for displaying multi-frequency bio-impedance

Abstract: A method and apparatus for displaying complex impedance and related physical characteristics of an object (30) at a plurality of sinusoidal frequencies over a wide frequency range. Excitation signal waveforms (10, 14, 18) are stored in digital form. Digital sampling and processing circuitry (84, 86) provide high form. Digital sampling and processing circuitry (84, 86) provide high noise immunity and accurate measurement at all impedance phase angles. The technique is particularly well-suited to the measurement of complex impedances in living biological tissue because the digital implementation results in highly accurate measurement values over a plurality of selectable frequencies within a wide sinusoidal frequency region. A novel adaptation of digital cross-correlation (84) and convolution (86) techniques is used to simultaneously display (72) real and imaginary electrical impedance of living biological tissue at several selectable sinusoidal frequencies.

FDTD calculation of radiation patterns, impedance, and gain for a monopole antenna on a conducting box

Abstract: The ability of the finite-difference-time-domain (FDTD) method to calculate radiation patterns, input impedance, and gain for a monopole antenna on a conducting box is demonstrated. Results are given for the bare box and with the box coated with a dielectric layer. Radiation patterns are compared with measurements and with the method of moments for the bare box. Radiation patterns for the dielectric-covered box and all impedance and gain results are compared with measurements only. Good agreement is obtained in all cases. The FDTD approach includes a dielectric covering quite easily, while this would be quite difficult for a method of moments approach. The FDTD method requires similar computer time as the method of moments for a single-frequency result, but produces wide-bandwidth impedance and gain results with much less computer time. >

Electrical characterization of radio‐frequency discharges in the Gaseous Electronics Conference Reference Cell

Abstract: Measurements of the electrical characteristics of radio‐frequency (rf) discharges can be subject to large errors due to limitations in the measurement instruments and the stray impedance of the discharge cell. This study reports electrical measurements of argon discharges in the GEC Reference Cell in which special care has been taken to identify and minimize these sources of error. Careful calibration of current and voltage probes was found to be essential. In addition, parasitic impedances in the cell were found to be large, sensitive to minor changes in electrical connections, and not adequately described by simple a priori models. A general technique for characterizing the stray impedance, including an analysis of the propagation of errors, is presented here. This technique assures accurate results with specified uncertainties. Error analysis demonstrated that large gains in the precision of the measurements can be obtained using an inductive shunt circuit. Together, these techniques should improve the utility of electrical measurements for gauging the reproducibility of plasma conditions among rf discharge cells, for testing theoretical results, and for monitoring plasma processing.

Review of Applications of Impedance and Noise Analysis to Uniform and Localized Corrosion

Abstract: Steady-state techniques are very limited when they are used for analyzing corrosion processes for uniform or for localized phenomena. Non-steady-state techniques give valuable information ...

Solid state isolation device using opto-isolators

Abstract: An improved solid state isolation device that uses opto-isolators to provide an electrical barrier between a telephone network and user telephone equipment includes a load and current control circuit, and an adjustable termination circuit to eliminate or reduce signal distortion and insensitivity to low signal levels resulting from low loop current, and lack of complex impedance compensation/termination for diverse telephone system parameters. An improved trans-hybrid return loss circuit reduces internal losses; while loop termination losses are reduced by the use of Shottky diodes in the line coupler circuit. A high pass filter improves common mode rejection ratios in the transmit and receive paths, and adjustable compensation is provided to account for manufacturing variances in the opto-isolators. An additional connection is also provided to adapt the opto-isolator circuit for use with 3-wire ring circuitry.

Analysis of a new power electronics interface with approximately sinusoidal 3-phase utility currents and a regulated DC output

Abstract: A three phase rectification scheme that draws line currents with reduced distortion and provides a regulated DC output voltage is analyzed. The scheme employs two boost DC-DC converters to modulate the DC link currents. The modulation current is injected into the AC side through an impedance network consisting of series tuned L-C branches. The theoretical analysis and the basic simulation results, which can be used to carry out the design of a system based on this approach, are presented. >

Equivalent circuit modeling of interconnects from time domain measurements

Abstract: A technique for the equivalent circuit modeling of interconnects having discontinuities such as bends, steps, and junctions in high-speed circuits and packages is developed. The circuit models are extracted from time domain reflection (TDR) measurements. The simulated results for the circuit models are compared with the measured data to validate the accuracy of the circuit model. The proposed method can be used to help validate circuit models based on field-theoretic techniques as well as used as an independent tool to synthesize circuit models for general nonuniform or interacting two- and three-dimensional interconnects. >

A general method for electric and magnetic coupled problem in 2D and magnetodynamic domain

Abstract: The authors present a novel formulation which allows the inclusion of the circuit equations in the finite element analysis in two dimensions. Two types of magnetic conductors are considered: massive conductors which can develop eddy currents and 'fine wire' without eddy current. Kirchhoff's voltage and current equations have been established for these conductors in accordance with magnetic equations. Furthermore, these conductors have been integrated into an external electric network. This formulation allows the solution of two-dimensional magnetic problems with all kinds of electric connections between conductors, which consist of impedances (R, L, and C) fed by sinusoidal sources. The modeling of an asynchronous machine with a squirrel cage (with simulation of end rings) is given to illustrate one application of the formulation. >

Dual frequency microstrip rectangular patches

Abstract: A dual frequency microstrip rectangular patch antenna resonating at frequencies 520 MHz and 2.5 GHz is presented. This antenna incorporates a matching structure to improve the impedance characteristic at one resonance frequency and uses symmetrically positioned varactor diodes to control resonance at a lower frequency. The use of varactor diodes also provides the benefit of a broad tuning range at the lower resonance frequency. A tuning range of 32%; at 520 MHz was measured.

Modified SiO2 aerogels as acoustic impedance matching layers in ultrasonic devices

Abstract: Acoustic impedance matching between a transducer and the irradiated medium can be achieved using quarterwave or impedance gradient layers. SiO 2 aerogels are suitable materials for both purposes. In order to investigate matched transducers for air applications, sound intensity measurements were performed with and without aerogel quarterwave layer for different types of piezoceramic transducers. The measured gains in sound pressure level are compared with theoretical data derived from Mason's equivalent circuit and from finite element simulations. Ways to employ aerogel gradient layers for the acoustic matching of piezoceramics and biological tissue are discussed. Such layers could be obtained by sintering aerogels in a temperature gradient. Under certain conditions, an energy transmission coefficient of more than 99% for all frequencies above a certain frequency, f 0 , is predicted with such systems.

Skin impedance measurements using simple and compound electrodes.

Abstract: We have studied the effect of the electrode configuration on the measurement of body impedance and found that the electrode configuration greatly affects the impedance measurement using the four-electrode method. We studied the characteristics of the compound electrode and found that the compound electrode provides the four-electrode method in a compact form. A new method of measuring the skin impedance using simple electrodes at low frequencies was developed. At high frequencies where the effect of internal tissue impedance is not negligible, we used the compensation method using compound electrodes, because they measure the voltage right under the skin. At 50 kHz, we measured the real part of the skin impedance of less than 80 Ω on the thorax. We propose a simple instrument which can measure accurate skin impedance at various frequencies.

Method for incipient failure detection in electric machines

Abstract: An improved method for detecting an incipient failure in a multiphase electric motor includes the following steps: periodically measuring or continuously monitoring voltage and current values at each input to the motor; determining negative sequence voltage and current values for each periodic measured input voltage and current value; calculating an effective negative sequence impedance phasor value angle from each of the determined negative sequence voltages and current values; and comparing the calculated negative sequence impedance phasor angles and/or real and imaginary components over a plurality of periodic measurements to detect a change therein, which change is indicative of an incipient failure mode.

Electrochemical measurement system having interference reduction circuit

Abstract: A system for electrically measuring certain chemical characteristics of electrically-conductive fluids, such as blood, located within a tube and subject to electrical current interference. The measurements are made by measuring the voltage potential between a reference electrode and a sensor electrode sensitive to a particular blood parameter such as pH or calcium, potassium or chloride concentration. A bypass path for the electrical current interference is provided by a pair of noise-reduction electrodes located on opposite sides of the reference and sensor electrodes and interconnected by an amplifier having a relatively low output impedance and a relatively high input impedance. The electrical current interference bypasses the signal electrodes by flowing directly into the amplifier's output terminal, such that the reference and sensor electrodes develop a potential between them that is independent of the electrical current interference.