Showing papers on "Electrical impedance published in 2006"

Impedance Models in Time Domain including the Extended Helmholtz Resonator Model

Abstract: The problem of translating a frequency domain impedance boundary condition to time domain involves the Fourier transform of the impedance function. This requires extending the definition of the impedance not only to all real frequencies but to the whole complex frequency plane. Not any extension, however, is physically possible. The problemshouldremain causal, the variables real, andthe wall passive. This leads to necessary conditions for an impedance function. Various methods of extending the impedance that are available in the literature are discussed. A most promising one is the so-called z-transform by Ozyoruk & Long, which is nothing but an impedance that is functionally dependent on a suitable complex exponent e −iωκ . By choosing κ a multiple of the time step of the numerical algorithm, this approach fits very well with the underlying numerics, because the impedance becomes in time domain a delta-comb function and gives thus an exact relation on the grid points. An impedance function is proposed which is based on the Helmholtz resonator model, called Extended Helmholtz Resonator Model. This has the advantage that relatively easily the mentioned necessary conditions can be satisfied in advance. At a given frequency, the impedance is made exactly equal to a given design value. Rules of thumb are derived to produce an impedance which varies only moderately in frequency near design conditions. An explicit solution is given of a pulse reflecting in time domain at a Helmholtz resonator impedance wall that provides some insight into the reflection problem in time domain and at the same time may act as an analytical test case for numerical implementations, like is presented at this conference by the companion paper AIAA-2006-2569 by N. Chevaugeon, J.-F. Remacle and X. Gallez. The problem of the instability, inherent with the Ingard-Myers limit with mean flow, is discussed. It is argued that this instability is not consistent with the assumptions of the Ingard-Myers limit and may well be suppressed.

Magnetodielectric Substrates in Antenna Miniaturization: Potential and Limitations

Abstract: We discuss patch antenna miniaturization using magnetodielectric substrates. Recent results found in the literature reveal that with passive substrates advantages over conventional dielectric substrates can only be achieved if natural magnetic inclusions are embedded into the substrate. This observation is revised and the physical background is clarified. We present a detailed discussion concerning magnetic materials available in the microwave regime and containing natural magnetic constituents. The effects of magnetic dispersion and loss are studied: constraints on the microwave permeability are used to estimate the effect of magnetic substrates on the achievable impedance bandwidth. Microwave composites filled with thin ferromagnetic films are considered as a prospective antenna substrate. We calculate the impedance bandwidth of a lambda/2-patch antenna loaded with the proposed substrate, and challenge the results against those obtained with conventional dielectric substrates. The results are verified using full-wave simulations, and it is shown that the radiation quality factor is strongly minimized with the proposed substrate even in the presence of realistic losses. Estimates for the radiation efficiency are given as a function of the magnetic loss factor

System and method for terminating treatment in impedance feedback algorithm

Abstract: A system and method for performing electrosurgical procedures are disclosed. The system includes an electrosurgical generator adapted to supply energy at an output level to tissue. The electrosurgical generator includes a microprocessor adapted to generate a desired impedance trajectory having at least one slope. The target impedance trajectory includes one or more target impedance values. The microprocessor is also adapted to drive tissue impedance along the target impedance trajectory by adjusting the output level to substantially match tissue impedance to a corresponding target impedance value. The microprocessor is further adapted to compare tissue impedance to a threshold impedance value and adjust output of the electrosurgical generator when the tissue impedance is equal to or greater than the threshold impedance. The system also includes an electrosurgical instrument including at least one active electrode adapted to apply electrosurgical energy to tissue.

Improved wide-band Schiffman phase shifter

Abstract: In this paper, one improved wide-band Schiffman phase shifter is presented by modifying the ground plane underneath the coupled lines. In this new design, with the ground plane under the coupled lines removed, the even-mode impedance will be increased substantially. Meanwhile, we propose that one additional isolated rectangular conductor is placed under the coupled lines to act as one capacitor so that the odd-mode impedance is decreased. The proposed new design was simulated by the full-wave electromagnetic software IE3D and validated by the measurement. Compared with the cascading microstrip multisection coupled-line configuration, our newly proposed planar one with a patterned ground plane is small in size and, meanwhile, has a good performance. As an example, one Schiffman phase shifter on a double-sided printed circuit board is designed, simulated, fabricated, and measured. The measured amplitude and phase imbalance between the two paths are within 0.5 dB and 5/spl deg/, respectively, over the frequency band from 1.5 to 3.1 GHz, or around 70% bandwidth. The measured return loss is found to be better than -12 dB over the operating frequency band.

On artificial magnetodielectric loading for improving the impedance bandwidth properties of microstrip antennas

Abstract: The effect of artificial magnetodielectric substrates on the impedance bandwidth properties of microstrip antennas is discussed. We review the results found in the literature and then focus on practically realizable artificial magnetic media operating in the microwave regime. Next, a realistic dispersive behavior of a practically realizable artificial substrate is embedded into the model. It is shown that frequency dispersion of the substrate plays a very important role in the impedance bandwidth characteristics of the loaded antenna. The impedance bandwidths of reduced size patch antennas loaded with dispersive magnetodielectric substrates and high-permittivity substrates are compared. It is shown that unlike substrates with dispersion-free permeability, practically realizable artificial substrates with dispersive magnetic permeability are not advantageous in antenna miniaturization. This conclusion is experimentally validated.

Electrode surface treatment and electrochemical impedance spectroscopy study on carbon/carbon supercapacitors

Abstract: Power improvement in supercapacitors is mainly related to lowering the internal impedance The real part of the impedance at a given frequency is called ESR (equivalent series resistance) Several contributions are included in the ESR: the electrolyte resistance (including the separator), the active material resistance (with both ionic and electronic parts) and the active material/current collector interface resistance The first two contributions have been intensively described and studied by many authors The first part of this paper is focused on the use of surface treatments as a way to decrease the active material/current collector impedance Al current collector foils have been treated following a two-step procedure: electrochemical etching and sol-gel coating by a highly-covering, conducting carbonaceous material It aims to increase the Al foil/active material surface contact leading to lower resistance In a second part, carbon-carbon supercapacitor impedance is discussed in term of complex capacitance and complex power from electrochemical impedance spectroscopy data This representation permits extraction of a relaxation time constant that provides important information on supercapacitor behaviour The influence of carbon nanotubes addition on electrochemical performance of carbon/carbon supercapacitors has also been studied by electrochemical impedance spectroscopy

Method of scaling navigation signals to account for impedance drift in tissue

Abstract: A method for scaling the impedance measured during the course of an electrophysiology study accounts for impedance drifts. By scaling the impedance there is greater assurance that previously recorded positional information can be used to accurately relocate an electrode at a prior visited position. The scale factor may be based upon a mean value across several sensing electrodes (e.g., 12, 14, 16, 17, 18, 19, or 22). Alternatively, the scale factor may be calculated specifically with respect to an orientation of a dipole pair of driven electrodes (e.g., 12, 14, 16, 17, 19, 19, 22).

Experimental characterization of collapse-mode CMUT operation

Abstract: This paper reports on the experimental characterization of collapse-mode operation of capacitive micromachined ultrasonic transducers (CMUTs). CMUTs are conventionally operated by applying a direct current (DC) bias voltage less than the collapse voltage of the membrane, so that the membrane is deflected toward the bottom electrode. In the conventional regime, there is no contact between the membrane and the substrate; the maximum alternating current (AC) displacement occurs at the center of the membrane. In collapse-mode operation, the DC bias voltage is first increased beyond the collapse voltage, then reduced without releasing the collapsed membrane. In collapse-mode operation, the center of the membrane is always in contact with the substrate. In the case of a circular membrane, the maximum AC displacement occurs along the ring formed between the center and the edge of the membrane. The experimental characterization presented in this paper includes impedance measurements in air, pulse-echo experiments in immersion, and one-way optical displacement measurements in immersion for both conventional and collapse-mode operations. A 205-mum times 205-mum 2-D CMUT array element composed of circular silicon nitride membranes is used in the experiments. In pulse-echo experiments, a custom integrated circuit (IC) comprising a pulse driver, a transmit/receive switch, a wideband low-noise preamplifier, and a line driver is used. By reducing the parasitic capacitance, the use of a custom IC enables pulse-echo measurements at high frequencies with a very small transducer. By comparing frequency response and efficiency of the transducer in conventional and collapse regimes, experimental results show that a collapsed membrane can be used to generate and detect ultrasound more efficiently than a membrane operated in the conventional mode. Furthermore, the center frequency of the collapsed membrane can be changed by varying the applied DC voltage. In this study, the center frequency of a collapsed transducer in immersion is shown to vary from 20 MHz to 28 MHz with applied DC bias; the same transducer operates at 10 MHz in the conventional mode. In conventional mode, the maximum peak-to-peak pressure is 370 kPa on the transducer surface for a 40-ns, 25-V unipolar pulse excitation. In collapse mode, a 25-ns, 25-V unipolar pulse generates 590 kPa pressure at the surface of the transducer

Quadrature Oscillators with Grounded Capacitors and Resistors Using FDCCIIs

Abstract: Two current-mode and/or voltage-mode quadrature oscillator circuits each using one fully-differential secondgeneration current conveyor (FDCCII), two grounded capacitors, and two (or three) grounded resistors are presented. In the proposed circuits, the current-mode quadrature signals have the advantage of high-output impedance. The oscillation conditions and oscillation frequencies are orthogonally (or independently) controllable. The current-mode and voltage-mode quadrature signals can be simultaneously obtained from the second proposed circuit. The use of only grounded capacitors and resistors makes the proposed circuits ideal for integrated circuit implementation. Simulation results are also included.

Magnetoacoustic tomography with magnetic induction for imaging electrical impedance of biological tissue

Abstract: An experimental feasibility study was conducted on Magnetoacoustic-Tomography with Magnetic Induction (MAT-MI). It is demonstrated that the 2-dimensional (2D) MAT-MI system can detect and image the boundaries between regions of different electrical conductivity with high spatial resolution. Utilizing a magnetic stimulation coil, MAT-MI evokes magnetically induced eddy current in an object which is placed in a static magnetic field. Because of the existence of Lorenz forces, the eddy current in turn causes acoustic vibrations, which are measured around the object in order to reconstruct the electrical impedance distribution of the object. The present experimental results from the saline and gel phantoms are promising and suggest the merits of MAT-MI in imaging electrical impedance of biological tissue with high spatial resolution.

High-frequency ultrasound annular-array imaging. Part I: array design and fabrication

Abstract: This is Part I of a series of two papers describing the development of a digital high-frequency, annular-array, ultrasonic imaging system. In this paper, the design and fabrication of a high-frequency annular array as well as its performance will be reported. A six-element, 50 MHz array, which incorporated an acoustic lens to provide an initial focal point, was designed and fabricated. A submicron size grain lead titanate piezoelectric ceramic was used to both reduce lateral coupling and keep the electrical impedance matched close to the 50 ohm receive electronics. The array elements were isolated using laser micromachining to fully separate the annuli, and electrical interconnection was achieved by directly soldering thin wires to the elements. The resulting array attained an average impulse response that exhibited a 43 MHz center frequency, 30% relative bandwidth, and an average insertion loss of 31 dB at 45 MHz. Maximum next-element crosstalk was -27 dB in water.

Integrated description of electrode/electrolyte interfaces based on equivalent circuits and its verification using impedance measurements.

Abstract: An integrated theory describing both faradaic and nonfaradaic currents obtained upon potential step at an electrified electrode/electrolyte interface has been developed based on equivalent circuits that had been used to explain electrochemical reactions and experimentally verified. The faradaic current is shown to consist of the mass transport-dependent and -independent parts, which is in general agreement with the expression previously derived from the diffusion equations. The decay of the capacitive current is determined by the time constant represented by the product of the resistance obtained from the parallel connection of the solution and polarization resistances and the double layer capacitance; this is not consistent with the current understanding of the capacitive current decay, which takes into account the double layer capacitance and the solution resistance only. Many insights into the electron-transfer reactions are discussed based on the interpretation of impedance representation of the system, which would not have been possible without the present theory.

Analytical equivalent impedance for a planar circular induction heating system

Abstract: A set of analytical expressions is derived for the equivalent impedance in a planar circular induction heating system, used for example in home appliances. The induction system consists of an n-turn planar winding loaded by a conductive material. Expressions that describe the frequency dependence of impedance are provided. The influence of load conductivity, load permeability, and geometrical dimensions is also considered. Validation of the proposed model is carried out through experimental measurements.

Theory of Switched RF Resonators

Abstract: Transient characteristics of switched resonators have been studied. The resonators exhibit controllable impedance and energy storage behaviors during an alternating charging and discharging process. It is discovered that these unique characteristics of switched resonators can lead to novel RF envelope signal processing functions. The paper discusses the basic theory of switched resonators and the designing methodology of relevant RF signal processing applications. Two applications are presented as examples. The first example is a variable, reflective attenuator which uses the controllable impedance of the switched resonators to achieve different attenuation/reflection levels. In the second example, a simple, compact and efficient RF pulse compression technique based on the switched resonator concept is introduced. It utilizes the charging/discharging characteristics of the circuit resonator by varying the resonator's Q-factor. The approach compresses the duration of the modulated pulse without changing the phase characteristics of the RF carrier. The pulse compression technique can also be implemented with other type of resonators including distributed resonators, as demonstrated through the experiments

System and method of operating double fed induction generators

Abstract: A protection system and a protection method are provided for protecting a double fed induction generator (16) and a gearbox (24) during a grid fault. The protection system includes a plurality of controlled impedance devices (44). Each of the controlled impedance devices is coupled between a respective phase of a stator winding of the double fed induction generator and a respective phase of a grid side converter (26). The protection system also includes a controller (32) configured for coupling and decoupling impedance in one or more of the plurality of controlled impedance devices in response to changes in at least one of a utility grid voltage and a utility grid current.

Investigation into the performance of proximity coupled stacked patches

Abstract: We investigate the parameters that control the impedance and radiation performance of proximity coupled stacked microstrip patch radiators. In particular we explore the relationship required between the dielectric layers to achieve broadband behavior and also how the dimensions of the stacked radiators and the relative location of the feed can influence the impedance response. Bandwidths in excess of 20% can be achieved with careful layer design. We also investigate the dielectric layer configurations required to achieve broadband impedance responses when higher dielectric constant feed material is used. This latter study is of particular importance when designing MMIC compatible printed antennas.

Development of a high-frequency (> 50 MHz) copolymer annular-array, ultrasound transducer

Abstract: The development of a high frequency (> 50 MHz) annular array ultrasonic transducer is presented. The array was constructed by bonding a 9 mum P(VDF-TrFE) film to a two-sided polyimide flexible circuit with annuli electrodes on the top layer. Each annulus was separated by a 30 mum kerf and had several electroplated microvias that connected to electrode traces on the bottom side of the flex circuit. In order to improve device sensitivity, each element was electrically matched to an impedance magnitude of 50 Omega and 0deg phase at resonance using a serial inductor and high impedance coaxial cable. The array's performance was evaluated by measuring the electrical impedance, pulse echo response, and cross talk between elements. The average round trip insertion loss was -33.5 dB after compensating for diffractive and attenuative losses. The measured average center frequency and bandwidth for an element was 55 MHz and 47%, respectively. The measured cross talk between adjacent elements remained below -29 dB at the center frequency in water. A vertical wire phantom was imaged using a single focus transmit beamformer and dynamic focusing receive beamformer. This image showed a significant improvement in lateral resolution over a range of 9 mm after the dynamic focusing receive algorithm was applied. These results correlated well with predictions from a field II simulation. After beamforming, the minimum lateral resolution achieved by the array (-6 dB) was 108 mum at the focus

A constant phase element sensor for monitoring microbial growth

Abstract: The construction and performance of a novel constant phase element (CPE) sensor for monitoring microbial growth has been reported in this paper. The signal conditioning circuit measures the changes in the phase angle of the impedance characteristics of the probe. The major advantages of the measurement system developed are the simplicity of the construction of the probe and ease in measurement. It is proposed that the sensor can be effectively used for monitoring bacterial and yeast growth.

Compact Wideband UHF Patch Antenna on a Reactive Impedance Substrate

Abstract: The goal of this letter is to design a compact, planar UHF antenna operating over the frequency range 420?450 MHz. A microstrip patch antenna is selected as the elementary radiating structure because it has a low profile and is simple to fabricate. Compact size and enhanced bandwidth are simultaneously achieved through the use of a reactive impedance surface (RIS) in place of a PEC ground plane. The inherently limited bandwidth of the patch is overcome by incorporating a U-shape resonant slot radiator within the patch area. The design, fabrication, and measurement results are presented. It is shown that a patch with a U-shape slot whose dimensions are only $0.136~lambda times 0.228~lambda$ at the lowest frequency of operation can provide more than 20% bandwidth, covering a frequency range of 410?485 MHz.