Showing papers on "Frequency response published in 2004"

Recycling ambient microwave energy with broad-band rectenna arrays

Abstract: This paper presents a study of reception and rectification of broad-band statistically time-varying low-power-density microwave radiation. The applications are in wireless powering of industrial sensors and recycling of ambient RF energy. A 64-element dual-circularly-polarized spiral rectenna array is designed and characterized over a frequency range of 2-18 GHz with single-tone and multitone incident waves. The integrated design of the antenna and rectifier, using a combination of full-wave electromagnetic field analysis and harmonic balance nonlinear circuit analysis, eliminates matching and filtering circuits, allowing for a compact element design. The rectified dc power and efficiency is characterized as a function of dc load and dc circuit topology, RF frequency, polarization, and incidence angle for power densities between 10/sup -5/-10/sup -1/ mW/cm/sup 2/. In addition, the increase in rectenna efficiency for multitone input waves is presented.

The PolyMAX Frequency-Domain Method: A New Standard for Modal Parameter Estimation?

Abstract: Recently, a new non-iterative frequency-domain parameter estimation method was proposed. It is based on a (weighted) least-squares approach and uses multiple-input-multiple-output frequency response functions as primary data. This so-called "PolyMAX" or polyreference least-squares complex frequency-domain method can be implemented in a very similar way as the industry standard polyreference (time-domain) least-squares complex exponential method: in a first step a stabilisation diagram is constructed containing frequency, damping and participation information. Next, the mode shapes are found in a second least- squares step, based on the user selection of stable poles. One of the specific advantages of the technique lies in the very stable identification of the system poles and participation factors as a function of the specified system order, leading to easy-to-interpret stabilisation diagrams. This implies a potential for automating the method and to apply it to "difficult" estimation cases such as high-order and/or highly damped systems with large modal overlap. Some real-life automotive and aerospace case studies are discussed. PolyMAX is compared with classical methods concerning stability, accuracy of the estimated modal parameters and quality of the frequency response function synthesis.

Comparison of the response of doubly fed and fixed-speed induction generator wind turbines to changes in network frequency

Abstract: Synchronous and fixed-speed induction generators release the kinetic energy of their rotating mass when the power system frequency is reduced. In the case of doubly fed induction generator (DFIG)-based wind turbines, their control system operates to apply a restraining torque to the rotor according to a predetermined curve with respect to the rotor speed. This control system is not based on the power system frequency and there is negligible contribution to the inertia of the power system. A DFIG control system was modified to introduce inertia response to the DFIG wind turbine. Simulations were used to show that with the proposed control system, the DFIG wind turbine can supply considerably greater kinetic energy than a fixed-speed wind turbine.

Measurement and modeling of an ultra-wide bandwidth indoor channel

Abstract: This paper describes the results of frequency-domain channel sounding in residential environments. It consists of detailed characterization of complex frequency responses of ultra-wideband (UWB) signals having a nominal center frequency of 5 GHz. A path loss model as well as a second-order autoregressive model is proposed for frequency response generation of the UWB indoor channel. Probability distributions of the model parameters for different locations are presented. Also, time-domain results such as root mean square delay spread and percent of captured power are presented.

Bandwidth enhancement for transimpedance amplifiers

Abstract: A technique for bandwidth enhancement of a given amplifier is presented. Adding several interstage passive matching networks enables the control of transfer function and frequency response behavior. Parasitic capacitances of cascaded gain stages are isolated from each other and absorbed into passive networks. A simplified design procedure, using well-known low-pass filter component values, is introduced. To demonstrate the feasibility of the method, a CMOS transimpedance amplifier (TIA) is implemented in a 0.18-/spl mu/m BiCMOS technology. It achieves 3 dB bandwidth of 9.2 GHz in the presence of a 0.5-pF photodiode capacitance. This corresponds to a bandwidth enhancement ratio of 2.4 over the amplifier without the additional passive networks. The transresistance gain is 54 dB/spl Omega/, while drawing 55 mA from a 2.5-V supply. The input sensitivity of the TIA is -18 dBm for a bit error rate of 10/sup -12/.

Source-independent full waveform inversion of seismic data

Abstract: A set of seismic trace data is collected in an input data set that is first Fourier transformed in its entirety into the frequency domain. A normalized wavefield is obtained for each trace of the input data set in the frequency domain. Normalization is done with respect to the frequency response of a reference trace selected from the set of seismic trace data. The normalized wavefield is source independent, complex, and dimensionless. The normalized wavefield is shown to be uniquely defined as the normalized impulse response, provided that a certain condition is met for the source. This property allows construction of the inversion algorithm disclosed herein, without any source or source coupling information. The algorithm minimizes the error between data normalized wavefield and the model normalized wavefield. The methodology is applicable to any 3-D seismic problem, and damping may be easily included in the process.

Amplitude and phase calibration of hydrophones up to 70 MHz using broadband pulse excitation and an optical reference hydrophone

Abstract: A substitution calibration technique for piezoelectric ultrasonic hydrophones is presented that uses an optical multilayer hydrophone as the reference receiver. Broadband nonlinearly distorted focused pulses are first measured with the reference hydrophone and then with the hydrophone to be calibrated. By Fourier transformation of the time wave forms and division of the frequency spectra, the complex-valued frequency response of the hydrophone under test is obtained in a broad frequency range in a very fast and efficient way and with high frequency resolution. The results obtained for a membrane hydrophone and a needle-type hydrophone are compared with those obtained by independent calibration techniques such as primary calibration using optical interferometry and secondary calibration using time-delay spectrometry, and good agreement is found. The calibration data obtained are apt to improve the results of ultrasound exposure measurements using broadband voltage-to-pressure conversion. This is demonstrated for standard pulse parameter determination from exemplar exposure measurements on a commercial diagnostic ultrasound machine. For the membrane hydrophone, the evaluation method commonly used leads to an overestimation of the positive peak pressure by up to 50%, an underestimation of the rarefactional peak pressure by up to 11%, and an overestimation of the pulse intensity integral by up to 28%.

Dynamic frequency control with increasing wind generation

Abstract: Frequency control is essential for the secure and stable operation of a power system. With wind penetration increasing rapidly in many power systems, ensuring continuous power system security is vital. The frequency response to a disturbance on the all Ireland system is simulated for a range of installed wind capacities under different system conditions. The purpose of this study is to assess the effects of increased wind generation on system frequency, and the security of the system following such disturbances.

Closed-form design and efficient implementation of variable digital filters with simultaneously tunable magnitude and fractional delay

Abstract: This paper proposes a closed-form solution for designing variable one-dimensional (1-D) finite-impulse-response (FIR) digital filters with simultaneously tunable magnitude and tunable fractional phase-delay responses. First, each coefficient of a variable FIR filter is expressed as a two-dimensional (2-D) polynomial of a pair of parameters called spectral parameters; one is for independently tuning the cutoff frequency of the magnitude response, and the other is for independently tuning fractional phase-delay. Then, the closed-form error function between the desired and actual variable frequency responses is derived without discretizing any design parameters such as the frequency and the two spectral parameters. Finally, the optimal solution for the 2-D polynomial coefficients can be easily determined through minimizing the closed-form error function. We also show that the resulting variable FIR filter can be efficiently implemented by generalizing Farrow structure to our two-parameter case. The generalized Farrow structure requires only a small number of multiplications and additions for obtaining any new frequency characteristic, which is particularly suitable for high-speed tuning.

Detection of islanded behavior and anti-islanding protection of a generator in grid-connected mode

Abstract: A method of controlling a generator system connected to an electric power system in which the output frequency characteristic of the generator system is measured, a first phase angle and frequency of the measured frequency characteristic is estimated using a first phase locked loop having a first bandwidth, and a second phase angle and frequency of the measured frequency characteristic is estimated using a second phase locked loop having a second bandwidth greater than the first bandwidth. Further, the method calculates a frequency difference between the first and second estimated frequencies, and an angle variation that is proportional to the calculated frequency difference. The estimated second phase angle is then added to the calculated angle variation so as to form an output current phase angle reference, and an output current phase angle of the generator system is controlled to be aligned with the output current phase angle reference. The method also determines whether or not the generator system is within a generation island based on the measured frequency characteristic.

Estimating soil electric properties from monostatic ground-penetrating radar signal inversion in the frequency domain

Abstract: [1] A new integrated approach for identifying the shallow subsurface electric properties from ground-penetrating radar (GPR) signal is proposed. It is based on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with a dielectric filled transverse electric and magnetic (TEM) horn antenna to be used off the ground in monostatic mode; that is, a single antenna is used as emitter and receiver. This radar configuration is appropriate for subsurface mapping and allows for an efficient and more realistic modeling of the radar-antenna-subsurface system. Forward modeling is based on linear system response functions and on the exact solution of the three-dimensional Maxwell equations for wave propagation in a horizontally multilayered medium representing the subsurface. Subsurface electric properties, i.e., dielectric permittivity and electric conductivity, are estimated by model inversion using the global multilevel coordinate search optimization algorithm combined sequentially with the local Nelder-Mead simplex algorithm (GMCS-NMS). Inversion of synthetic data and analysis of the corresponding response surfaces proved the uniqueness of the inverse solution. Laboratory experiments on a tank filled with a homogeneous sand subject to different water content levels further demonstrated the stability and accuracy of the solution toward measurement and modeling errors, particularly those associated with the dielectric permittivity. Inversion for the electric conductivity led to less satisfactory results. This was mainly attributed to the characterization of the frequency response of the antenna and to the high frequency dependence of the electric conductivity.

Piezoelectric resonator, filter, and duplexer

Abstract: A piezoelectric resonator of the present invention is structured such that on a substrate 5 having a cavity 4 formed therein, a lower electrode 3 , a piezoelectric body 1 , a spurious component control layer 16 , and an upper electrode 2 are formed in this order from bottom up. The spurious component control layer 16 is a layer for controlling a spurious frequency, and composed of, for example, a metallic material, a dielectric material, or a piezo electric material. By additionally providing the spurious component control layer 16 , it is made possible to cause variation of the spurious frequency due to unwanted variation to become greater than variation in resonance frequency of the main resonance of the piezoelectric resonator. Thus, it is possible to realize a piezoelectric resonator having an admittance frequency response where no spurious component occurs between resonance frequency fr and antiresonance frequency fa.

Frequency Modeling of Open-Channel Flow

Abstract: A good model is necessary to design automatic controllers for water distribution in an open-channel system. The frequency response of a canal governed by the Saint-Venant equations can be easily obtained in the uniform case. However, in realistic situations, open-channel systems are usually far from the uniform regime. This paper provides a new computational method to obtain a frequency domain model of the Saint-Venant equations linearized around any stationary regime (including backwater curves). The method computes the frequency response of the Saint-Venant transfer matrix, which can be used to design controllers with classical automatic control techniques. The precision and numerical efficiency of the proposed method compare favorably with classical numerical schemes (e.g., Runge–Kutta). The model is compared in nonuniform situations to the one given by a finite difference scheme applied to Saint-Venant equations (Preissmann scheme), first in the frequency domain, then in the time domain. The proposed ...

Frequency Response of a Piecewise Linear Vibration Isolator

Abstract: Piecewise linear vibration isolators are designed to optimally balance the competing goals of motion control and isolation. The piecewise linear system represents a hard nonlinearity, which cannot be assumed small, and hence standard perturbation methods are unable to provide a complete analytical solution. To date there is no frequency response equation reported for piecewise linear isolator systems to include both dual damping and stiffness behavior. In this investigation an averaging method was adopted to explore the frequency response of a symmetric piecewise linear isolator at resonance. The result obtained by an averaging method is in agreement with numerical simulation and experimental measurements. Preliminary sensitivity analysis is conducted to find the effect of system parameters. It appears that the damping ratio plays a more dominant role than stiffness in piecewise linear vibration isolators.

A study on resonant frequency and Q factor tunings for MEMS vibratory gyroscopes

Abstract: A new approach for improving the performance of MEMS vibratory gyroscopes was developed. The methodology suggests a simple way of improving the performance such as the overshoot, settling time and shock immunity by tuning the resonant frequency and the quality factor. The difference in the resonant frequency in two modes (driving and sensing mode) and the quality factors were found to be key factors in determining the dynamics of the gyroscopes. The difference in the frequency could be easily controlled by the electrical stiffness but it was difficult to control the quality factor because it is determined by vacuum level and the shape of structure. An electrostatic feedback technique allowed the control of the quality factor of the micro-gyroscopes. The experimental results show that the magnitude of the resonant peak in the frequency response of the gyroscope is reduced by 58% when the equivalent quality factor of the sensing system is tuned from 264 to 100 at a 100 Hz frequency difference between the driving and sensing modes. The time domain estimation was an approximate 50% reduction in the overshoot and an approximate threefold shortening of the settling time in that case. The estimation in the time domain was based on the simulation because there is no method to measure the transient response of gyroscopes directly.

Broad-band microstrip-to-CPW transition via frequency-dependent electromagnetic coupling

Abstract: An improved broad-band microstrip-to-coplanar-waveguide (CPW) transition is developed on a basis of the frequency-dependence characteristic of an electromagnetic surface-to-surface coupling. A self-calibrated method of moments is extended to model this unbounded two-port discontinuity with the two dissimilar microstrip/CPW feeding lines. Numerical results are provided to demonstrate its frequency response of transmission under varied strip/slot dimensions and further exhibit its attractive ultra-broad-band transmission with low radiation loss. Next, the back-to-back transition circuits with the two different lengths are fabricated and measured to deembed in experiment the S-parameters of two single-transition structures. Predicted and measured results show good agreement with the return loss less than -10dB over the frequency of 3.2-11.2 GHz.

Energy Transfer Properties of Nonlinear Systems in the Frequency Domain

Abstract: In this paper, an analysis of the energy transfer properties of nonlinear systems in the frequency domain is studied based on a new concept known as Nonlinear Output Frequency Response Functions (NOFRFs). The new concept allows the analysis to be implemented in a manner similar to the analysis of linear systems in the frequency domain

New approach for the measurement of damping properties of materials using the Oberst beam

Abstract: The Oberst method is widely used for the measurement of the mechanical properties of viscoelastic or damping materials. The application of this method, as described in the ASTM E756 standard, gives good results as long as the experimental set-up does not interfere with the system under test. The main difficulty is to avoid adding damping and mass to the beam owing to the excitation and response measurement. In this paper, a method is proposed to skirt those problems. The classical cantilever Oberst beam is replaced by a double sized free-free beam excited in its center. The analysis is based on a frequency response function measured between the imposed velocity at the center (measured with an accelerometer) and an arbitrary point on the beam (measured with a laser vibrometer). The composite beam (base beam + material) properties are first extracted from the measurement by an optimization algorithm. Young’s modulus and structural damping coefficient of the material under test can be deduced using classical...

Oscillatory flow in microchannels Comparison of exact and approximate impedance models with experiments

Abstract: Commonly used, lumped-parameter expressions for the impedance of an incompressible viscous fluid subjected to harmonic oscillations in a channel were compared with exact expressions, based on solutions of the Navier-Stokes equations for slots and channels of circular and rectangular cross-section, and were found to differ by as much as 30% in amplitude These differences resulted in predicted discrepancies by as much as 400% in frequency response amplitude for simple second-order systems based on size scales and frequencies encountered in microfluidic devices These predictions were verified experimentally for rectangular microchannels and indicate that underdamped fluidic systems operating near the corner frequency of any included flow channel should be modeled with exact expressions for impedance to avoid potentially large errors in predicted behavior List of symbols A Channel cross-sectional area (m 2 ) Ac Membrane area (m 2 ) a Rectangular duct and slot half-width or radius (m) b Rectangular duct half-depth and slot depth (m) C Capacitance (m 3 /Pa)

Edge adaptive image expansion and enhancement system and method

Abstract: An edge adaptive system and method for image filtering. The method maps each output pixel onto input image coordinates (110) and then prefilters and resamples the input image pixels around this point to reduce noise and adjust the scale corresponding to a particular operation. Then the edge in the input image is detected based on local (125) and average (127) signal variances in the input pixels. According to the edge detection parameters (129), including orientation, anisotropy and variance strength, the method determines a footprint and frequency response for the interpolation of the output pixel. In a more particular implementation, the method divides the input pixel space into a finite number of directions called skews, and estimates the edge orientation with the nearest skew direction. This further facilitates pixel inclusion in the interpolation of the output pixel.

Voltage to Pressure Conversion: Are you getting "phased" by the problem?

Abstract: The process of calibration provides the user with the sensitivity data necessary to convert the voltage measured at the end of the hydrophone cable to an acoustic pressure. This paper addresses the precise nature of how this data is used. Most regulatory standards simply require the user to use the sensitivity figure at the acoustic working frequency of the source in conjunction with a known (or pre-specified) “flatness” criterion for the overall frequency response. This paper questions the validity of this approach, and looks at the differences that can be obtained when the voltage to pressure conversion is accomplished by means of a deconvolution process that utilises all of the frequency response curve (as opposed a single data point). Theoretical and experimental comparisons between single frequency and full waveform deconvolution are presented. Currently, hydrophone calibration is a magnitude only measurement, but metrological improvements offer the possibility for measurement of hydrophone phase. As a logical extension to waveform deconvolution, the influence of phase response and the impact that this has on measured data is also presented. Clearly the proposed methods require greater computation during the measurement process, but this does not necessarily require longer measurement. This paper discusses practical methods of implementing these enhanced voltage to pressure conversion techniques in a commercial environment.

Pavement material microwave density measurement methods and apparatuses

Abstract: A method of obtaining a material property of a pavement material from a microwave field generally includes generating a microwave frequency electromagnetic field of a first mode about the pavement material. The frequency response of the pavement material in the electromagnetic field can be measured, such as by a network analyzer. The measurement of the frequency response permits correlating the frequency response to a material property of the pavement material sample, such as the density. A method of correcting for the roughness of a pavement material divides the pavement into a shallow layer and a deep layer. Two planar microwave circuits measure the permittivity of the shallow and deep layer. The permittivities are correlated to correct for roughness. An apparatus for obtaining the density of a pavement sample includes a microwave circuit and a network analyzer. The network analyzer measures the frequency response to determine the density of the pavement material.

Coplanar quarter-wavelength quasi-elliptic filters without bond-wire bridges

Abstract: This paper presents the design and experimental results of two coplanar quarter-wavelength microwave filters operating at a frequency of approximately 3 GHz. Coplanar designs are of particular interest because they are less sensitive to the thickness of the dielectric substrate than other transmission-line types such as a microstrip. One of the designs has a quasi-elliptic frequency response. In addition, the design also eliminates problems with unbalancing of the ground planes normally present in coplanar structures. Bond wires between the ground planes are not required. This paper describes in detail the structure of the resonators and how they are coupled together to form a filter.

Simplifying and interpreting two-tone measurements

Abstract: We develop a mathematical description of the response of weakly nonlinear systems to second-order memory mechanisms. Our description is based on a time-varying gain-modulation function. We show that intermodulation (IM) products arising from interactions at baseband have phase symmetries different from both interactions at second harmonic frequencies and gain compression and, thus, may be readily distinguished through measurement. We also demonstrate a technique for measuring and aligning the phase of IM products outside the measurement bandwidth of our instrumentation to identify contributions to memory with broad frequency response.

Analysis of segmented traveling-wave optical modulators

Abstract: A simple and comprehensive modeling approach is developed for analyzing the frequency response of segmented traveling-wave optical modulators. The approach is based on the microwave transmission (ABCD) matrix theory. The case study for a GaAs traveling-wave Mach-Zehnder modulator (MZM) verifies this analysis approach with excellent agreement to the reported experimental results; the analyses for the quantum-well-based MZMs and electroabsorption modulators indicate that the segmented traveling-wave design can provide much better bandwidth than the lumped-element or the continuous-traveling-wave counterparts, with a few decibels penalty in the electrical-to-optical (E/O) conversion gain if low-loss optical waveguides are available. Meandered transmission line design, which provides more design freedom, is also analyzed using this modeling approach.