Showing papers on "Frequency response published in 1991"

A new, continuous-time model for current-mode control (power convertors)

Abstract: A current-mode control power convertor model that is accurate at frequencies from DC to half the switching frequency is described for constant-frequency operation. Using a simple pole-zero transfer function, the model is able to predict subharmonic oscillation without the need for discrete-time z-transform models. The accuracy of sampled-data modeling is incorporated into the model by a second-order representation of the sampled-data transfer function which is valid up to half the switching frequency. Predictions of current loop gain; control-to-output; output impedance; and audio susceptibility transfer functions were confirmed with measurements on a buck converter. The audio susceptibility of the buck converter can be nulled with the appropriate value of external ramp. The modeling concentrates on constant-frequency pulse-width modulation (PWM) converters, but the methods can be applied to variable-frequency control and discontinuous conduction mode. >

Control oriented system identification: a worst-case/deterministic approach in H/sub infinity /

Abstract: The authors formulate and solve two related control-oriented system identification problems for stable linear shift-invariant distributed parameter plants In each of these problems the assumed a priori information is minimal, consisting only of a lower bound on the relative stability of the plant, an upper bound on a certain gain associated with the plant, and an upper bound on the noise level The first of these problems involves identification of a point sample of the plant frequency response from a noisy, finite, output time series obtained in response to an applied sinusoidal input with frequency corresponding to the frequency point of interest This problem leads naturally to the second problem, which involves identification of the plant transfer function in H/sub infinity / from a finite number of noisy point samples of the plant frequency response Concrete plans for identification algorithms are provided for each of these two problems >

A frequency-dependent finite-difference time-domain formulation for transient propagation in plasma

Abstract: Previous FDTD (finite-difference time-domain) formulations were not capable of analyzing plasmas for two reasons. First, FDTD requires that at each time step the permittivity and conductivity be specified as constants that do not depend on frequency, while even for the simplest plasmas these parameters vary with frequency. Second, the permittivity of a plasma can be negative, which can cause terms in FDTD expressions to become singular. A novel FDTD formulation for frequency-dependent materials (FD)/sup 2/TD has been developed. It is shown that (FD)/sup 2/TD can be applied to compute transient propagation in plasma when the plasma can be characterized by a complex frequency-dependent permittivity. While the computational example presented is for a pulse normally incident on an isotropic plasma slab, the (FD)/sup 2/TD formulation is fully three-dimensional. It can accommodate arbitrary transient excitation, with the limitation that the excitation pulse must have no zero frequency energy component. Time-varying electron densities and/or collision frequencies could also be included. The formulation presented is for an isotropic plasma, but extension to anisotropic plasma should be fairly straightforward. >

Application of Prony analysis to the determination of modal content and equivalent models for measured power system response

Abstract: It has been shown by J.F. Hauer et al. (ibid., vol.5, p.80-90, Feb. 1990) that Prony analysis can be very effective in response-based modal analysis of noise-free linear time-invariant (LTI) systems. This methodology extends Fourier analysis by directly estimating the frequency, damping, strength, and relative phase of the modal components present in a recorded signal. This earlier work is extended, concentrating upon power system planning applications, for stability program outputs. Results are presented for modal analysis and detailed model construction based on response data obtained through large-scale tests of the western US power system. An optimal modeling program, SYSFIT, is used to supplement the measurements. >

Direct determination of the amplitude and the phase of femtosecond light pulses.

Abstract: A direct measurement of the amplitude and the phase of a femtosecond light pulse is performed for the first time to our knowledge The measurement is made in the frequency domain, and the time dependence of the field can be easily obtained by a Fourier transform The technique relies on a pulse synthesis scheme to unravel the frequency dependence of the phase A mask filters the spectrum, which gives rise to a pulse with a measurable temporal profile related to the frequency dependence of the phase In particular, with a rectangular slit the time delay of the synthesized pulse is the first derivative of the phase with respect to the frequency of the original pulse at the central frequency of the filter The amplitude of the spectrum is obtained from the power spectrum

High-frequency equivalent circuit of GaAs FETs for large-signal applications

Abstract: The application of GaAs field effect transistors in digital circuits requires a valid description by an equivalent circuit at all possible gate and drain bias voltages for all frequencies from DC up to the gigahertz range. An equivalent circuit is presented which takes into account the gate current of positively biased transistors as well as the symmetrical nature of the devices at low drain voltages. A fast method of determining the elements of the equivalent circuit at all bias points without frequency limitations is presented. Direct computation from analytical expressions, without iteration, allows this parameter extraction procedure to be used for real-time on-wafer parameter extraction. Large-signal calculations are possible by inserting the voltage dependences evaluation for the elements into suitable simulation programs, such as SPICE. >

Convoluted array elements and reduced size unit cells for frequency-selective surfaces

Abstract: When the curvature of a surface that is to be made frequency selective is not small, it modifies the array lattice and element geometry, altering the transmission performance of the surface. For a given degree of curvature, small unit cells are distorted less. The transmission response of a typical array of conducting elements has a main reflection resonance at a wavelength λ1, with further nulls at shorter wavelengths. We have attempted to reduce the unit cell dimension p as much as possible for a given reflection wavelength λ1, thereby increasing its separation from grating responses. The ratio λ1/p has been used as a figure of merit. An array of linear dipoles on a square lattice of side p is a standard of comparison, a typical case giving a figure of merit of 1.6. A Jerusalem cross array gives a value of about 4.5. By convoluting the conductors of a square loop element, or by using, as the array elements, the low order generations of space filling curves, such as that described by Hilbert, we have obtained values of λ1/p in the range 7–12. The unit cell areas are lower than that of the dipole standard by factors averaging about 40.

Accurate computation of wide-band response of electromagnetic systems utilizing narrow-band information

Abstract: Cauchy's technique for interpolating a rational function from samples of frequency responses and/or their derivatives is investigated. This technique can be used to speed up the numerical computations of parameters, including input impedance and RCS of any linear time-invariant electromagnetic system. This technique is utilized to find the far field of a slit conducting cylinder (TM incidence) over a bandwidth utilizing the information about the current and its derivatives at a few sample points. The numerical results are presented are in good agreement with exact computational data. This technique is a true interpolation/extrapolation technique as it provides the same defective result as the original electric field integral equation at a frequency which corresponds to the internal resonance of the closed structure. >

Response of the shear layers separating from a circular cylinder to small-amplitude rotational oscillations

Abstract: The frequency response of the shear layers separating from a circular cylinder subject to small-amplitude rotational oscillations has been investigated experimentally in water for the Reynolds number (Re) range 250 to 1200. A hot-film anemometer was placed in the separated shear layers from 1 to 1.5 diameters downstream of the cylinder, and connected to a lock-in analyser. by referencing the lock-in analyser to the cylinder oscillations, the amplitude and phase of the response to different frequency oscillations were measured directly. It is shown that rotational oscillations corresponding to cylinder peripheral speeds between 0.5 and 3% of the free stream can be used to influence the primary (Karman) mode of vortex generation. For Re greater than ≈ 500, such oscillations can also force the shear-layer vortices associated with the instability of the separating shear layers. Corresponding to the primary and shear-layer modes are two distinct peaks in response amplitude versus frequency curves, and two very different phase versus frequency curves. The response of the shear layers (and near wake) in the range of Karman frequency suggests qualitative similarities with the response of an oscillator near resonance. Forced oscillations in the higher-frequency shear-layer mode range are simply convected by the shear layers. Close to the cylinder, the shear-layer response is shown to be comparable to that of generic free shear layers studied by others.

Wide bandwidth passive probe

Abstract: A passive test probe apparatus 100 which provides high impedance and a relatively flat frequency response over a wide bandwidth. The probe apparatus 100 uses standard, non-lossy coaxial cable 130. The probe apparatus 100 employs a low capacitance probe tip 102 designed to minimize stray probe tip capacitance. The probe apparatus 100 uses a front-end (tip) resistor R t in series with a conventional RC tip network 240. The tip resistor R t provides two functions: First, it establishes a minimum input impedance for the probe input, and secondly, it provides approximately 80% of the high frequency attenuation when working into the cable characteristic impedance. In addition, a technique is described for determining an optimum length of coaxial cable 130 to be used in the probe apparatus 100. When the cable length is selected in accordance with the present invention, the crossover dip and other transmission line signal distortion effects are minimized.

Shifting the frequency response of switched-capacitor filters by nonuniform sampling

Abstract: A clock with nonuniformly spaced sampling times can be used to shift the frequency response of switched-capacitor filters. By using such a clock, which deviates only slightly from a uniform sampling clock, small shifts in the frequency response of a switched-capacitor filter can be realized. The nonuniform sampling generates undesired modulation sidebands which are small if the deviation from uniform sampling is small. Computer simulations and measured data are presented to support equations that predict the frequency response shift and the amplitude of the undesired sidebands. This technique can be used to correct frequency response errors in monolithic switched-capacitor filters caused by capacitor ratio inaccuracies. It may also be useful in applications such as spectrum analyzers and tone generators. >

Control System Design and Simulation

Abstract: Introduction to Control Systems, Modelling of Dynamic Systems, System Time Response, Frequency Response, Frequency Domain Compensator Design, Root Locus, Process Control, Discrete Time Systems, Computer Control Systems, Nonlinearities in Control Systems. Appendices.

Application of the time varying transfer function for exact small-signal analysis

Abstract: An analysis method for determining the control-to-output linearized describing function of time-interval-modulated switched networks using time-varying system theory was previously proposed by the author. A simplification is now presented which eases the analysis considerably. Use of the simplified approach is demonstrated in the derivation of the control-to-output frequency response of pulse-width-modulated (PWM) DC-to-DC switching power converters operating in discontinuous conduction mode (DCM) and current programmed converters operating in continuous conduction mode (CCM) as well as in DCM. Experimental results are presented which verify the modeling approach. >

Analysis of forced oscillations in a multimachine power system

Abstract: The origin of forced, low frequency oscillations introduced in power systems by low-speed diesel generators is investigated. The linear models of a multimachine power system are presented for both interconnected and autonomous systems. Particular emphasis is given to the calculation of the electromechanical oscillation frequencies. A three-machine test system exhibiting a very strong resonance is analysed. The method of analysis is based upon the computation of the frequency response of a multivariable system. Finally, the electromechanical oscillation analysis and the frequency response computation are applied to the electric power system of the island of Crete, where a number of large low-speed diesel engine driven generators have recently been installed.< >

Frequency response and path loss measurements of indoor channel

Abstract: An improved method of operation of an HP network analyser to perform measurements of the frequency response of the indoor RF propagation channel and the relative path loss envelope and phase of a CW signal is described. An improved postprocessing procedure is also described. Test results obtained for indoor measurements in the 1-2 GHz band (2.3 ns resolution after windowing) and 945 MHz path loss measurements are presented as examples.

Experimental Identification of Mechanical Joint Parameters

Abstract: The dynamic behavior of a mechanical system generally are strongly affected by the properties of mechanical joints. An identification method which directly used the measured frequency response functions (FRFs) to identify the joint properties was introduced in this work. Because the measurement noise in the frequency response functions is unavoidable in practice and may lead to very faulty results, the proposed method has been developed especially to overcome this problem. The accuracy and feasibility of the proposed method were verified and demonstrated by theoretical simulation and experiments. The results show that the joint properties can be identified accurately from the FRFs even with noise effect.

A two-dimensional time domain near zone to far zone transformation

Abstract: In a previous paper, a time domain transformation useful for extrapolating 3-D near zone finite difference time domain (FDTD) results to the far zone was presented. In this paper, the corresponding 2-D transform is outlined. While the 3-D transformation produced a physically observable far zone time domain field, this is not convenient to do directly in 2-D, since a convolution would be required. However, a representative 2-D far zone time domain result can be obtained directly. This result can then be transformed to the frequency domain using a Fast Fourier Transform, corrected with a simple multiplicative factor, and used, for example, to calculate the complex wideband scattering width of a target. If an actual time domain far zone result is required it can be obtained by inverse Fourier transform of the final frequency domain result.

Improved mode-superposition technique for modal frequency response analysis of coupled acoustic-structural systems

Abstract: A new formulation is derived by using an orthogonality condition of a coupled system. An improved compensation technique is then proposed for compensating for the effect of truncated modes, which are the lower and/or higher modes beyond the frequency domain of an MFR analysis

Analysis of eddy currents in nuclear magnetic resonance imaging.

Abstract: The eddy currents in nuclear magnetic resonance (NMR) imaging are analyzed from the solutions of Maxwell's equations and their effects are examined over various experimental conditions from whole-body diagnostic imaging to recently developed NMR microscopy. The analysis is focused mainly on the frequency characteristics and intensity variations of the eddy-current-induced field which depends on the overall system size, ratio of the gradient coil size to the magnet bore diameter, and the pulse-sequence-dependent parameters such as input current waveform and repetition time. From the analysis, the frequency response of the eddy-current-induced field is that of a high-pass filter whose cutoff frequency is inversely proportional to the square of the overall system size. The intensity ratio of the generated field to the induced field is not affected by the overall system size, but is sensitively related to the ratio of the gradient coil size to the magnet bore diameter.

Comparison of fast photodetector response measurements by optical heterodyne and pulse response techniques

Abstract: Characterization of high-speed photodetectors by optical heterodyne and pulse response techniques is reported, and the results are compared. Scalar deconvolution was used to obtain photodetector response from the pulse response measurements with effects of the measurement apparatus removed. Linearity of the detector response is verified, validating comparison in the frequency domain using a Fourier transform of the pulse response data. The comparison is limited to the magnitude of the transfer function in the frequency domain, since the optical heterodyne technique does not give phase information. Both sets of results are corrected for the effects of their respective measurement systems. After these corrections, close agreement between results obtained by using the two techniques was found. >

Parametric system identification on logarithmic frequency response data

Abstract: Gradient search methods that fit the parameters of a user-defined transfer function to experimental logarithmic frequency response data are presented. The methods match a model based on physically significant parameters, including natural frequencies of poles and zeros and damping ratios of complex poles and zeros. The algorithms construct and utilize their own analytical gradient descent functions, based on the desired model. One method attempts to fit both log magnitude and phase, while another identifies a minimum phase transfer function model from only log magnitude frequency response data. The log magnitude algorithm is shown to be superior to traditional methods using nonlogarithmic frequency response data, including those used in commercially available frequency response analyzers. The algorithms are shown to perform well, especially for systems with lightly-damped dynamics. >

Potentiostatic apparatus and methods

Abstract: A potentiostat for an amperometric sensor uses a voltage-controlled current source (VCCS), which may be either unidirectional or, preferably, bidirectional. An error amplifier compares the potential of the sensor's reference or counter electrode, relative to ground potential, to a set-point potential. The error voltage serves as an input to the VCCS. The output of the VCCS--a current proportional to the error voltage--is supplied to the sensor's auxiliary electrode. In a steady state, the error voltage is just large enough to supply current that is equal to the current through the sensor's working and auxiliary (or counter) electrodes. The error voltage can either serve directly as an output voltage or it can be amplified without introducing a feed-back loop disturbance. The VCCS permits direct grounding of the sensor's working electrode and operation of the error amplifier at a low or moderate gain, so as to increase the circuit's frequency response and reduce the probability of oscillation. Also, in conjunction with an adjustable low-pass filter, the VCCS permits optimization of the system's frequency response so as to yield improved stability.

The response of water level in a well to a time series of atmospheric loading under confined conditions

Abstract: The water level in a well that penetrates a confined aquifer can fluctuate in response to changes in atmospheric pressure. The response varies with the well casing and screen dimensions, the transmissivity and compressibility of the aquifer, and to a small extent, its storativity. Recently this loading-response problem has been solved in terms of a frequency response function that characterizes how attenuation and phase shifts in the response signal vary with frequency. The counterpart of this solution in the time domain is an impulse response function. This solution has the immediate appeal that it can be used directly to filter raw loading and water level records via serial convolution, where the response at any time is a weighted aggregate of the previous loading signal. The impulse response function derives from well-known solutions to the slug test problem. When based on the Cooper-Bredehoeft-Papadopulos solution, the response function is precise but computationally intensive. Based on the Hvorslev solution, the response function is computationally simple, but applicable only to aquifers with small storativity. Simulations and a field example clearly illustrate the dominating influence of transmissivity in dampening and lagging the response of a confined aquifer to a loading signal, and how both effects increase with higher frequency. The results also illustrate the weak influence of storativity in modulating the response signal.

Frequency response analysis and modeling of measurement transformers under distorted current and voltage supply

Abstract: The time and frequency response of current and potential transformers (CTs and PTs) operating under fundamental frequency conditions are well known. However, in the presence of harmonic frequencies, these transformers can exhibit significant errors. The authors present CT and PT comprehensive frequency domain equivalent circuits which, in addition to previous study features, include the hysteresis and eddy current effects. Furthermore, a practical approach which calculates the stray capacitances is proposed. By comparing theoretical and practical results the importance of considering parameters which are usually ignored in such studies is highlighted. >

Silicon condenser microphone with integrated field-effect transistor

Abstract: A silicon condenser microphone is described, which works with an integrated field-effect transistor (FET). The gate of the transistor corresponds to the membrane of the microphone. Between the membrane and the gate oxide is a small air gap. The drain current of the transistor is controlled by the deflections of the membrane. The structure, which carries the FET and which is placed beyond the membrane, can have very small lateral dimensions. This results in small values of air-gap streaming losses and high air-gap compliances, thus yielding a good acoustic behaviour. The design of a silicon microphone with suspended-gate FET is described and experimental results of frequency response and noise are presented. The measured sensitivities are in the range 0.1–1 mV/Pa, which is about 15 dB lower than the calculated values. The reduction in sensitivity is caused by the silicon fabrication process of the microphones and can be eliminated. The frequency response is smooth up to 30 kHz. The noise measurement shows a 1/ f slope, which is typical for the noise behaviour of the FET.

Frequency response and tunability of grating-assisted directional couplers

Abstract: The spectral properties of grating-assisted directional couplers are studied using an improved coupled mode formulation. Key parameters for the design of these structures, such as the grating period, the coupling length, and other structural parameters, are calculated. The frequency response, the filter bandwidth, and the tuning range are analyzed. The technique is used to examine a specific case of InGaAsP-InP tunable filters, and the results are compared to a prior experiment. >

Nonuniformly coupled microstrip transversal filters for analog signal processing

Abstract: The Fourier transform relationship between frequency response and impedance profile for single nonuniform transmission lines is used to derive the time-domain step response of single and coupled nonuniform lines. The expression for the step response of a characteristically terminated nonuniformly coupled transmission line structure is shown to correspond to the characteristic impedance profile. By using this relationship, any arbitrary step response can be realizing by utilizing nonuniformly coupled strip or microstrip lines for possible applications as waveform-shaping networks and chirp filters. A numerical procedure to compute the step response of the nonuniform coupled line four-port is also formulated in terms of frequency-domain parameters of an equivalent cascaded uniform coupled line model with a large number of sections. Sinusoidal and chirp responses are presented as examples that are readily implemented using coupling microstrip structures. The step response of an experimental nonuniformly coupled microstrip structure is presented to validate the theoretical results. >