Showing papers on "Electrical impedance published in 1989"

Electrode models for electric current computed tomography

Abstract: A mathematical model for the physical properties of electrodes suitable for use in electric current computed tomography is discussed. The model includes the effects of discretization, shunt, and contact impedance. The complete model was validated by experiment. Bath resistivities of 284.0, 139.7, 62.3, and 29.5 Omega -cm were studied. Values of effective contact impedance used in the numerical approximations were 58.0, 35.0, 15.0, and 7.5 Omega -cm/sup 2/, respectively. Agreement between the calculated and experimentally measured values was excellent throughout the range of bath conductivities studied. It is desirable in electrical impedance imaging systems to model the observed voltages to the same precision as they are measured in order to be able to make the highest-resolution reconstructions of the internal conductivity that the measurement precision allows. The complete electrode model, which includes the effects of discretization of the current pattern, the shunt effect due to the highly conductive electrode material, and the effect of an effective contact impedance, allows calculation of the voltages due to any current pattern applied to a homogeneous resistivity field. >

Improved field probes for soil water content and electrical conductivity measurement using time domain reflectometry

Abstract: Volumetric water content θ and soil electrical conductivity σ may be measured in situ using time domain reflectometry (TDR). The parallel-wire or two-wire transmission line TDR probes currently in field use suffer from unwanted noise and information loss due to impedance mismatch between the probe and the coaxial connecting cable. Here we describe symmetric, multiwire probes designed to minimize these problems and eliminate the need for a balancing transformer between probe and TDR device. Analysis of the electric field distributions around these new probes shows that they emulate a coaxial transmission line, and their measured characteristic probe impedances approach that of coaxial probes. Signals from the new probes permit more reliable and accurate θ and σ measurement and are superior to those of two-wire probes with balancing transformer. The enhanced signal clarity of the new probes extends to sample diameters of at least 0.2 m. We show that electrical conductivity determined with the new probes is identical to that found with a coaxial cell and substantially different from that measured by a two-wire probe. Our results indicate that values of σ, determined using the Giese-Tiemann thin sample approach and measured characteristic probe impedances of coaxial or multiwire probes, agree with values of σ measured using an ac bridge for both electrolyte solutions and soil samples to within ±10%, provided σ exceeds 10 mS m−1. Finally, we give an example of the use of multiplexed three-wire probes in following rainfall infiltration and redistribution during and after a simulated rainfall event in the field. Infiltrated quantities of water estimated from the TDR water content profiles agreed within ±10% with the amount applied.

Controlling impedance at the man/machine interface

Abstract: Recent work on the dynamics of interactions between humans and machined is reviewed. An experimental apparatus for simulating virtual environments and investigating the man/machine interface is described. Relevant experiments on the mechanical impedance of the human arm are reviewed. Despite active neuromuscular feedback control, the human arm exhibits the impedance of a passive object. Preliminary results of some experiments on the adaptability of human arm impedance are presented. These results indicate limitation in the rapidity of parameter adaptation in humans. >

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

Abstract: We report room‐temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1‐nm‐thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound‐state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak‐to‐valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak‐to‐valley ratio, should allow oscillations up to about 1 THz.

Relationships between antennas as scatterers and as radiators

Abstract: The field scattered by an antenna contains a component that is the short circuit scattered field normalized by the short circuit current and a second component that is the radiation field normalized by the transmitting current and multiplied by a factor (1- Gamma ). The RCS is the magnitude squared of the difference between two terms, one being the square root of a complex 'structural' cross section, and the other (1- Gamma ) times the square root of a complex 'antenna' cross section. These relationships, and the role of the load impedance are clarified. The connection between antenna directivity and load effective receiving area is also derived. >

Transfer function method for measuring characteristic impedance and propagation constant of porous materials

Abstract: A method for measuring the characteristic impedance and propagation constant of porous materials is described in this paper. Measurements were performed based on a surface impedance method that required a set of distinct acoustic impedances derived at the material surface. This requirement is satisfied by arbitrarily changing the air space depth behind the material, and then a new formulation is derived so that a recently developed method of determination, called the transfer function method, can be applied. An appropriate set of air space depths is also discussed. Glass wool and porous aluminum were used to assess the usefulness of the present method. The normal acoustic impedance and normal absorption coefficient of the test materials with arbitrary thicknesses or with an arbitrary air space depth behind them were calculated from the obtained characteristic impedance and propagation constant and were compared with the measured values that were obtained directly by using the transfer function method. The good agreement achieved suggests that the present method is reliable and effective enough to measure the characteristic impedance and propagation constant over a broadband frequency range.

Analysis of the cylindrical-rectangular patch antenna

Abstract: An analysis is presented of a thin cylindrical-rectangular microstrip patch antenna. After obtaining the electric field under the curved patch and the resonant frequencies using the cavity model, the far-field is found by considering the equivalent magnetic current radiating in the presence of a cylindrical surface. The input impedance and the total Q-factor are then calculated. Numerical and graphical results are presented to illustrate the effect of curvature on the characteristics of the TM/sub 10/ and TM/sub 01/ modes. >

Analysis of microstrip patch antennas using finite difference time domain method

Abstract: The study of microstrip patch antennas is directly treated in the time domain, using a modified finite-difference time-domain (FDTD) method. Assuming an appropriate choice of excitation, the frequency dependence of the relevant parameters can readily be found using the Fourier transform of the transient current. The FDTD method allows a rigorous treatment of one or several dielectric interfaces. Different types of excitation can be taken into consideration (coaxial, microstrip lines, etc.). Plotting the spatial distribution of the current density gives information about the resonance modes. The usual frequency-dependent parameters (input impedance, radiation pattern) are given for several examples. >

Radio Frequency Energy And Impedance Feedback

Abstract: The object of this study was to show the potential use of tissue electrical impedance as a feedback parameter for the control of therapeutic Radio Frequency (R.F.) energy. Radio Frequency energy is a form of electromagnetic energy with an usable frequency range between 200KHZ and 2MHZ. The low frequency limit is determined by the physiological stimulation of muscle, and the high frequency is limited by the physics of the delivery system.

Technique for measuring the frequency‐dependent complex dielectric constants of liquids up to 20 GHz

Abstract: A technique for the measurement of the frequency‐dependent complex dielectric constant e=e’−je‘ of liquids applicable to frequencies up to at least 20 GHz is described. The technique utilizes a coaxial probe dipped into the liquid. The reflection coefficient of the probe was measured using a network analyzer between 45 MHz and 20 GHz. A new de‐imbedding procedure for eliminating the connector and coax mismatches is described, during which the impedance Z(e) of the probe end was determined. The dielectric constant e was determined from Z(e) by modeling the coax–liquid interface as a capacitance. The de‐imbedding procedure, which utilizes three calibrations, directly eliminates the (unknown) fringe‐field impedance. Radiation effects were minimized by using narrow (0.047‐in.) semirigid coax. The technique yields accurate results for e’ and e‘ of liquids such as methanol and water over the entire frequency range up to 20 GHz, and can be used to determine the relaxation spectra of liquid and liquidlike samples.

Analog drive for ultrasonic probe with tunable phase angle

Abstract: The system uses a tunable inductor in series with the piezoelectric crystal excitation transducer in the probe which has a flux modulation coil. The bias current through this flux modulation coil is controlled by the system. It is controlled such that the inductance of the tunable inductor cancels out the capacitive reactance of the load impedance presented by the probe when the probe is being driven by a driving signal which matches the mechanical resonance frequency of the probe. The resulting overall load impedance is substantially purely resistive. The system measures the phase angle and monitors the power level. The system uses this information to adjust the bias current flowing through the flux modulation coil to maintain the substantially purely resistive load impedance for changing power levels. This information is also used to adjust the frequency of the driving signal to track changing mechanical resonance conditions for the probe at different power levels. This method of operation insures substantially maximum power transfer efficiency and substantially linear power control over a range of power dissipation levels. There is also disclosed an analog circuit to measure the phase angle for the load driving signal and to adjust the frequency of the driving signal for best performance. This system includes an integrator to eliminate the effect of offset errors caused by operational amplifiers. There is also disclosed a system to determine the mechanical resonance frequency by sweeping the drive frequency and monitoring the drive current for the frequency at which the drive current is a maximum.

Spectral-domain analysis of E-plane waveguide to microstrip transitions

Abstract: The spectral-domain technique and a residue calculus theorem are used to compute the input impedance of a microstrip transition to a rectangular waveguide. The transition consists of a printed circuit board inserted into a waveguide housing along the E-plane. The effects of the dielectric layer are considered in the present analysis. The behavior of the input impedance of the transition is studied with respect to the critical dimensions of the probe length and backshort location. Calculated results by the new formulation agree well with those computed using an integral equation and those measured at Ka-band frequencies. >

Composition monitor and monitoring process using impedance measurements

Abstract: Radio frequency bridge techniques are used to parameterize the complex dielectric properties of solids, liquids, gasses and mixtures thereof. This parameterization is performed in an electrically isolated, physically open structure (Fig. 12) which allows continuous or batch monitoring of the materials and their mixtures. A method and apparatus are provided for measuring the composition of multicomponent process streams flowing in pipes or ducts. The method uses the pipe in which the mixture flows as a waveguide in which propagating radio frequency electromagnetic energy (Fig. 15) is induced through dielectric loaded apertures (137). The dielectric measurement is performed in an electrically isolated, flow through test section (135) which induces constructive or destructive interference patterns at characteristic frequencies. The characteristic frequency determines the dielectric constant of the mixture. The dielectric properties are used in turn to determined mixture composition. A density measurement is also provided for three component streams such as oil, water, and gas. Temperature and pressure measurements are made to correct for temperature and pressure induced variations in calibrated component impedance and density values.

Dependence of electrical impedance of cement-based materials on their moisture condition

Abstract: This paper outlines an investigation into monitoring the moisture condition of cement-based materials using impedance spectroscopy techniques. Impedance measurements are made over the frequency range 20 Hz-110 MHz, and several parameters identified which could characterise the moisture condition of such materials.

Principles of the impedance technique

Abstract: A simplified treatment of the electrical impedance of biological tissues is presented, examining the sources of impedance changes and the requirements for recording these changes. The author covers small electrodes, simple models, resistivity of blood, the reactive component in bioimpedance, the use of bioadmittance, the use of the finite-element technique and impedance matching. >

Power line communication system

Abstract: A power line communication system which includes an electrical power line having one or more loads which feed objectionable electrical noise back into the power line. The electrical noise is attenuated across a broad frequency range to enable effective communication over the power line by a directional common mode trap located between the point where the communication signals are applied to the power line and the source of the electrical noise. The trap requires only a capacitor for each electrical phase and a 1:1 transformer having a magnetic core and single turn, straight through windings. The trap is connected to provide a low impedance path to ground for the electrical noise, while providing a high impedance to ground for the communication signals.

Ultrasound transducer models for piezoelectric polymer films

Abstract: A method is presented for determining the piezoelectric constants and the frequency-dependent dielectric properties of the polymers from a five-step algorithm based on analysis of air-loaded broadband impedance measurements. It is shown how to account for the frequency-dependent lossy properties of these films in an equivalent impedance circuit model and a modified Mason's model. Comparisons between the models and actual film transducers show excellent broadband simulation of both electrical input impedance and ultrasonic pulse-echo performance. >

A review of image reconstruction techniques for electrical impedance tomography.

Abstract: There has recently been an increasing interest in the possibility of producing images of electrical impedance within the human body. When an electric current is applied to the body of a voltage distribution is developed across the body surface. This distribution is in part dependent on the internal impedance distribution within the body and its is possible to estimate this distribution from a suitable set of voltage measurements. Because of the nonlinear relationship between the impedance distribution and the voltage distribution at the surface of the body, the reconstruction problem is much more difficult than for other tomographic imaging techniques, but a significant amount of progress has been made, and it is now possible to produce tomographic images of i n v i v o distributions of impedance, albeit with low spatial resolution. Future developments should improve image quality.

Electrical power control apparatus and methods

Abstract: Electrosurgery apparatus includes an r.f. power amplifier with an output to a patient electrode. The amplifier is controlled by the voltage of input signals from a control unit which receives a sample of the voltage and current of the output signal. The voltage and current signals are supplied to A/D converters to provide two digital signals that are used to address a look-up table in an EPROM and provide a digital output representative of impedance. A switch is set by the user to the desired mode/power curve and this provides a digital signal which together with the impedance signal is used to address a second look-up table in an EPROM which contains digital representations of the required output voltage of the amplifier to produce the desired power level. An adder compares the required voltage with a feedback of the actual voltage to produce an error signal that is supplied to the amplifier to control its output.

Output impedance considerations for switching regulators with current-injected control

Abstract: The output impedance of current-injection-controlled (CIC) buck regulators can be significantly reduced by deriving the current control signal from the output capacitor instead of the inductor. This technique is shown to be mathematically equivalent to CIC with load-current feedforward. Performance comparisons with normally implemented CIC and voltage control are given. >

A dual-frequency electrical impedance tomography system

Abstract: An electrical impedance tomography (EIT) system has been constructed, operating at two frequencies, 40.96 and 81.92 kHz, for investigating the practicability of the dual-frequency imaging method discussed theoretically in a previous paper (Griffiths and Ahmed, 1987). For testing the system, a phantom with a frequency-dependent electrical conductivity was designed. The properties of the phantom can be adjusted to match the frequency dependence observed in a given type of tissue. Dual-frequency images were obtained from a phantom simulating liver and also from 200 g of porcine liver in a saline tank. Prior to image reconstruction, it was necessary to apply a correction to the data to cancel the effects of stray capacitance within the electronics.

Spectral-domain computation of characteristic impedances and multiport parameters of multiple coupled microstrip lines

Abstract: A numerical procedure based on the spectral-domain techniques is formulated to compute all the frequency-dependent normal-mode parameters of general multiple coupled line structures in an inhomogeneous medium. In addition to the phase and attenuation constants for all the normal modes, these parameters include the line-mode and decoupled line modal impedances and the current and equivalent voltage eigenvector matrices of the coupled system. The multiport admittance (and impedance) matrices and coupled line equivalent-circuit model parameters are evaluated in terms of these normal-mode parameters. Numerical results for these normal-mode parameters for typical asymmetric two-, three-, and four-line microstrip structures are included to demonstrate the procedure and the frequency dependence of these parameters. >

Microwave wafer probe having replaceable probe tip

Abstract: A microwave wafer probe having a replaceable planar transmission line probe tip (14) which detachably connects to a planar transmission line circuit board (26) within the probe head. The circuit board (26) may include passive and/or active electrical circuit components interconnecting its conductors which, due to the detachable interconnection with the probe tip (14), do not have to be replaced if the probe tip should be damaged. The detachable interconnection between the probe tip (14) and the circuit board (26) is tolerant to misalignment of the two elements because the interconnected end portions of the respective conductors are shaped so as to maintain the impedance between the two elements substantially constant despite misalignment. Preferably, both the circuit board (26) and the detachable tip (14) include coplanar transmission lines interconnected by compressing the overlapping end portions of their conductors together.

Two terminal incandescent lamp controller

Abstract: A two-terminal, alternating current power control device connected in an electrical path between a load and a remote switch, comprising a current switching device, a controlling device and a direct current power supply device. The current switching device provides a low impedance electrical path in response to the application of triggering signals thereto, and provides a high impedance electrical path in the absence of the triggering signals. A control circuit is provided for applying triggering signals to the current switching device as the proper time in the AC cycle, and is responsive to a momentary interruption in the applied AC voltage so as to effect change in the power intensity or timed duration of power delivered to the load. A remote switch in the electircal path can be the source of a momentary power interruption. The entire power control device can be constructed to directly attach to the electrical screw shell base of a gas dischage lamp, so the lamp and power control device can be mounted into the normal accommodating electrical fixture without the need for an intermediary fixture or additional electrical wiring.

Capacitive pressure sensor

Abstract: A pressure sensor (10) includes a pair of capacitors (C1, C2), at least one of which is variable responsive to an applied fluid pressure (DELTAP) to predictably change capacitance value. A tuned oscillator circuit (14) having an inductive impedance includes the variable capacitor and simulates a resonant inductive-capacitive circuit not including an inductor. The tuned circuit (14) oscillates to provide a signal with a period which is a function of the variable capacitance value, and of the applied fluid pressure. Additional elements are provided for precise measurement of the variable signal, correction for temperature effects, correction of measurement aberrations, and production of an output signal in a preferred format.

Inhomogeneous Biot waves in layered media

Abstract: A description, using transfer matrices, is given of the propagation of the three inhomogeneous Biot waves in layered porous media. This is applied to predict the surface impedance of porous layered materials at oblique incidence, and an example is presented.

Enhanced line powered amplifier

Abstract: An enhanced amplifier for interconnecting a two-wire, bidirectional cable side transmission path and a two-wire bidirectional equipment side transmission path in a communication system includes a first hybrid splitter and a second hybrid splitter. The first hybrid splitter is formed of a cable-to-equipment voltage sense amplifier, a shunt current driver, and a series current compensator for generating a first simulated terminating impedance. The second splitter is formed of an equipment-to-cable voltage sense amplifier, the shunt current driver and the series current compensator for generating a second simulated terminating impedance. Other features disclosed include automatic gain adjustment circuitry for automatically setting the gain in gain/equalizer circuits to a fixed level. Further, auto-balancing circuitry is provided for adjusting automatically and continuously both equipment and cable side balance networks.

An integral equation method for the evaluation of conductor and dielectric losses in high-frequency interconnects

Abstract: An integral equation method is developed to solve for the complex propagation constant in multilayer planar structures with an arbitrary number of strip conductors on different levels. Both dielectric losses in the substrate layers and conductor losses in the strips and ground plane are considered. The Green's function included in the integral equation is derived by using a generalized impedance boundary formulation. The microstrip ohmic losses are evaluated by using an equivalent frequency-dependent impedance surface which is derived by solving for the fields inside the conductors. This impedance surface replaces the conducting strips and takes into account the thickness and skin effect of the strips at high frequencies. The effects of various parameters such as frequency, thickness of the lines, and substrate surface roughness on the complex propagation constant are investigated. Results are presented for single strips, coupled lines, and two-level interconnects. Good agreement with data available in the literature is shown. >