Small-signal Approximation Research Articles

Modeling and Design of Polymer-Based Tunneling Accelerometers by ANSYS/MATLAB

A prototype design of an inexpensive polymer-based tunneling accelerometer is described in this paper. Instead of silicon, polymethyl methacrylate (PMMA) is used as the mechanical material. By using silicon molds fabricated by conventional lithography and wet-etching techniques in hot embossing, PMMA structures can be replicated within 20 min. The performance of the tunneling sensor can be estimated and improved based on mechanical-level analysis by ANSYS and system-level analysis by MATLAB. The nonlinear tunneling mechanism and electrostatic actuation are linearized using small-signal approximation. To enhance the stability and broaden the bandwidth of the tunneling accelerometer system, a feedback control system with one zero and two poles is designed. The dynamic range of the system is greatly enhanced. The bandwidth of the closed-loop system is approximately 15 kHz.

Timing jitter smoothing by Talbot effect II Intensity spectrum

The power spectral density of the intensity of jittery trains after an integer temporal Talbot dispersive line is computed in the small-signal approximation. The influence in the spectrum of the optical linewidth and chirp of the Gaussian pulses of the train and also of different pulse-to-pulse timing jitter correlations is addressed. Before entering the Talbot dispersive line, timing jitter produces noise sidebands around the harmonics of the train. The temporal Talbot effect adds a multiplicative factor to the noise spectral density that depends on the characteristics of both the pulses and the dispersive line but not on the pulse-to-pulse correlation or the value of the timing jitter's standard deviation. The structure of this multiplicative term is peaked, resulting in narrowband noise patterns in specific locations of the spectrum and, in particular, around the harmonics of the train. Thus the temporal Talbot effect provides a dispersive mechanism for noise filtering. The bandwidth of the dispersion-induced noise peaks is ∼1 order of magnitude below the repetition-rate frequency.

Elliptically polarized pump-probe spectroscopy and its application to observation of electron-spin relaxation in GaAs quantum wells

An elliptically polarized pump-probe spectroscopy is developed. A theoretical model is derived from the rate equations of two-level system with small signal approximation to describe the experimental signal of the spectroscopy. The spectroscopy eliminates the systematic errors between theories and experimental results in present circularly polarized pump-probe spectroscopy, and is implemented easily in experiments. The initial degree of spin polarization of carriers in an excited state can be extracted by means of the spectroscopy, which is an important parameter in the investigation of spatial transport of spin polarization. The electron-spin relaxation in GaAs quantum wells is studied by using this spectroscopy.

Optimal design and applications for quasi-phase-matching three-wave mixing

Solutions of quasi-phase-matching three-wave mixing (TWM) for a nonuniform grating structure with phase-shifted segments are derived by use of a matrix operator, for the first time to the authors' knowledge, under the small-signal approximation. Compared with iterative methods of coupled-mode equations, using these expressions can significantly shorten the calculation time, and they can be effectively applied to all-optical signal processing in TWM. The signal and pump bandwidths of wavelength conversion in difference-frequency mixing, as well as the all-optical gating bandwidth in sum-frequency mixing, are optimized with the matrix solutions. By comparison with loss-free waveguide, the propagation loss a of the waveguide decreases the conversion efficiency /spl eta/, but has little influence on the bandwidth. Numerical results show that /spl alpha/ can be ignored for nonlinear lengths L<20 mm, and the optimal bandwidth /spl Delta//spl lambda/ is sensitive to /spl eta/ and the variation in the grating period, but not to /spl alpha/.

Synthesis of the modeling and control systems of a tunneling accelerometer using the MatLab simulation

In this paper, we construct the function model of a tunneling accelerometer using MatLab Simulink. With small input signal approximation, the tunneling current and electrostatic actuator nonlinear blocks are analyzed and linearized. From the derived relation between open and close loop transfer functions, we synthesize a simple but effective way to design the control system. By choosing a control system, the accelerometer bandwidth is broadened and the damping is enhanced. The system stability, root locus, step response, gain and phase margins, and pole-zero distribution are all plotted and discussed. With the designed control and feedback systems, a close loop system is stable on both parameter disturbance and frequency response. Compared with the real system at 50 ng Hz−1/2 noise level, we also present tunneling accelerometer characteristics, such as the exponential relationship between tunneling current and displacement change, time history record, dynamics and frequency response.

Model of Schottky junction admittance taking into account incomplete impurity ionization and large-signal effects

A general model for the Schottky junction reverse bias admittance is created. The model takes into account incomplete impurity ionization and does not rely on the small-signal approximation, so that impurity rate equations are not linearized. The model is valid at arbitrary temperatures for an arbitrary periodic bias amplitude and harmonic content as long as the free-carrier relaxation time is much shorter than the bias period. Impurity ionization is treated in the framework of the Shockley-Read-Hall statistics; free-carrier distribution is assumed to be an equilibrium one. The model allows calculation of the junction potential distribution and thus the junction transfer function and admittance. Junction admittance is calculated for different ac bias amplitudes and the results are compared with the data obtained from the small-signal model. It is shown that the model fits experimental junction admittance more accurately than the small-signal model when the ac component of the potential is large with respect to the thermal potential k B T/q.

Optimal design for the quasi-phase- matching three-wave mixing

Expressions for the quasi-phase-matching (QPM) three-wave mixing (TWM) with arbitrary grating structure and phase shift are obtained in this paper, for the first time, under the small-signal approximation. The expressions can be extensively applied to the all-optical signal processing for TWM, in which the signal and pump bandwidth of the wavelength conversion in DFM and the all-optical gate (AOG) bandwidth in SFM are all optimized. The optimal results from our expressions are compared with the results from the coupled-mode equations of QPM-TWM. Compared with loss free, the propagation loss in waveguides can decrease the conversion efficiency, but only a little change for the bandwidth.

The dependence of the oscillation frequency of an FEM on beam voltage

A waveguide free electron maser oscillator operating in the X-band frequency range has been constructed. It has been observed that the frequencies of oscillation, although close to the longitudinal modes of the cavity, vary slightly with electron beam voltage. It is proposed that this effect is due to the voltage-dependent variation in the phase of the radiation caused by the interaction with the electron beam. An initial comparison has been made between the observed variation of frequency with voltage and simple calculations carried out using the small signal approximation.

Semiconductor impurity parameter determination from Schottky junction thermal admittance spectroscopy

The problem of interpretation of thermal admittance spectroscopy data for semiconductor impurity parameter extraction is considered. Traditional analysis predicts that the Arrhenius plot for conductance peak temperatures is a straight line with the slope proportional to impurity activation energy and the intercept determining its capture cross section. Using a general model of the Schottky junction admittance we show that conductance peak positions strongly depend on the impurity bulk occupation number and potential distribution in the space-charge region, and, as a result, the Arrhenius plot is nonlinear for some semiconductor parameters and experimental conditions, in particular for relatively shallow impurities. In this case, the traditional linear approximation of the Arrhenius plot yields inaccurate values of activation energy and capture cross section. We propose a more accurate procedure for admittance spectroscopy data analysis involving least-squares fitting using the general and the small-signal models of the junction admittance. Although much more computationally intensive, the general model is shown to provide a better fit to the data at low temperatures, where the small-signal approximation is invalid. This approach is applied for an example admittance data and yields a better fit of the theoretical curve to the data and an improved value of activation energy for the nitrogen donor in 6H-SiC.

Analytical theory of frequency-multiplying gyro-traveling-wave-tubes

The theory is developed which describes analytically the gain and bandwidth in frequency-multiplying gyro-traveling-wave-tubes. In this theory the input waveguide is considered in the small-signal approximation. Then, in the drift region separating the input and output waveguides, the electron ballistic bunching evolves which causes the appearance in the electron current density of the harmonics of the signal frequency. The excitation of the output waveguide by one of these harmonics is considered in a specified current approximation. This makes the analytical study of a large-signal operation possible. The theory is illustrated by using it to analyze the performance of an existing experimental tube.

Theoretical Study for Wavelength Conversion of Cascaded Second-Order Nonlinearity in Silica Fiber

In this paper, A novel wavelength divison multiplexing (WDM) nets is proposed by use of cascaded second-order nonlinearities (χ(2):χ(2)) in the D-fiber grating after periodically thermal/electric-field poling, and is theoretically analyzed for the first time. The coupled mode equations of the χ(2):χ(2) nonlinearities are derived, and the analytic expressions for the electric-field amplitude of converted light wave and the conversion efficiency η are obtained under the small signal approximation, which are well consistent with numerical calculations. Both analytic expressions and numerical results show that, under the phase matching, η is proportional to the logarithm of the square of pump light power, and the 4th power of the grating length L and the second-order nonlinearity d. The calculated results also show that η and Δλ of this fiber grating can be reached over -17dB and 120nm, respectively. With the increase of L, η increases rapidly while Δλ decreases quickly. The results of simulated calculations and theoretical analysis show that the cascaded χ(2):χ(2) process is different from quasi-phases-matched difference-frequency generation.

Diffusion length in CdTe by measurement of photovoltage spectra in CdS/CdTe solar cells

Modified method of surface photovoltage (SPV), spectral response measurement, and constant photocurrent method (CPM) were applied to thin film CdS/CdTe solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated without constraints of absorption coefficients for the incident radiation and thickness of the sample assuming one space charge region (SCR) located on the CdS/CdTe interface. In addition to the diffusion length, the SPV is a function of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar cell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measurement using light penetrating from the CdS side strongly depends on the thickness of the SCR. The small signal approximation model presented here successfully explains both measured spectra and permits extraction of the diffusion length of minority carriers and thickness of the SCR in CdTe absorber. The CPM is used for determination of absorption coefficients in the CdTe layer. The absorption of this material depends on its preparation and must be known for correct evaluation of experimental data.

A generalized method for calculating the RMS values of switching power converters

In this paper an analytical method is presented which allows the exact calculation of the RMS value of the state space variables of a switching power converter, whose starting point is the unified state-space representation of the switched networks and the end result is a complete form of equations that include both the DC and RMS values. The method proposed in this paper bridges the gap which exists between the state-space technique and the small signal approximation that departs from the steady state. Ripple values are negligible, compared to the steady state values themselves. A new extended circuit model is proposed whose fixed topology contains all the elements of any DC-to-DC converter, regardless of its detailed configuration, and by which different converters can be characterized in the form of a table conveniently stored in a computer data bank to provide a useful tool for computer aided design and optimization. Finally, examples are given for buck, boost, and buck-boost regulator topologies.

Properties of the peaks of second harmonic light through Fibonacci-class ferroelectric domains

After establishing the method of constructing a class of one-dimensional (1D) Fibonacci-class quasiperiodic (FC(n)) ferroelectric domains system, we have studied the properties of the electric field of the second harmonic generation (SHG) by means of the small-signal approximation in the case of the vertically transmission. It was found that only the second harmonic light (SHL) peaks which were indexed by two special integers q and p would be the brightest and the spectra whose positions were decided by successive FC(n) integers \(\) and \(\) were perfect self-similar without considering the dispersive effect of the refractive index on SHL. The effect of the vacancies for some special spectral lines was also studied generally. The analytic results were confirmed by the numerical simulations.

Reflection of transverse waves in a structure “dielectric-piezoelectric semiconductor with current”

Reflection of a plane monochromatic transverse wave by the boundary of the acoustic contact of a dielectric with a hexagonal piezoelectric semiconductor in the presence of a longitudinal charge drift is treated in the small-signal approximation within the framework of the hydrodynamic description of a charge carrier plasma. A procedure of selecting the branches with allowance for the conversion of the quasi-acoustic mode (a refracted transverse wave) into plasma-acoustic disturbances, both in-leaking at the boundary or out-leaking from it, is proposed for the determination of the solution under the conditions of a supersonic drift and “overcritical” angles of incidence. Beyond the restrictions of White’s theory of ultrasonic wave propagation in piezoelectric semiconductors, it is demonstrated that this technique removes the defects of the solutions obtained earlier in the form of discontinuities in angular dependences of the modulus of the reflection coefficient of a transverse wave in the vicinity of the “critical” angle of incidence and leads to a solution that does not contain a resonance singularity of a polar type.

Self-normalizing method to measure the detective quantum efficiency of a wide range of x-ray detectors.

The detective quantum efficiency (DQE) is widely accepted as the most relevant parameter to characterize the image quality of medical x-ray systems. In this article we describe a solid method to measure the DQE. The strength of the method lies in the fact that it is self-normalizing so measurements at very low spatial frequencies are not needed. Furthermore, it works on any system with a response function which is linear in the small-signal approximation. We decompose the DQE into several easily accessible quantities and discuss in detail how they can be measured. At the end we lead the interested reader through an example. Noise equivalent quanta and normalized contrast values are tabulated for standard radiation qualities.

Giant magnetoimpedance in a cylindrical magnetic conductor

A rigorous treatment of the giant magnetoimpedance (GMI) in soft magnetic wires is presented. A small-signal approximation is used for a cylindrical magnetic conductor which is saturated along its axis by a static magnetic field. The general analysis of GMI includes a discussion of the influence of different parameters on the GMI and of how the calculation can be extended to nonsaturating fields. The comparison with high frequency impedance spectra of CoFeSiB wires measured with a network analyzer, including the observation of the ferromagnetic resonance peaks, confirms that the proposed model gives a satisfactory explanation for the linear GMI effect over a broad frequency range and opens the way to more refined calculations.

Computer-aided analysis of power electronic circuits by stepwise topological identification

A new approach for the time-domain simulation of power electronic circuits is developed. The methodology is based on stepwise identification of circuit topology. The simulation process automatically looks for a valid topology in every step. At each simulation instant, the circuit topology is transformed into a resistive network, giving another superior feature of this method: the avoidance of solving complicated s-domain or differential formulas. No small-signal approximation is assumed. Parasitic resistance of the reactive element is included in the circuit model. Several examples illustrating the generality and computational efficiency of this new method are presented. The simulated results are favourably verified with experimental measurements and available literature. © 1997 John Wiley & Sons, Ltd.

Analysis and design of a multiple feedback loop control strategy for single-phase voltage-source UPS inverters

This paper presents the analysis and design of a multiple feedback loop control scheme for single-phase voltage-source uninterruptible power supply (UPS) inverters with an L-C filter. The control scheme is based on sensing the current in the capacitor of the load filter and using it in an inner feedback loop. An outer voltage feedback loop is also incorporated to ensure that the load voltage is sinusoidal and well regulated. A general state-space averaged model of the UPS system is first derived and used to establish the steady-steady quiescent point. A linearized small signal dynamic model is then developed from the system general model using perturbation and small-signal approximation. The linearized system model is employed to examine the incremental dynamics of the power circuit and select appropriate feedback variables for stable operation of the closed-loop UPS system. Experimental verification of a laboratory model of the UPS system under the proposed closed-loop operation is provided for both linear and nonlinear loads. It is shown that the control scheme offers improved performance measures over existing schemes, It is simple to implement and capable of producing nearly perfect sinusoidal load voltage waveform at moderate switching frequency and reasonable size of filter parameters. Furthermore, the scheme has excellent dynamic response and high voltage utilization of the DC source.

Theory of low temperature capacitance measurements on amorphous silicon thin film solar cells

A theory of low temperature capacitance measurements on thin film amorphous silicon structures, like p-i Schottky diodes or p-i-n solar cells, is presented. As the interface effects and the glow discharge deposition process dramatically affect the intimate structure of thin film materials, the material properties of thin films are extremely different from the material properties of thick layers. The theory takes into account the effect of the finite dimensions of the semiconductor layers in order to explain the experimental results. A small signal approximation leads to a simple analytical expression of the capacitance which can be easily used for measurement fitting. An extensive analysis of parameter sensitivity is presented together with some examples of how experimental results can be interpreted. To date, low temperature capacitance measurements appear to be the first characterization technique of thin doped layers included in a thin film structure.