Frequency-domain Simulations Research Articles

Dispersionless tunneling of slow light in antisymmetric photonic crystal couplers

We suggest a novel and general approach to the design of photonic-crystal directional couplers operating in the slow-light regime. We predict, based on a general symmetry analysis, that robust tunneling of slow-light pulses is possible between antisymmetrically coupled photonic crystal waveguides. We demonstrate, through Bloch mode frequency-domain and finite-difference time-domain (FDTD) simulations that, for all pulses with strongly reduced group velocities at the photonic band-gap edge, complete switching occurs at a fixed coupling length of just a few unit cells of the photonic crystal.

Mixed-signal matched filter for high-rate communication systems

A mixed-signal programmable filter suitable for high-rate communication systems is addressed. The proposed filter has analogue input and analogue-sampled outputs. The filter taps are stored in a digital memory and can be changed on the fly which is desirable in many practical applications such as adaptive filtering. The filter structure is based on a bank of digitally controlled transconductors along with small capacitors. The employed transconductors are based on simple Complementary Metal-Oxide Semiconductor (CMOS) transistors and thus can be integrated efficiently with the digital parts of systems. A cosine roll-off filter is designed and investigated by simulation in time and frequency domains. The results show that the proposed structure has a good speed-complexity-consumption trade-off.

Frequency domain simulator for mobile radio channels and for IEEE 802.16-2004 standard using measured channels

Two radio channel simulators based on the time variant frequency transfer function have been implemented using digital signal processing techniques in SIMULINK. The first simulator uses a two ray channel model to determine the number of taps per coherent bandwidth of the channel by comparing the bit error rate (BER) with the tapped delay line model for differential quadrature phase shift keying. The results show that ten taps per coherent bandwidth are appropriate for a very close approximation of the channel. The second simulator utilises real channel measurements to estimate the BER for the IEEE 802.16 -2004 wireless metropolitan area network orthogonal frequency division-multiplexing standard with 256 carriers. Channel measurements at three frequencies in the 2-6-GHz frequency band in rural/semi rural environment demonstrate the simulator and relate the BER performance of the standard to the frequency selectivity of the channel.

NOISE-BIAS COMPENSATION METHOD FOR SYSTEM IDENTIFICATION OF BUILDING STRUCTURES USING SMALL-AMPLITUDE STATIONARY RANDOM INPUT

A physical-parameter system identification method was proposed in a previous paper which enables one to eliminate the noise effects in the case of containing a noise at either one of floors just above or below a specified story. In order to examine the effect of the level and correlation of noises on the identification accuracy, a new frequency-domain simulation method is proposed for generating two stationary random processes with a specified level and correlation. It is shown that, when the previous system identification method is applied to the case including noises at both floors, the method fails to evaluate the true stiffness and damping coefficients depending on the level and correlation of noises. To resolve this problem, a new noise-bias compensation method is proposed which enables the evaluation of the level and correlation of noises before identifying the story stiffness and damping.

Frequency domain simulations of charge-density-wave strains: Comparison with electro-optic measurements

We have studied changes in charge-density-wave strain under application of square-wave currents of variable amplitude and frequency by numerically solving the phase-slip augmented diffusion model introduced by Adelman et al. [T.L. Adelman, M.C. de Lind van Wijngaarden, S.V. Zaitsev-Zotov, D. DiCarlo, R.E. Thorne, Phys. Rev. B 53 (1996) 1833]. The frequency dependence of the strain, at each position and amplitude, was fitted to a modified harmonic oscillator expression, and the position and current dependence of the fitting parameters determined. In particular, the delay time (1 /resonant frequency) vanishes adjacent to the contact and grows with distance from the contact, and both the delay time and relaxation time decrease rapidly with increasing current (and phase-slip rate), as experimentally observed in the electro-optic response of blue bronze. We have also found that pinning the phase at the contacts causes more rapid changes in strain between the contacts than allowing the phase to flow outside the contacts.

Modeling the Nonlinear Response of Multitones With Uncorrelated Phase

Traditional simulation approaches for predicting the frequency-domain response of nonlinear devices to multiple tone excitation enforce correlation of the phases of the individual tones. This is the case with time-domain simulators and transform-based schemes such as Harmonic balance as the waveform must be single valued, thus enforcing correlation. Previous efforts in frequency-domain simulators using the arithmetic operator method (AOM) also produced results in good agreement with measurements for correlated-phase input signals. Here, the AOM is applied to directly determine the spectral response of nonlinear systems to excitation by multiple uncorrelated tones in a single simulation. Verification is provided using measurements of a nonlinear amplifier excited by 15 independent tones and comparison to the average of the ensemble of results from multiple correlated-phase simulations.

Active <formula formulatype="inline"><tex>$Q$</tex></formula>-Factor and Equilibrium Stability Formulation for Sinusoidal Oscillators

Quality factor (Q-factor) and stability criterion of sinusoidal oscillators are formulated from the viewpoints of energy equilibrium and spectrum purity. Complex power works as an objective function to determine the steady-state oscillation amplitude and frequency. Conjugate product of amplitude and frequency slopes of the function dominates equilibrium stability. Logarithmic derivative of oscillator's output impedance defines active Q-factor, which takes into account effects of positive power generated by active devices, while keeping its non-divergent property. Transfer Q -factor is also defined for oscillating networks involving noise sources on different branches from the port of spectrum observation. Presented formulas can be applied to the entire oscillator circuit without de-embedding active devices, and thus they enable frequency-domain CAD simulators to numerically evaluate oscillator performance.

Efficient Cartesian-Grid-Based Modeling of Rotationally Symmetric Bodies

Axially symmetric waveguides, resonators, and scatterers of arbitrary cross section and anisotropy in the cross section can be modeled rigorously with use of 2-D Cartesian-grid-based codes by means of mere redefinition of material permittivity and permeability profiles. The method is illustrated by the frequency-domain simulations of resonant modes in a circular-cylinder cavity with perfectly conducting walls, a shielded uniaxial anisotropic dielectric cylinder, and an open dielectric sphere for which, after proper implementation of the perfectly matched layer boundary conditions, the radiation quality factor Q is also determined.

Correction of picosecond voltage pulses measured with external electro-optic sampling tips

We quantify the distortion that an external electro-optic sampling tip imprints on a measured picosecond voltage pulse propagating on a coplanar stripline. In order to characterize the sampling tip, we take advantage of the electro-optic effect of the GaAs substrate onto which the stripline is fabricated. In these measurements, the electro-optic tip just acts as a disturbance of the transmission line. We derive a correction function that allows us to eliminate the influence of the invasive sampling tip on a measured voltage pulse up to a frequency of 400 GHz. The corrected voltage pulse shows good agreement with the true, undisturbed voltage pulse obtained from a sampling measurement using the electro-optic effect of the GaAs substrate. Moreover, the experimental results are supported by finite-difference frequency-domain simulations. Our work paves the way towards a more accurate characterization of high-speed electronic devices using external electro-optic sampling tips.

Determination of the spatial TDR-sensor characteristics in strong dispersive subsoil using 3D-FEM frequency domain simulations in combination with microwave dielectric spectroscopy

The spatial sensor characteristics of a 6 cm TDR flat band cable sensor section was simulated with finite element modelling (high frequency structure simulator—HFSS) under certain conditions: (i) in direct contact with the surrounding material (air, water of different salinities, different synthetic and natural soils (sand–silt–clay mixtures)), (ii) with consideration of a defined gap of different size filled with air or water and (iii) the cable sensor pressed at a borehole-wall. The complex dielectric permittivity ε⋆(ω, τi) or complex electrical conductivity σ⋆(ω, τi) = iωε⋆(ω, τi) of the investigated saturated and unsaturated soils was examined in the frequency range 50 MHz–20 GHz at room temperature and atmospheric pressure with a HP8720D-network analyser. Three soil-specific relaxation processes are assumed to act in the investigated frequency–temperature–pressure range: one primary α-process (main water relaxation) and two secondary (α′, β)-processes due to clay–water–ion interactions (bound water relaxation and the Maxwell–Wagner effect). The dielectric relaxation behaviour of every process is described with the use of a simple fractional relaxation model. 3D finite element simulation is performed with a λ/3 based adaptive mesh refinement at a solution frequency of 1 MHz, 10 MHz, 0.1 GHz, 1 GHz and 12.5 GHz. The electromagnetic field distribution, S-parameter and step responses were examined. The simulation adequately reproduces the spatial and temporal electrical and magnetic field distribution. High-lossy soils cause, as a function of increasing gravimetric water content and bulk density, an increase in TDR signal rise time as well as a strong absorption of multiple reflections. An air or water gap works as a quasi-waveguide, i.e. the influence of the surrounding medium is strongly reduced. Appropriate TDR-travel-time distortions can be quantified.

State-space average-value modelling of capacitor-based switching stages: including conduction losses

State-space average-value models of a second-order pulse-width modulated DC-DC converter have been proposed in the literature to represent the non-ideal inductor-based switching stages. This Letter extends previous works and proposes a new full-order, state-space average-value model of fourth-order DC-DC converters, which considers the non-ideal capacitor-based switching stages. The proposed model is seamlessly functional in all operational modes and has been verified by hardware measurement and detailed simulation in both time and frequency domains.

Simulation tool for shock absorber noise prediction in time and frequency domains

This paper investigates the validity of a hydraulic simulation model to calculate the piston rod acceleration for further frequency domain analysis. At the present time, simulation results analyses of shock absorber characteristics are mostly restricted to the low frequency range for the hydraulic behaviour and the high frequency range for Noise, Vibration and Harshness (NVH) studies. However increasing demand on vehicle noise reduction is focusing more attention on high frequency matters caused by shock absorbers hydraulic effects. The most important noise concern related to shock absorbers is knocking noise. This is a structural noise caused by the vibration of chassis components excited by the shock absorber's piston rod vibrations. These vibrations are caused mainly by valves opening-closing and friction during stroke changes. The validation process is based on quantitative and qualitative comparison results between test and simulation both in time and frequency domain.

From Frequency-Domain Physics-Based Simulation to Time-Domain Modeling of Traveling-Wave Tube Amplifiers for High Data-Rate Communication Applications

We report here on a methodology to use frequency-domain physics-based nonlinear simulations of traveling-wave tubes (TWTs) in block time-domain models. We present examples using an improved version of the large-signal code CHRISTINE and a two-block baseband time-dependent model with a feedback loop that takes into account reflections internal to the TWT. The novelty in this approach is that it enables us to relate digital performance to the physical characteristics of the device, and it has the potential to significantly impact the analysis of TWT amplifiers for high data-rate applications.

Inverter EMI modeling and simulation methodologies

A numerical prediction of electromagnetic interference (EMI) allows evaluation of EMI performances at the design stage and before prototyping. It can also help reduce the post-prototype electromagnetic compatibility cost by minimizing late redesign and modifications of a drive implementation. This paper describes two simulation approaches with time- and frequency-domain simulations and verifies them with experimental results. Both time- and frequency-domain simulation approaches are found effective as long as the noise source and propagation path are properly modeled. The three-dimensional (3-D) finite-element-analysis (FEA)-based parasitic parameter extraction tool-Ansoft Spicelink has been used substantially. To gain additional degree of confidence, the results obtained from FEA are verified with closed-form solutions and actual measurements.

Dynamic simulations of hydro turbine and its state estimation based LQ control

The effect of the elasticity of the water column in the penstock of a hydro power plant represents an irrational mathematical function. In this paper, this function is reduced to a lower order using the H-infinity approximant method. The gate position-turbine power nonlinear steady state characteristic is modeled by this reduced order function, and then, the time domain and frequency domain simulations of turbine power are presented and compared with the widely known Padé approximant method for order reduction of irrational functions.

Frequency-domain method for evaluating the ride comfort of a motorcycle

In many European towns, the demand for fast and efficient mobility is frequently satisfied by means of two-wheeled vehicles. The improvement of comfort of two-wheeled vehicles used by tired and busy workers can increase safety in ground transport. Nowadays, multibody codes make it possible to predict the ride comfort of two-wheeled vehicles by means of time-domain or frequency-domain simulations. Comfort indices can be developed by post-processing the results of numerical simulations. This task is difficult, because the indices should depend on vehicle characteristics and should be independent of road quality and vehicle speed. Poor quality roads may generate nonlinear effects. Speed influences the trim of the vehicle and the wheelbase filtering, which takes place because the same road unevenness excites the front and rear wheel with a time delay which depends on the vehicle’s speed. In this paper, the comfort of two-wheeled vehicles is studied by means of a frequency-domain approach. The wheelbase filtering is averaged considering typical missions of the vehicle. The missions are journeys with a forward speed that assumes different values according to a probability density function. Indices of comfort are calculated taking into account the human sensitivity. The examples show that the proposed comfort indices depend on suspensions’ characteristics and, hence, are useful design tools. Finally, some time-domain calculations are carried out to give emphasis to nonlinear effects and to show the limits of the frequency-domain analysis.

Electromagnetic fields in the presence of an infinite dielectric wedge

Electromagnetic fields excited by a line source in the presence of an infinite dielectric wedge with refractive index N are determined by application of the Kontorovich–Lebedev transform. Singular integral equations for spectral functions are solved by perturbation procedure, and the solution is obtained in the form of a Neumann series in powers of . The devised numerical scheme permits evaluation of the higher-order terms and, thus, extends the perturbation solution to values of N not necessarily close to unity. Asymptotic approximations for the near and far fields inside and outside the dielectric wedge are derived. The combination of the Neumann-type expansion of the transform functions with the representation of the field as a Bessel function series extends solutions derived with the Kontorovich–Lebedev method to the case of real-valued wavenumbers and arbitrarily positioned source and observer. Numerical results showing the influence of wedges with various values of dielectric and magnetic constants on the directivity of a line source are presented and verified through finite-difference frequency-domain simulations.

Representations with poles and cuts for the time-domain simulation of fractional systems and irrational transfer functions

Fractional differential systems are infinite-dimensional systems which are difficult to study and simulate: they can be represented with poles and cuts. This representation applies to a wider class of irrational transfer functions, and is most useful for signal processing purposes, such as frequency-domain and time-domain simulations: the approximations in low dimension which give the most striking numerical results are obtained through an optimization procedure, the parameters of which are meaningful from a signal point of view. Ten such systems of increasing complexity are thoroughly investigated.

Design of an active suspension system for a quarter-car road vehicle model using model reference control

The use of skyhook damping for reduction of the vibration in the suspension systems is well documented. The drawback is that it is not practically feasible to obtain the equivalent of a reference point in the sky. The alternative method of using a damper between the sprung mass (the vehicle hull) and the unsprung mass (the wheel assembly) leads to a deterioration in the performance of the unsprung mass. In this paper a new method is proposed that tries to overcome the limitation of the practical skyhook damping method by the use of the model reference control (MRC) method, where the control input is utilized to achieve near-ideal skyhook sprung mass performance in a practical skyhook damping set-up. The MRC method is also applied to a quarter-car suspension model, where the control input is used to achieve the response of a system with damping and stiffness values different from those of the actual system. This allows for variation of the system constants as dictated by dynamic response needs. The proposed methods are applied on a typical quarter-car suspension system and the necessary time and frequency domain simulations are carried out to validate the theoretical predictions.

Minimizing the discrete reflectivity of perfectly matched layers

The perfectly matched layer (PML) method is widely used for truncating unbounded domains in simulations of optical waveguiding structures. In this letter, we develop a practical procedure for selecting a PML profile for a given set of discretization parameters. Our approach is designed for frequency domain simulations and it is based on minimizing the average discrete reflectivity of the PML