Showing papers on "Frequency response published in 1996"

The dielectric properties of biological tissues: II. Measurements in the frequency range 10 Hz to 20 GHz

Abstract: Three experimental techniques based on automatic swept-frequency network and impedance analysers were used to measure the dielectric properties of tissue in the frequency range 10 Hz to 20 GHz. The technique used in conjunction with the impedance analyser is described. Results are given for a number of human and animal tissues, at body temperature, across the frequency range, demonstrating that good agreement was achieved between measurements using the three pieces of equipment. Moreover, the measured values fall well within the body of corresponding literature data.

Subspace-based multivariable system identification from frequency response data

Abstract: Two noniterative subspace-based algorithms which identify linear, time-invariant MIMO (multi-input/multioutput) systems from frequency response data are presented. The algorithms are related to the recent time-domain subspace identification techniques. The first algorithm uses equidistantly, in frequency, spaced data and is strongly consistent under weak noise assumptions. The second algorithm uses arbitrary frequency spacing and is strongly consistent under more restrictive noise assumptions, promising results are obtained when the algorithms are applied to real frequency data originating from a large flexible structure.

Damping controller design for power system oscillations using global signals

Abstract: This paper describes a new power system stabilizer (PSS) design for damping power system oscillations focusing on interarea modes. The input to the PSS consists of two signals. The first signal is mainly to damp the local mode in the area where PSS is located using the generator rotor speed as an input signal. The second is an additional global signal for damping interarea modes. Two global signals are suggested; the tie-line active power and speed difference signals. The choice of PSS location, input signals and tuning is based on modal analysis and frequency response information. These two signals can also be used to enhance damping of interarea modes using SVC located in the middle of the transmission circuit connecting the two oscillating groups. The effectiveness and robustness of the new design are tested on a 19-generator system having characteristics and structure similar to the Western North American grid.

Analysis of dynamic spectra in ferret primary auditory cortex. I. Characteristics of single-unit responses to moving ripple spectra

Abstract: 1. Auditory stimuli referred to as moving ripples are used to characterize the responses of both single and multiple units in the ferret primary auditory cortex. Moving ripples are broadband complex sounds with a sinusoidal spectral profile that drift along the logarithmic frequency axis at a constant velocity. 2. Neuronal responses to moving ripples are locked to the phase of the ripple, i.e., they exhibit the same periodicity as that of the moving ripple profile. Neural responses are characterized as a function of ripple velocity (temporal property) and ripple frequency (spectral property). Transfer functions describing the response to these temporal and spectral modulations are constructed. Temporal transfer functions are inverse Fourier transformed to obtain impulse response functions that reflect the cell's temporal characteristics. Ripple transfer functions are inverse Fourier transformed to obtain the response field, a measure analogous to the cell's response area. These operations assume linearity in the cell's response to moving ripples. 3. Transfer functions and other response functions are shown to be fairly independent on the overall level or depth of modulation of the ripple stimuli. Only downward moving ripples were used in this study. 4. The temporal and ripple transfer functions are found to be separable, in that their shapes remain unchanged for different test parameters. Thus ripple transfer functions and response fields remain statistically similar in shape (to within an overall scale factor) regardless of the ripple velocity or whether stationary or moving ripples are used in the measurement. The same stability in shape holds for the temporal transfer functions and the impulse response functions measured with different ripple frequencies. Separability implies that the combined spectrotemporal transfer function of a cell can be written as the product of a purely ripple and a purely temporal transfer functions, and thus that the neuron can be computationally modeled as processing spectral and temporal information in two separate and successive stages. 5. The ripple parameters that characterize cortical cells are distributed somewhat evenly, with the characteristic ripple frequencies ranging from 0.2 to > 2 cycles/octave and the characteristic angular frequency typically ranging from 2 to 20 Hz. 6. Many responses exhibit periodicities in the spectral envelope of the stimulus. These periodicities are of two types. Slow rebounds, not found in the spectral envelope, and with a period of approximately 150 ms, appear with various strengths in approximately 30% of the cells. Fast regular firings with interspike intervals of approximately 10 ms are much less common and appear to correspond to interactions between the component tones that make up a ripple.

Control topology options for single-phase UPS inverters

Abstract: Several control topologies for single-phase UPS inverters are presented and compared, with the common objective of providing a dynamically stiff, low THD, sinusoidal output voltage. Full state feedback, full state command controllers are shown utilizing both filter inductor current and filter capacitor current feedback to augment output voltage control. All controllers presented include output voltage decoupling in a manner analogous to "back-EMF" decoupling in DC motor drives. Disturbance input decoupling of the load current and its derivative is presented. An observer-based controller is additionally considered, and is shown to be a technically viable, economically attractive option. The accuracy transfer function of the observer estimate is used to evaluate its measurement performance. Comparative disturbance rejection is evaluated by overlaying the dynamic stiffness (inverse of output impedance) frequency response of each controller on a single plot. Experimental results for one controller are presented.

FIR filter design via semidefinite programming and spectral factorization

Abstract: We present a semidefinite programming approach to FIR filter design with arbitrary upper and lower bounds on the frequency response magnitude. It is shown that the constraints can be expressed as linear matrix inequalities (LMIs), and hence they can be easily handled by interior-point methods. Using this LMI formulation, we can cast several interesting filter design problems as convex or quasi-convex optimization problems, e.g. minimizing the length of the FIR filter and computing the Chebychev approximation of a desired power spectrum or a desired frequency response magnitude on a logarithmic scale.

Output Frequency Characteristics of Nonlinear Systems

Abstract: Some interesting properties of output frequencies of Volterra-type nonlinear systems are particularly investigated. These results provide a very novel and useful insight into the super-harmonic and inter-modulation phenomena in output frequency response of nonlinear systems, with consideration of the effects incurred by different nonlinear components in the system. The new properties theoretically demonstrate several fundamental output frequency characteristics and unveil clearly the mechanism of the interaction (or coupling effects) between different harmonic behaviors in system output frequency response incurred by different nonlinear components. These results have significance in the analysis and design of nonlinear systems and nonlinear filters in order to achieve a specific output spectrum in a desired frequency band by taking advantage of nonlinearities. They can provide an important guidance to modeling, identification, control and signal processing by using the Volterra series theory in practice.

Cross-phase modulation in fiber links with multiple optical amplifiers and dispersion compensators

Abstract: We have theoretically and experimentally investigated the cross-phase modulation (XPM) effect in optical fiber links with multiple optical amplifiers and dispersion compensators. Our theory suggests that the XPM effect can be modeled as a phase modulator with inputs from the intensity of copropagating waves. The frequency response of the phase modulator corresponding to each copropagating wave depends on fiber dispersion, wavelength separation, and fiber length. The total XPM-induced phase shift is the integral of the phase shift contributions from all frequency components of copropagating waves. In nondispersive fibers, XPM is frequency-independent; in dispersive fibers, XPM's frequency response is approximately inversely proportional to the product of frequency, fiber dispersion, and wavelength separation. In an N-segment amplified link, the frequency response of XPM is increased N-fold, but only in very narrow frequency bands. In most other frequency bands, the amount of increase is limited and almost independent of N. However, in an N-segment amplified link with dispersion compensators, the frequency response of XPM is increased N-fold at all frequencies if the dispersion is compensated for within each fiber segment. Thus, the XPM-induced phase shift is smaller in systems employing lumped dispersion compensation than in systems employing distributed dispersion compensation.

Frequency response of sampled-data systems

Abstract: This paper introduces the concept of frequency response for sampled-data systems and explores some basic properties as well as its computational procedures. It is shown that 1) by making use of the lifting technique, the notion of frequency response can be naturally introduced to sampled-data systems in spite of their time-varying characteristics, 2) it represents a frequency domain steady-state behaviour, and 3) it is also closely related to the original transfer function representation via an integral formula. It is shown that the computation of the frequency response can be reduced to a finite-dimensional eigenvalue problem, and some examples are presented to illustrate the results.

Microwave imaging for tissue assessment: initial evaluation in multitarget tissue-equivalent phantoms

Abstract: A prototype microwave imaging system is evaluated for its ability to recover two-dimensional (2-D) electrical property distributions under transverse magnetic (TM) illumination using multitarget tissue equivalent phantoms. Experiments conducted in a surrounding lossy saline tank, demonstrate that simultaneous recovery of both the real and imaginary components of the electrical property distribution is possible using absolute imaging procedures over a frequency range of 300-700 MHz. Further, image reconstructions of embedded tissue-equivalent targets are found to be quantitative not only with respect to geometrical factors such as object size and location but also electrical composition. Quantitative assessments based on full-width half-height criteria reveal that errors in diameter estimates of reconstructed targets are less than 10 mm in all cases, whereas, positioning errors are less than 1 mm in single object experiments but degrade to 4-10 mm when multiple targets are present. Recovery of actual electrical properties is found to be frequency dependent for the real and imaginary components with background values being typically within 10-20% of their correct size and embedded object having similar accuracies as a percentage of the electrical contrast, although errors as high as 50% can occur. The quantitative evaluation of imaging performance has revealed potential advantages in a two-tiered receiver antenna configuration whose measured field values are more sensitive to target region changes than the typical tomographic type of approach which uses reception sites around the full target region perimeter. This measurement strategy has important implications for both the image reconstruction algorithm where there is a premium on minimizing problem size without sacrificing image quality and the hardware system design which seeks to economize on the amount of measured data required for quantitative image reconstruction while maximizing its sensitivity to target perturbations.

Pilot spacing in orthogonal frequency division multiplexing systems on practical channels

Abstract: The performance of two different channel interpolation methods to be used with orthogonal frequency division multiplexing systems are investigated. The considered schemes use constant pilot frequencies for channel response estimation. The interpolation techniques are piecewise-constant and piecewise-linear methods which due to their inherent simplicity are straightforward to implement. The results are given as error probability vs. pilot separation for a channel with exponential type power-delay profile and M-ary quadrature amplitude (MQAM) submodulation with M=4, 16 and 64.

Experimental Results of Adaptive Periodic Disturbance Cancellation in a High Performance Magnetic Disk Drive

Abstract: This paper considers the implementation of an adaptive algorithm for periodic disturbance cancellation. It is shown that the maximum rate of adaptation can be calculated precisely based on measurements of the system's frequency response. The response of the closed-loop system to additional disturbances can also be computed on that basis. The results are verified experimentally on a high track density magnetic disk drive. Excellent matching between the theoretical and experimental results is observed. An improved method is also proposed that leads to faster convergence of the adaptive algorithm and better disturbance rejection capabilities. The results of this paper significantly enhance the ability of the control engineer to design and analyze adaptive feedforward algorithms for a variety of applications where periodic disturbances are encountered.

Efficient AC and noise analysis of two-tone RF circuits

Abstract: In this paper we present a preconditioned recycled Krylov-subspace method to accelerate a recently developed approach for ac and noise analysis of linear periodically-varying communication circuits. Examples are given to show that the combined method can be used to analyse switching filter frequency response, mixer 1/f noise frequency translation, and amplifier intermodulation distortion. In addition, it is shown that for large circuits the preconditioned recycled Krylov-subspace method is up to forty times faster than the standard optimized direct methods.

PID tuning for integrating and unstable processes

Abstract: A systematic PI and PID tuning method is developed for integrating and unstable processes. The method, based on a maximum peak-resonance specification, leads to simple tuning parameter expressions and is graphically supported by the Nichols chart. The common characteristic of integrating and unstable processes is that their open-loop frequency response, with the controller in cascade, has a phase maximum. The controller parameters are adjusted such that this maximum is located on the right-most point of the ellipse, corresponding to the maximum peak resonance on the Nichols chart. For these types of processes, making the open-loop frequency-response curve tangent to a specified ellipse is an efficient method of controlling the overshoot, the stability and the dynamics of the system. Charts that give the optimal peak resonance, according to the ITAE criterion, are given. The results obtained with this method are presented for typical design examples. A comparison of the performances of the proposed design settings with those given by earlier workers for unstable processes is presented.

The design of log-domain filters based on the operational simulation of LC ladders

Abstract: A technique for the design of log-domain filters is introduced which is based on the operational simulation of LC ladders. The method is used to design a fifth-order Chebyshev and elliptic filter. HSPICE simulation results and experimental results are shown with emphasis on frequency behavior and linearity. The filters showed good correlation between the original filter specifications and the measured frequency response. They proved to be easily tunable and could be operated up to 1 MHz or one tenth of the f/sub T/ of the slowest transistor. Total harmonic distortion and intermodulation distortion was measured with results up to -47 and -55 dB, respectively.

Biophysics of the cochlea. II: Stationary nonlinear phenomenology.

Abstract: Nonlinearities affecting cochlear mechanics produce appreciable compression in the basilar membrane (BM) input/output (I/O) functions at the characteristic frequency for sound‐pressure levels (SPLs) as low as 20 dB (re: 20 μPa) This is thought to depend upon saturation of the outer hair cell (OHC) mechanoelectrical transducer (MET) This hypothesis was tested by solving a nonlinear integrodifferential equation that describes the BM vibration in an active cochlea The equation extends a previously developed linear approach [Mammano and Nobili, J Acoust Soc Am 93, 3320–3332 (1993)], here modified to include saturating MET, with a few corrections mainly concerning tectorial membrane resonance and OHC coupling to the BM Stationary solutions were computed by iteration in the frequency domain for a wide range of input SPLs, generating BM I/O functions, frequency response envelopes, and two‐tone distortion products Traveling‐wave amplitude envelopes were also computed for a fixed suppressor and several suppressed tones in order to evidence the phenomenon of two‐tone suppression (frequency masking) at the mechanical level All results accord nicely with experimental data

Power system reduction to simplify the design of damping controllers for interarea oscillations

Abstract: This paper describes the application of a reduction method to power systems to obtain simplified models that facilitate the design of damping controllers. The reduced system retains the lightly damped swing of interest and exhibits modal characteristics similar to the unreduced system in the frequency range associated with swing modes. A power system damping controller is designed using a reduced system. The modal characteristics of the reduced and unreduced system with the damping controller included, are compared. The validity of the control design is assessed via nonlinear simulations. The reduction method is simple to implement and is based on the computation of the observability and controllability Gramians.

Active frequency-selective surfaces

Abstract: Two different varieties of active frequency-selective surfaces (FSS) are described. In one type, active FSS elements incorporating switched PIN diodes are discussed. Waveguide simulation studies show that the frequency response of the surface can be electronically switched from that of a reflecting structure to a transmitting structure. In the other type, FSS are printed on ferrite substrates which are biased with a DC magnetic field. The application of this bias field to the ferrite allows tuning of the resonant frequency by several gigahertz. In addition, the frequency characteristics can be switched from transmitting to reflecting, as for the active element FSS.

Estimation of Mass, Stiffness and Damping Matrices from Frequency Response Functions

Abstract: A frequency-domain method for estimating the mass, stiffness and damping matrices of the model of a structure is presented. The developed method is based on our previous work on the extraction of normal modes from the complex modes of a structure. A transformation matrix is obtained from the relationship between the complex and the normal frequency response functions ofa structure. The transformation matrix is employed to calculate the damping matrix of the system. The mass and the stiffness matrices are identified from the normal frequency response functions by using the least squares method. Two simulated systems are employed to illustrate the applicability of the proposed method. The results indicate that the damping matrix can be identified accurately by the proposed method. The reason for the good results is that the damping matrix is identified independently from the mass and the stiffness matrices. In addition, the robustness of the new approach to uniformly distributed measurement noise is also addressed.

Development of a piezoelectric servoflap for helicopter rotor control

Abstract: A servoflap that uses a piezoelectric bender to deflect a trailing edge flap for use on a helicopter rotor blade was designed, built, and tested. This servoflap design is an improvement over a design developed previously at MIT. The design utilizes a new flexure mechanism to connect the piezoelectric bender to the control surface. The efficiency of the bender was improved by tapering its thickness with length. Also, the authority of the actuator was increased by implementing a nonlinear circuit to control the applied electric field, allowing a greater range of actuator voltages. Experiments were carried out on a bench test article to determine the frequency response of the actuator, as well as hinge moment and displacement capabilities. Flap deflections of or more were demonstrated while operating under no-load conditions at frequencies up to 100 Hz. The data indicate that, if properly scaled, the actuator will produce flap deflections greater than at the 90% span location on a full-scale helicopter. In addition, the first mode of the actuator was at frequency of the target model rotor. Proper inertial scaling of this actuator could raise this modal frequency to greater than on an operational helicopter, which is adequate for most rotor control purposes. A linear state space model of the actuator was derived. Comparisons of this model with the experimental data highlighted a number of mild nonlinearities in the actuator's response. However, the agreement between the experiment and analysis indicate that the model is a valid tool for predicting actuator performance.

System Dynamics: An Introduction

Abstract: 1. Introduction. 2. Energy and Power Flow in State-Determined Systems. 3. Summary of One-Port Primitive Elements. 4. Formulation of System Models. 5. State Equation Formulation. 6. Energy Transducing System Elements. 7. Operational Methods for Linear Systems. 8. System Properties and Solution Techniques. 9. First- and Second-Order System Response. 10. General Solution of the Linear State Equations. 11. Solution of System Response by Numerical Simulation. 12. The Transfer Function. 13. Impedance-Based Modeling Methods. 14. Sinusoidal Frequency Response of Linear Systems. 15. Frequency Domain Methods. Appendix A - Introduction to Matrix Algebra. Appendix B - Complex Numbers. Appendix C - Partial Fraction Expansion of Rational Functions.

High-frequency quantum-well infrared photodetectors measured by microwave-rectification technique

Abstract: We explore the high-frequency capability of quantum-well infrared photodetectors using a microwave-rectification technique. We characterize a variety of devices with barrier thicknesses from 234 to 466 /spl Aring/ and number of wells from 4 to 32. Our packaged detectors have a relatively flat frequency response up to about 30 GHz. These experiments indicate that the intrinsic photoconductive lifetime for these devices in the high-biasing field regime is in the range of 5-6 ps.

Wideband modeling of arbitrarily shaped E-plane waveguide components by the "boundary integral-resonant mode expansion method"

Abstract: The paper describes a very fast and flexible algorithm for the wideband modeling of arbitrarily shaped H-plane waveguide components. The algorithm is based on the evaluation of the poles and the residues of the Y-parameters by the "boundary integral-resonant mode expansion method." It also permits the fast evaluation of the effect of a deformation on the frequency response, a feature very useful either for optimization or for setting the mechanical tolerances. Some examples demonstrate the efficiency, flexibility, and reliability of the method. They show that the frequency response of complicated structures, such as multicavity filters, can be calculated in times of the order of one minute (or less) on ordinary workstations.

System identification using nonstationary signals

Abstract: The conventional method for identifying the transfer function of an unknown linear system consists of a least squares fit of its input to its output. It is equivalent to identifying the frequency response of the system by calculating the empirical cross-spectrum between the system's input and output, divided by the empirical auto-spectrum of the input process. However, if the additive noise at the system's output is correlated with the input process, e.g., in case of environmental noise that affects both system's input and output, the method may suffer from a severe bias effect. We present a modification of the cross-spectral method that exploits nonstationary features in the data in order to circumvent bias effects caused by correlated stationary noise. The proposed method is particularly attractive to problems of multichannel signal enhancement and noise cancellation, when the desired signal is nonstationary in nature, e.g., speech or image.

Design of low-power and low-voltage log-domain filters

Abstract: This paper describes a component-level mapping between linear- and log-domain filters. A modified multiple input class AB log-integrator with both f- and Q-tuning is presented. A 3/sup rd/-order elliptic filter derived from a LC ladder prototype is designed using the proposed log building blocks and including the log-domain implementation of the transmission zero. Simulated frequency responses, THD and SNR, obtained from large signal transient analysis, are given. The filter is designed for a 2 V supply and consumes 1.3 mW for a cutoff frequency of 10 MHz. It can easily be frequency tuned from 1 MHz to 30 MHz.

A frequency response, harmonic distortion, and intermodulation distortion test for BIST of a sigma-delta ADC

Abstract: Built-In-Self-Test (BIST) for VLSI systems is desirable for production-line testing and in-the-field diagnostics. This brief discusses a Mixed Analog-Digital BIST (MADBIST) for a frequency response test, a harmonic distortion test and an intermodulation distortion test of an analog-to-digital converter. The MADBIST strategy for the frequency response, harmonic distortion, and intermodulation distortion tests of the ADC is introduced, accuracy issues are discussed, and preliminary experimental results are presented.

Pendulum as Vibration Absorber for Flexible Structures: Experiments and Theory

Abstract: The dynamic response ofa beam-tip mass-pendulum system subjected to a sinusoidal excitation is investigated. A simple pendulum mounted to a tip mass of a beam is used as a vibration absorber. The nonlinear equations of motion are developed to investigate the autoparametric interaction between the first two modes of the system. The nonlinear terms appear due to the curvature of the beam and the coupling effect between the beam and pendulum. Complete energy transfer between modes is shown to occur when the beam frequency is twice the pendulum frequency. Experimental results are compared with a theoretical solution obtained using numerical integration. The experimental results are in qualitative agreement with the theory.