Showing papers on "Output impedance published in 2013"

Measurement techniques for piezoelectric nanogenerators

Abstract: Electromechanical energy harvesting converts mechanical energy from the environment, such as vibration or human activity, into electrical energy that can be used to power a low power electronic device. Nanostructured piezoelectric energy harvesting devices, often termed nanogenerators, have rapidly increased in measured output over recent years. With these improvements nanogenerators have the potential to compete with more traditional micro- or macroscopic energy harvesting devices based on piezoelectric ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) or electrostatic, electret or electromagnetic kinetic energy harvesters. Power output from a nanogenerator is most commonly measured through open-circuit voltage and/or short-circuit current, where power may be estimated from the product of these values. Here we show that such measures do not provide a complete picture of the output of these devices, and can be misleading when attempting to compare alternative designs. In order to compare the power output from a nanogenerator, techniques must be improved in line with those used for more established technologies. We compare ZnO nanorod/poly(methyl methacrylate) (PMMA) and ZnO nanorod/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) devices, and show that despite an open-circuit voltage nearly three times lower the ZnO/PEDOT:PSS device generates 150 times more power on an optimum load. In addition, it is shown that the peak voltage and current output can be increased by straining the device more rapidly and therefore time-averaged power, or time-integrated measures of output such as total energy or total charge should be calculated. Finally, the internal impedance of the devices is characterised to develop an understanding of their behaviour and shows a much higher internal resistance but lower capacitive impedance for the ZnO/PMMA device. It is hoped that by following more rigorous testing procedures the performance of nanostructured piezoelectric devices can be compared more realistically to other energy harvesting technologies and improvements can be rapidly driven by a more complete understanding of their behaviour.

Adaptive Active Capacitor Converter for Improving Stability of Cascaded DC Power Supply System

Abstract: Connecting converters in cascade is a basic configuration of dc distributed power systems (DPS). The impedance interaction between individually designed converters may make the cascaded system unstable. The previous presented approaches of stabilizing the cascaded systems need to modify the source and/or load converter's internal structure such as the topology and control circuit that are contradictory to the modularization characteristic of dc DPS. In this paper, an adaptive active capacitor converter (AACC) is introduced to stabilize the cascaded system. The AACC is connected in parallel with the cascaded system's intermediate bus and only needs to detect the bus voltage without any change of the existing subsystems. Hence, it can be designed as a standard module for dc DPS. The AACC serves as an equivalent bus capacitor to reduce the output impedance of the source converter, thus avoiding the intersection with the load converter's input impedance, and as a result, the cascaded system becomes stable. The equivalent bus capacitor emulated by the AACC is adaptive according to the output power of the cascaded system, and thus, the power loss of AACC is minimized and the dynamic response of the system is better than that of the system using a passive capacitor. Furthermore, since no electrolytic capacitor is needed in the AACC, the cascaded system's lifetime is prolonged. The operation principle, control, and design consideration of the AACC are discussed in this paper, and a 480 W cascaded system comprising two phase-shifted full-bridge converters has been built and evaluated. The experimental results verify the validity of the proposed AACC.

Influence of phase-locked loop on input admittance of three-phase voltage-source converters

Abstract: A regulated power converter can cause instability issues at its terminals with its input filter or the source. The Generalized Nyquist stability Criterion (GNC), when applied to the product of the source impedance and load admittance in the dq frame, can be used to analyze the stability of balanced three-phase ac systems. This paper presents the influence of the Phase-Locked Loop (PLL) on the input admittance of three-phase Voltage-Source Converters (VSC) for open loop, current-feedback and voltage feedback control conditions. Analytical results show that the PLL introduces a parallel admittance to the open loop admittance of the VSC, and that the current and voltage control loop have additional effects on it. Both simulation and experimental results are presented to verify this analysis. The possible instability due to different PLL design is also shown.

A Current Decoupling Parallel Control Strategy of Single-Phase Inverter With Voltage and Current Dual Closed-Loop Feedback

Abstract: The output characteristics of a single-phase inverter with voltage and current dual closed-loop feedback control are analyzed, and the equivalent circuit model of a parallel single-phase inverter system is introduced. By taking both resistance and inductance components of the equivalent output impedance into consideration, a current decoupling control strategy of the parallel inverter system is then proposed. Furthermore, by constructing a three-phase balanced current according to the output current of the single-phase inverter, an active and reactive current decomposition method is presented to decompose the output current of the single-phase inverter into active and reactive currents according to the instantaneous reactive power theory. The block diagram of the active and reactive current decomposition method is given, and current decomposition simulation results are shown to verify the analysis results. The prototype is developed, and the control of two parallel-connected 2-kVA inverters is realized. The experimental results show its feasibility and effectiveness.

A High-Efficiency Solar Array Simulator Implemented by an LLC Resonant DC–DC Converter

Abstract: In this paper, a high-efficiency solar array simulator (SAS) implemented by an LLC resonant dc-dc converter is proposed to save the cost and energy of photovoltaic (PV) system testing. The proposed converter has zero-voltage switching (ZVS) operation of the primary switches and zero-current switczero-current switching (ZCS) operation of the rectifier diodes. By frequency modulation control, the outputhing (ZCS) operation of the rectifier diodes. By frequency modulation control, the output impedance of an LLC resonant converter can be regulated from zero to infinite without shunt or serial resistors. Therefore, the efficiency of the proposed SAS can be significantly increased. The circuit operations are analyzed in detail to derive the theoretical equations. Circuit parameters are designed based on the practical considerations. Finally, an illustrative example is implemented to demonstrate the feasibility of the proposed SAS.

Biocompatible, High Precision, Wideband, Improved Howland Current Source With Lead-Lag Compensation

Abstract: The Howland current pump is a popular bioelectrical circuit, useful for delivering precise electrical currents. In applications requiring high precision delivery of alternating current to biological loads, the output impedance of the Howland is a critical figure of merit that limits the precision of the delivered current when the load changes. We explain the minimum operational amplifier requirements to meet a target precision over a wide bandwidth. We also discuss effective compensation strategies for achieving stability without sacrificing high frequency output impedance. A current source suitable for Electrical Impedance Tomography (EIT) was simulated using a SPICE model, and built to verify stable operation. This current source design had stable output impedance of 3.3 MΩ up to 200 kHz, which provides 80 dB precision for our EIT application. We conclude by noting the difficulty in measuring the output impedance, and advise verifying the plausibility of measurements against theoretical limitations.

Resistance Selection of High Impedance Surface Absorbers for Perfect and Broadband Absorption

Abstract: High impedance surface absorbers comprising of lossless dielectric substrate and resistive pattern were studied based on equivalent circuit method Resonant conditions in terms of potential resonant frequencies and corresponding equivalent resistance were deduced Theoretical calculation, numerical simulation and experiment showed that expected absorption was obtained by matching the capacitance, inductance and the resistance We demonstrated that the optimal resistance of frequency-dispersive was between 0 and 377 Ω For square, dipole and square ring pattern, a mean resistance value was required to achieve broadband absorption It offered a way to create broadband and perfect absorbers by controlling the surface resistance of sub-wavelength structures

Three-phase AC system impedance measurement unit (IMU) using chirp signal injection

Abstract: Impedance defined in synchronous coordinates is a useful tool to predict and evaluate stability of three-phase AC systems. For online impedance measurement in AC systems, the widely used frequency sweep method takes a long time and may not be practical in systems where the operating point cannot be maintained for a long time. A wide bandwidth signal could be used to significantly reduce the measurement time. This paper suggests a chirp signal and describes its advantages over other signals. Experimental results demonstrate the effectiveness of this approach.

Modeling, Analysis, and Suppression of the Impact of Full-Scale Wind-Power Converters on Subsynchronous Damping

Abstract: This paper presents modeling, analysis, and suppression of the impact of a voltage-source converter (VSC)-based full-scale wind turbine (FSWT) on subsynchronous damping in power systems. First, a detailed output impedance model of the grid-interfacing VSC is presented and analyzed under different converter operating modes, control functions, and variations in operating point. Second, the output impedance model is used to analyze the impact of the grid-interfacing VSC on the electrical damping characteristics in the subsynchronous frequency range and interactions with an electrically nearby synchronous generator. The IEEE first benchmark for subsynchronous resonance studies is used for theoretical analysis and simulation results. It is found that the grid-interfacing VSC of the FSWT can have a negative impact on overall electrical damping and there is a risk of subsynchronous torsional oscillation especially at lower frequency modes. Third, to mitigate such interactions, an active damping controller is developed to reshape the output impedance of the interfacing VSC and yield positive electrical damping at torsional modes. Time-domain simulation results are presented to validate the theoretical analysis and show the effectiveness of the proposed active damping controller.

Scalable periphery tunable matching power amplifier

Abstract: A scalable periphery tunable matching power amplifier is presented. Varying power levels can be accommodated by selectively activating or deactivating unit cells of which the scalable periphery tunable matching power amplifier is comprised. Tunable matching allows individual unit cells to see a constant output impedance, reducing need for transforming a low impedance up to a system impedance and attendant power loss. The scalable periphery tunable matching power amplifier can also be tuned for different operating conditions such as different frequencies of operation or different modes.

An Islanding Detection Method Based on Dual-Frequency Harmonic Current Injection Under Grid Impedance Unbalanced Condition

Abstract: This paper proposes a three-phase grid impedance detection method based on dual-frequency harmonic current injection for islanding detection. Grid impedance detection based on single harmonic current injection is reliable in three-phase impedance balanced grid. However, under impedance unbalanced condition, the harmonic voltage caused by the injected symmetric harmonic current is asymmetric, which affects the calculation of grid impedances and even leads to failed detection. The method based on dual-frequency harmonic current injection is injecting two non-characteristic symmetric harmonic currents, and then according to the different harmonic voltages caused by different frequency harmonic currents, all three-phase impedances can be calculated accurately. The implementing algorithm of the presented method is derived and its performance is analyzed in detail. Simulation and experiments are carried out under grid impedance balanced and unbalanced conditions. Theoretical analysis and experiment results proved that the proposed method is feasible.

Multi-Port Driven Radiators

Abstract: Integrated multi-port driven (MPD) radiator design is presented as an approach that takes advantage of the increased design space offered by using a hybrid design of an antenna with multiple ports and its driver circuitry integrated together on a single substrate. This reduces costly losses by eliminating independent elements for power combination, output impedance matching networks, and power transfer by engineering current patterns on a chip based on the desired far field pattern. The electromagnetic radiation produced by a circularly polarized MPD antenna is calculated analytically to provide design intuition, with supporting electromagnetic simulations. A single element 160 GHz MPD antenna and the supporting driver circuitry is designed and fabricated in a 0.13 μm SiGe BiCMOS process. A tuned 8 phase ring oscillator generates the signal with each phase feeding class A power amplifiers that drive the antenna. The radiator achieves 4.6 dBm single element effective isotropically radiated power (EIRP) and total radiated power of -2.0 dBm at 161 GHz while consuming 117.5 mA DC current from a 3.3 V source. Measurements of three frequency bands at 145, 154 and 161 GHz show greater than 0 dBm EIRP for each band, demonstrating the wide band nature of the antenna.

Impedance matching circuit with tunable notch filters for power amplifier

Abstract: An impedance matching circuit with at least one tunable notch filter for a power amplifier is disclosed The power amplifier amplifies an input radio frequency (RF) signal and provides an amplified RF signal The impedance matching circuit performs output impedance matching for the power amplifier and includes at least one tunable notch filter Each tunable notch filter has a notch that can be varied in frequency to provide better attenuation of an undesired signal The at least one tunable notch filter attenuates at least one undesired signal in the amplified RF signal The at least one tunable notch filter may include (i) a first tunable notch filter to attenuate a first undesired signal at a second harmonic of the amplified RF signal and/or (ii) a second tunable notch filter to attenuate a second undesired signal at a third harmonic of the amplified RF signal

Modeling the grid synchronization induced negative-resistor-like behavior in the output impedance of a three-phase photovoltaic inverter

Abstract: Photovoltaic power has to be converted from DC into AC in grid-connected applications. The conversion is done by using a single or three-phase inverter. Phase angle and frequency of the injected current and the grid voltage have to match to achieve unity power factor. This has been commonly accomplished by using a phase-locked-loop (PLL). The PLL has a tendency to make the output impedance of the inverter to appear as a negative resistor which can introduce harmonics in the grid current or even make the inverter-grid interface unstable. This paper presents a general small-signal model of a PV inverter in the synchronous reference frame which includes the PLL. Due to the negative resistance, the inverter can become unstable when the grid has high inductance. The derived small-signal model can be used to predict the exact conditions where the instability will take place by utilizing the impedance ratio and Nyquist stability criterion.

An Efficient, Watt-Level Microwave Rectifier Using an Impedance Compression Network (ICN) With Applications in Outphasing Energy Recovery Systems

Abstract: We present a transmission line based impedance compression network (ICN) for application in RF-to-dc conversion. We show that the ICN is able to significantly compress the undesired input impedance variation that occurs when there is a large variation of input power. We designed and measured an ICN for a 4 W, 4.6 GHz rectifier and show that the impedance is significantly compressed with the ICN. We then demonstrate the ICN in an outphasing system where we achieve up to a 37% improvement in efficiency at back-off power and an overall more efficient design over large input power variation.

Power Amplifier Integrated With Bandpass Filter for Long Term Evolution Application

Abstract: This letter presents a high-power amplifier (PA) matched by bandpass filter (BPF) for long term evolution (LTE) applications. The phase shift of the BPF is found to retain ±90° at the working frequency. It can be employed as the output impedance transformer which transforms 50 Ω to the desired maximum power point. By using this method, the high-PA integrated with the BPF avoids extreme wide line and obtain three functions which could boost the radio signal, select the working frequency band and eliminate the harmonics at the same time. The system dimensions are consequently reduced as the following BPF after PA is omitted. A high-PA matched by BPF working at 2.6 GHz is designed. The analysis is given and the measured results and comparisons are presented. The good agreement confirms the validity of the proposed method.

Electric field coupling technique of wireless power transfer for electric vehicles

Abstract: An electric-field coupling wireless-power-transfer technique is proposed based on the concept of impedance matching. In the proposed electric-field coupling technique, the impedance matching can be easily achieved by controlling the matching inductance. Also, the shielding of the electric field is easy. A metallic plate implemented at the bottom surface of a vehicle is proposed to maximize the coupling capacitance. A simplified experimental setup has been developed, and the maximum power transfer efficiency of about 78% has been measured.

Design and implementation of three-phase AC impedance measurement unit (IMU) with series and shunt injection

Abstract: Electric power systems based on power electronics technology are prone to instability due to the constant power nature of power electronics converters. The impedances defined in synchronous coordinates are useful tools to predict and evaluate three-phase small-signal system stability. This paper presents the design and implementation of an impedance measurement unit used in three-phase systems up to 100kW. It extracts the system impedance online at any specific operating point. The unit utilizes both series and shunt injection modes to perform impedance identification for any load and source conditions, even if the source is very stiff. Experimental results demonstrate the effectiveness of the designed unit with both linear and nonlinear loads.

Rfid chip package and rfid tag

Abstract: An RFID chip package includes an RFID chip including a voltage booster circuit and processing an RF signal in a UHF band and a power supply circuit connected to the RFID chip and including at least one inductance element. A reactance component of an input/output impedance at an antenna-connecting input/output terminal of the power supply circuit is substantially 0Ω.

Internal and external force-based impedance control for cooperative manipulation

Abstract: An asymptotically stable cascaded control algorithm is proposed for cooperative manipulation of a common object. This algorithm controls motion and internal forces of the object, as well as the contact forces between the object and environment. The motion of each manipulator is controlled using an inverse dynamics type of controller. Only knowledge of the kinematics of the manipulated object is required, since the interaction forces and moments between the object and manipulators are measured. The internal stresses in the object are controlled based on enforced impedance relationships between the object and each manipulator. The internal forces and moments are computed using the object kinematics. Contact with the environment is controlled with an enforced impedance relationship between the object and the environment. For both internal and external forces, reference trajectories can be specified. Asymptotic stability of each controller is proven using Lyapunov stability theory and LaSalle's invariance principle. Guidelines are suggested to compute control parameters of the internal impedance parameters. Merits of the control algorithm are demonstrated in simulations.

A New Method for Matching Network Adaptive Control

Abstract: This paper presents a novel direct calculation method for matching network adaptive control utilizing an analytic algorithm based on an integrated measurement of load impedance. In principle, this approach has no limitations on the match tuned load impedance and the operating frequency, but in practice its operation is restricted by the matching network tolerances and the accuracy and dynamic range of the load impedance measurement circuitry. The matching network consisting of microelectromechanical systems tunable capacitors and high-Q fixed inductors is one of the best choices for this loop since it has a broad match tuning range across loads condition and frequencies.

Analysis and Active Suppression of AC- and DC-Side Instabilities in Grid-Connected Current-Source Converter-Based Photovoltaic System

Abstract: Current source converters (CSCs) can be a viable option to interface large photovoltaic (PV) systems to the utility grid. However, interaction dynamics in both the ac- and dc-sides might be yielded and affect the converter stability. 1) On the ac side, interactions of the grid impedance with the LC ac-side filter might cause uncertain resonant frequency modes that should be damped without affecting the converter efficiency. 2) On the dc-side, under uncertain characteristics of the PV source impedance (e.g., number of PV modules connected and/or uncertainty in source circuit parameters), the Nyquist stability criterion might be violated due to equivalent source/load impedance mismatch. Moreover, the real part of the dc impedance of the CSC can be negative which contributes to instabilities of the PV system. In this paper, the ac-side LC filter dynamics are robustly damped by an improved active compensator in the control structure of the CSC. More importantly, the dc-side interactions dynamics are effectively stabilized by proposing active reshaping techniques for the dc impedance of the CSC so that the Nyquist stability criterion is maintained and positive damping is added. Time-domain model is implemented under Matlab/Simulink environment to validate the analytical results.

High Accurate Howland Current Source: Output Constraints Analysis

Abstract: Howland circuits have been widely used as powerful source for exciting tissue over a wide frequency range. When a Howland source is designed, the components are chosen so that the designed source has the desired characteristics. However, the operational amplifier limitations and resistor tolerances cause undesired behaviors. This work proposes to take into account the influence of the random distribution of the resistors in the modified Howland circuit over the frequency range of 10 Hz to 10 MHz. Both output current and impedance of the circuit are deduced either considering or the operational amplifiers parameters. The probability density function due to small changes in the resistors of the circuit was calculated by using the analytical modeling. Results showed that both output current and impedance are very sensitive to the resistors variations. In order to get higher output impedances, high operational amplifier gains are required. The operational amplifier open-loop gain increases as increasing the sensitivity of the output impedance. The analysis done in this work can be used as a powerful co-adjuvant tool when projecting this type of circuit in Spice simulators. This might improve the implementations of practical current sources used in electrical bioimpedance.

Resonance analysis in parallel voltage-controlled Distributed Generation inverters

Abstract: Thanks to the fast responses of the inner voltage and current control loops, the dynamic behaviors of parallel voltage-controlled Distributed Generation (DG) inverters not only relies on the stability of load sharing among them, but subjects to the interactions between the voltage control loops of the inverters and the remaining system dynamics. This paper addresses the later interactions and the consequent resonances through the frequency-domain analysis of the inverters output impedances and the remaining equivalent network impedance. Furthermore, impacts of the virtual output impedance loop and the voltage feedforward loop in the current controller are evaluated based on such an impedance interactions analysis. Simulation results are presented to confirm the validity of the theoretical analysis.

Low-Field Behavior of Source-Gated Transistors

Abstract: A physical description of low-field behavior of a Schottky source-gated transistor (SGT) is outlined where carriers crossing the source barrier by thermionic emission are restricted by JFET action in the pinch-off region at the drain end of the source. This mode of operation leads to transistor characteristics with low saturation voltage and high output impedance without the need for field relief at the edge of the Schottky source barrier and explains many characteristics of SGT observed experimentally. 2-D device simulations with and without barrier lowering due to the Schottky effect show that the transistors can be designed so that the current is independent of source length and thickness variations in the semiconductor. This feature together with the fact that the current in an SGT is independent of source-drain separation hypothesizes the fabrication of uniform current sources and other large-area analog circuit blocks with repeatable performance even in imprecise technologies such as high-speed printing.

Multi-Frequency Impedance Transformers for Frequency-Dependent Complex Loads

Abstract: In this paper, a general synthesis method is proposed for the design of multi-frequency impedance transformers (MFITs) for arbitrary frequency-dependent complex loads (FDCLs) by adopting the concept of multi-frequency inverters (MFIs). An MFI, which is placed between two susceptance blocks, is constructed with a transmission line and two-side multi-frequency susceptances (MFSs), whose values at multiple frequencies are independently specified. By merging neighboring susceptances, we get a very simple Pi-shaped topology of MFITs, which in theory has no limitation on the number of matching frequencies. The MFS blocks are realized with one or more parallel shunt stubs, providing needed susceptance values at several specified frequencies. A genetic algorithm is used in extracting the circuit parameters of the parallel stubs. Several dual-, triple-, and quad-frequency impedance transformers for FDCLs are designed for illustrating the design methods. Experiment and simulation results are compared with good agreement, validating the feasibility of the theory. The designed impedance transformers are concise in circuit and compact in dimensions.

High-Performance Bridge Modular Switched-Capacitor Converter With Small Component Requirement Based on Output Impedance Analysis for Low Loss

Abstract: In this paper, a novel multilevel bridge modular switched-capacitor converter is proposed for the application in high voltage gain. Compared with the conventional switched-capacitor converters, it utilizes fewer components (switching device and capacitor) to obtain the same voltage conversion, as well as some advantages including low total power device rating, output capacitor requirement, and low output voltage ripple. In order to illustrate its superiority, an output impedance analysis method is developed, which can comprehensively evaluate a two-phase switched-capacitor converter's steady-state performance by judging its output impedance. The simple formulation developed not only permits optimization of the capacitor sizes to meet several constraints such as a total capacitance or total energy storage limit, but also permits optimization of the switch sizes subject to constraints on total switch conductance or total switch volt-ampere products. Consequently, the output impedance under the optimizations has contributed to the comparison of the latest switched-capacitor converters. As a result, the performance (based on conduction loss) of the proposed converter is proved to be optimum in fast-switching-limit impedance and does well in slow-switching-limit output impedance as well. Simulation and experimental results have validated the principle and properties of this converter and the analysis method.

Impedance matching considering cross coupling for wireless power transfer to multiple receivers

Abstract: In this paper, a method of impedance matching for wireless power transfer (WPT) to multiple receivers is presented considering cross coupling effects between non-adjacent coils. For maximizing power transfer efficiency, design of a source coil and optimal load impedance are achieved based on circuit analysis and mutual inductance calculation. For verification, a WPT system of a transmitter and three receivers is fabricated. The fabricated WPT system can obtain reflection coefficient of under -11 dB for the maximum three receivers at once by using a circular single loop coil without capacitance compensation or additional impedance matching circuit. The simulated results show that the WPT system using the proposed method has more than 85% efficiency regardless of the number of receivers while efficiency for the conventional method is severely changed according to the number of the receivers. Measured results of the proposed method are consistent with calculation results.

Modeling and Analysis of the Fractional Order Buck Converter in DCM Operation by using Fractional Calculus and the Circuit-Averaging Technique

Abstract: By using fractional calculus and the circuit-averaging technique, the modeling and analysis of a Buck converter with fractional order inductor and fractional order capacitor in discontinuous conduction mode (DCM) operations is investigated in this study. The equivalent averaged circuit model of the fractional order Buck converter in DCM operations is established. DC analysis is conducted by using the derived DC equivalent circuit model. The transfer functions from the input voltage to the output voltage, the duty cycle to the output voltage, the input impedance, and the output impedance of the fractional order Buck converter in DCM operations are derived from the corresponding AC-equivalent circuit model. Results show that the DC equilibrium point, voltage ratio, and all derived transfer functions of the fractional order Buck converter in DCM operations are affected by the inductor order and/or capacitor order. The fractional order inductor and fractional order capacitor are designed, and PSIM simulations are performed to confirm the correctness of the derivations and theoretical analysis.

Four-electrode impedance spectrometer for investigation of solid ion conductors

Abstract: An impedance spectrometer capable of accurately measuring solid ion conducting sample impedance spectra by two- or four-electrode methods in either time or frequency domain has been built. The four-electrode measurement mode is implemented by constructing a differential amplifier with a very high input impedance and common mode rejection ratio over a wide frequency range. All of the measurements can be performed in frequencies ranging from 10 Hz to 2 MHz and sample temperatures up to 800 K. The working principle of the spectrometer as well as its technical parameters and accuracy estimation are presented in this paper. The advantage of four-electrode over two-electrode measurement mode is shown by an example of Ce0.9Gd0.1O1.95 solid electrolyte ceramic impedance measurements.