Frequency response

About: Frequency response is a research topic. Over the lifetime, 25705 publications have been published within this topic receiving 332249 citations.

Frequency-Domain Analysis of Linear Time-Periodic Systems

Abstract: In this report we study how a time-varying system with a time-periodic integral kernel (impulse response), g(t,\tau)=g(t+T,\tau+T), can be expanded into a sum of essentially time-invariant systems. This allows us to define a linear frequency response operator for periodic systems, called the Harmonic Transfer Function (HTF). The HTF is a direct analog of the transfer function for time-invariant systems, but it captures the frequency coupling of a time-periodic system. It can, for example, be used to compute the induced L_2-norm of periodic systems. The report also includes analysis of convergence of truncated HTFs, which is essential for practical computations as the HTF is an infinite-dimensional operator.

Response of a planar microstrip line excited by an external electromagnetic field

Abstract: The voltages and currents induced by external electromagnetic fields on a planar microstrip line have been studied with the use of a distributed-source transmission-line model. The frequency response of the microstrip line has been analyzed by simulating the illuminating field with a plane wave arbitrarily incident on the line. The influence of the microstrip geometrical and electrical characteristics on the voltages and currents induced on the line has been examined, and indications for reducing the circuit susceptibility have been obtained. The model adopted can be used for studying the response of the line to any type of external field arbitrarily varying in space time. Numerical results show that for lines loaded with the characteristic impedance at both terminals, voltage amplitudes on the order of some millivolts and currents of some hundreds of microamperes can be induced at f=3 GHz by an incident plane wave with an electric-field intensity of 1 V/m and for various angles of incidence. The voltages and currents induced on a microstrip circuit can be reduced by using substrates of sufficiently high permittivity. >

A Frequency Domain Approach to Water Quality Modeling in Groundwater: Theory

Abstract: A method for the analysis of complex temporal variations of environmental tracers or pollution time series in groundwater is examined using spectral analysis and linear filter theory for stationary stochastic processes. The interpretation of solute fluctuations subject to a time varying source is accomplished via frequency domain solutions to stochastic differential equations for three widely applied transport models: (1) a lumped parameter or linear reservoir model; (2) convective (advective) transport in a curvilinear flow field; and (3) convective-dispersive transport in a uniform flow field. Frequency domain solutions are presented in terms of the theoretical transfer function and phase spectra which describe the amplitude attenuation and phase lag between frequencies in the input and output. A comparison of the frequency response of the three models indicates that the unique filtering characteristics of each may provide a diagnostic tool for matching the appropriate theory to a sampled water quality “signal.” A procedure is suggested for parameter estimation which involves comparison of the theoretical and field estimated transfer function and phase spectra.

A frequency domain correlation technique for model correlation and updating

Abstract: Up to now, existent FRF based model updating methods use the differences between measured and analytical FRFs at a fixed frequency, as residual to minimize. This approach does not take into account that FRFs between a reference model (experimental) and a perturbed one (a finite elements model not yet updated), displace in two axes: amplitude and frequency. A more physical correlation, then, uses also the frequency shift. The problem is how to find it. Taking as base the well known Modal Assurance Criterion (MAC), the Modal Scale Factor (MSF), and the concept of frequency shift, two correlation techniques, the Frequency Domain Assurance Criterion (FDAC) and the Frequency Response Scale Factor (FRSF), are presented. They help to quantify the level of correlation between responses coming from the two models, determine the frequency shift at all measured frequencies, and establish a suitable set of frequencies to use during the updating procedure.

A power-efficient audio amplifier combining switching and linear techniques

Abstract: Integrated class-D audio amplifiers are very power efficient but require an external LC reconstruction filter, which prevents further integration. Also due to this filter, large feedback factors are hard to realize, so that the load influences the distortion and transfer characteristics. The 30 W amplifier presented in this paper consists of a switching part that contains a much simpler filter and a linear part that ensures a low distortion and flat frequency response. The switching part of the amplifier was integrated in a BCD process. Combined with a linear part and with a loudspeaker as load, it has a flat frequency response /spl plusmn/0.3 dB, a dissipation that is up to five times lower than a traditional class-AB audio amplifier, and a distortion of <0.02% over power and frequency range.