Fundamental Oscillation Frequency Research Articles

Frequency-doubling optoelectronic oscillator with fundamental frequency signal cancellation

A frequency-doubling optoelectronic oscillator (OEO) with fundamental frequency signal cancellation based on a dual-polarization Mach–Zehnder modulator (DPol-MZM) is proposed and experimentally demonstrated. In the proposed structure, two different modulation modes can be utilized to realize the frequency-doubling OEO. One is the carrier-suppressed double-sideband (CS-DSB) modulation, and the other is double-sideband (DSB) modulation. In both cases, one path is used to achieve the fundamental frequency oscillation in the OEO loop through the utilization of an equivalent or conventional DSB modulation, and the other path is employed to realize the frequency-doubled signal. In the realization of the frequency-doubled signal, the fundamental frequency signal can be cancelled by introducing an additional pure optical carrier with proper power and its polarization direction opposite to that of the modulated optical signal. A high sideband suppression ratio of the frequency-doubled signal up to 40 dB is achieved.

Impact of a dark boson mediated feeble interaction between dark matter and hadronic matter on the f -mode oscillation of neutron stars

We studied the possible presence of dark matter (DM) in neutron stars (NSs) and the structural properties of the DM admixed NSs (DMANSs) in one of our recent works [1]. The feeble interaction between the fermionic DM (χ) with the hadronic matter is introduced through a dark scalar (ϕ) and a dark vector (ξ) boson as mediators. The allowed range of the mass of the fermionic DM (mχ), for a particular range of DM Fermi momentum (kFχ), was obtained in the same work [1] with respect to the various astrophysical constraints on the structural properties of compact stars viz., the mass, radius, and tidal deformability. The present work is dedicated to the calculation and study of nonradial oscillation of the DMANSs using Cowling approximation. We particularly investigate the effect of the presence of DM on the fundamental (f) mode oscillation frequencies of the DMANSs utilizing the previously obtained range of mχ for four different hadronic models. In this work we thoroughly investigate how the individual and combined effects of mχ and kFχ affect the f-mode oscillation frequency. Within the framework of our DMANS models, for a particular value of kFχ, the range of fmaxDMANS, with respect to the allowed range of mχ, is also obtained in the present work for four different hadronic models. Since in the present era, the 1.4 and 2.01M⊙ NSs are of special interest after the detection of GW170817 and PSR J0740+6620, we particularly investigate, for the four hadronic models, the range of f1.4DMANS and f2.01DMANS with respect to the acceptable range of mχ corresponding to the range of kFχ. Published by the American Physical Society 2024

Fault detection method of new energy vehicle engine based on wavelet transform and support vector machine

In order to solve the problem that existing methods are affected by fundamental frequency and harmonic frequency oscillation of new energy vehicle engine, a new energy vehicle engine fault detection method based on wavelet transform and support vector machine is proposed. Firstly, a detection model of abnormal noise signal of automobile engine fault is established, and the time-frequency parameters of basis function are adjusted adaptively. Then, the mechanical excitation component and the battery excitation component in the engine surface radiation noise are separated, and the new energy vehicle engine fault signal is decomposed by feature decomposition and multi-scale separation. Finally, wavelet transform combined with support vector machine algorithm was used to extract fault features of new energy vehicles, fuzzy clustering was carried out, and time-frequency analysis of fault signals was carried out in fractional Fourier domain to realize fault detection of new energy vehicle engines. The test results show that the method has high fault identification level and good clustering of fault feature points for new energy vehicle engine fault detection, and can effectively improve the ability of engine fault detection with good application effect.

Novel cyclic homogeneous oscillation detection method for high accuracy and specific characterization of neural dynamics.

Determining the presence and frequency of neural oscillations is essential to understanding dynamic brain function. Traditional methods that detect peaks over 1/f noise within the power spectrum fail to distinguish between the fundamental frequency and harmonics of often highly non-sinusoidal neural oscillations. To overcome this limitation, we define fundamental criteria that characterize neural oscillations and introduce the cyclic homogeneous oscillation (CHO) detection method. We implemented these criteria based on an autocorrelation approach to determine an oscillation's fundamental frequency. We evaluated CHO by verifying its performance on simulated non-sinusoidal oscillatory bursts and validated its ability to determine the fundamental frequency of neural oscillations in electrocorticographic (ECoG), electroencephalographic (EEG), and stereoelectroencephalographic (SEEG) signals recorded from 27 human subjects. Our results demonstrate that CHO outperforms conventional techniques in accurately detecting oscillations. In summary, CHO demonstrates high precision and specificity in detecting neural oscillations in time and frequency domains. The method's specificity enables the detailed study of non-sinusoidal characteristics of oscillations, such as the degree of asymmetry and waveform of an oscillation. Furthermore, CHO can be applied to identify how neural oscillations govern interactions throughout the brain and to determine oscillatory biomarkers that index abnormal brain function.

Formula for two-sided estimation of the fundamental frequency of oscillations of a lattice truss

Formula for two-sided estimation of the fundamental frequency of oscillations of a lattice truss

Dynamic performance of terahertz quantum cascade laser with double optical feedbacks: Role of the external cavity

The dynamic performance of a terahertz (THz) quantum cascade laser (QCL) with double optical feedbacks (DOFs) is carefully studied within the framework of single-mode rate equations. The results have shown that the system displays three types of nonlinear dynamic phenomena under the control of different feedback strengths, which are envelope oscillations, spiking oscillations, and high-frequency small-amplitude oscillations, respectively. The fundamental frequency of oscillation is related to the ratio of the round-trip times from two external cavities and the ratio of their feedback strengths. By solving the steady-state equations, we find that the high-frequency, small-amplitude oscillations arise because a pair of modes appear at almost the same point. The output performance of THz QCL system is also studied with square wave signal modulation. The results show that the fundamental frequency of oscillation at the output is only related to the frequency of the square wave signal. And low frequency fluctuations (LFFs) dynamics can be excited by combination of DOFs and adjusting the duty cycle of the square wave signal. This work provides a novel insight into high data-bit THz communication based on QCLs.

The improved 2DoFs-PLL for MMC-HVDC transmission system

The performance of a modular multilevel converter (MMC) is highly related to the three-phase phase-locked loop (PLL). The presence of the DC component, the harmonic component, and the negative sequence component results in the poor dynamic and steady-state performance of the PLL, such as fundamental frequency oscillations in the phase estimated by the PLL. In order to suppress the influence of parameters variation and disturbance of the three-phase grid voltage, an improved PLL with a moving average filter with two degrees of freedom PID (2DoFs-PID) is proposed. Moreover, to improve the phase margin and gain margin, an extra compensator with a pole and a zero is introduced into the 2DoFs-PLL. The improved 2DoFs-PLL can track the frequency and phase with superior accuracy, stability, and rapidity. Furthermore, the methodology for the parameters design of the 2DoFs PID controller is introduced, based on the index of integrated time and absolute error. A simulated model and experimental platform are established to verify the performance of the improved 2DoFs-PLL. The results show that the improved 2DoFs-PLL can meet the needs of both steady-state performance and dynamic performance by employing the 2DoFs-PID controllers.

Multiple frequency response points identification through single asymmetric relay feedback experiment

In this paper a methodology to identify several points of the frequency response of a system using a single experiment is proposed. The identification is performed using the information obtained from an asymmetric relay feedback experiment. The frequency response points that are estimated correspond to the fundamental oscillation frequency induced by the asymmetric relay and its harmonics. The method is easy to implement since it only requires linear algebra operations, but relies on a proper data selection, which is largely studied, to obtain the most accurate results. The proposed method allows a Least Squares formulation, which has also been studied, and presents some benefits in terms of accuracy for certain cases. The presented results are validated experimentally using a practical identification case.

Strong flame interaction-induced collective dynamics of multi-element lean-premixed hydrogen flames

Multi-element injector configurations, typically comprising numerous small-scale jet nozzles, will be required to enable reliable lean-premixed gas turbine combustion operating with pure hydrogen or high hydrogen content fuels. The integration of large numbers of millimeter-scale injectors in tightly clustered arrays is highly likely to produce peculiar topological states determined by the collective dynamics of strongly interacting premixed hydrogen flames. To understand the influence of inter-element flame interactions in a multi-element nozzle environment, here we experimentally investigate the combustion dynamics of two different pure hydrogen flame ensembles, one relatively coarse array of 293 round jet nozzles, and the other a dense array of 421 nozzles. Measurements of self-induced instability were conducted for both configurations under 60–130 thermal power conditions, in conjunction with phase-synchronized OH PLIF and high-speed OH∗ chemiluminescence emission measurements. We demonstrate that for both cases the characteristic dimension of a single injector element is the main determinant of the fundamental frequency of self-induced pressure oscillations, and that the oscillations are primarily driven by the combination of coherent structure-related flame surface rollup and local extinction/pinch-off. Whereas the coarse injector arrangement exhibits well-organized parallel propagation and deformation of isolated vortical structures, the dense array case manifests strong repulsive interactions between adjacent coherent structures, naturally creating lateral flame surface modulations as well as limited streamwise oscillations. Importantly, collision-induced lateral flame dynamics play a mechanistic role in the substantial growth of higher harmonics in the frequency range of 1–2 kHz, leading to the creation of multiple frequency excitation states of commensurate amplitude, which uniquely define the thermoacoustic state of clustered lean-premixed hydrogen flames.

Influence of the properties of the plate surface on the oscillations of the cramped drop

We consider free and forced oscillations of a clamped liquid drop. The drop is surrounded by an incompressible fluid of a different density. In equilibrium, the drop has the form of a circular cylinder bounded axially by parallel solid planes, and the contact angle is right. These plates have different surface (chemical, mechanical, and geometrical) properties. The solution is represented as a Fourier series in eigenfunctions of the Laplace operator. The resulting system of complex equations for unknown amplitudes was solved numerically. The fundamental frequency of free oscillations can vanish in a certain interval of values of the Hocking parameter. The length of this interval depends on the aspect ratio of the drop. Frequencies of other eigenmodes of the drop decrease monotonically with increasing Hocking parameters.

A definition of the asymptotic phase for quantum nonlinear oscillators from the Koopman operator viewpoint.

We propose a definition of the asymptotic phase for quantum nonlinear oscillators from the viewpoint of the Koopman operator theory. The asymptotic phase is a fundamental quantity for the analysis of classical limit-cycle oscillators, but it has not been defined explicitly for quantum nonlinear oscillators. In this study, we define the asymptotic phase for quantum oscillatory systems by using the eigenoperator of the backward Liouville operator associated with the fundamental oscillation frequency. By using the quantum van der Pol oscillator with a Kerr effect as an example, we illustrate that the proposed asymptotic phase appropriately yields isochronous phase values in both semiclassical and strong quantum regimes.

Critical Angle and Fundamental Frequency of Symmetric Airfoils at Low Reynolds Numbers

The unsteady vortex evolution of NACA 0012 airfoil is numerically investigated at low angles of attack ranging from 0° to 10° where the separation is performed from the trailing edge. The Reynolds number ranges between 1000 and 4000. The laminar separation bubble at the trailing edge is observed and the main flow features are presented. It is found that, the increase of the angle of attack and Reynolds number result higher lift to drag ratio by an extensive decrease of the drag coefficient below 8° angle of attack. The transition from the steady condition to periodic force evolution has been revealed with a detailed flow field analysis at different angles of attack and Reynolds numbers. The critical angle is defined as the angle of attack where the onset of oscillations starts with a dominant fundamental frequency of oscillation. The angle of attack where the laminar separation bubble (LSB) is observed is also revealed in the current study. Both the LSB formation angle and the critical angle of attack is found to decrease with the increase of the Reynolds number from 1000 to 4000.

АНАЛИТИЧЕСКАЯ ОЦЕНКА ПЕРВОЙ ЧАСТОТЫ СОБСТВЕННЫХ КОЛЕБАНИЙ МНОГОПРОЛЕТНОЙ ФЕРМЫ

The task is to obtain the dependence of the lower limit of the fundamental frequency of natural oscillations of a statically determinate truss on the number of spans. The upper belt of the truss is straight, the lower belt of spans is arched. The inertial properties of the truss are modeled by the same concentrated masses in the nodes. Each mass has two degrees of freedom. The forces in the rods and the reactions of the supports are found by cutting out the nodes in symbolic form from the solution of the general system of equations for the equilibrium of the truss nodes. All transformations are made in the Maple computer mathematics system. To find the structural stiffness matrix, the Maxwell – Mohr formula is used. The first natural frequency of the structure is found using the approximate Dunkerley method. The generalization of a series of individual solutions found for trusses with a different number of spans to an arbitrary number of spans is performed by induction using Maple system operators. For comparison, the lowest oscillation frequency of a system with many degrees of freedom obtained numerically is used. A formula is derived for the lower estimate of the first frequency depending on the size of the truss and the number of spans. Comparison of the analytical solution with the numerical one shows a good degree of approximation of the formula for the first frequency. It is shown that the error significantly depends on the height of the truss. Cases of kinematic variability of the structure with an even number of spans are revealed.

Radial Oscillations of Quark Stars Admixed with Dark Matter

We investigated compact stars consisting of cold quark matter and fermionic dark matter treated as two admixed fluids. We computed the stellar structures and fundamental radial oscillation frequencies of different masses of the dark fermion in the cases of weak and strong self-interacting dark matter. We found that the fundamental frequency can be dramatically modified and, in some cases, stable dark strange planets and dark strangelets with very low masses and radii can be formed.

Monte Carlo evaluation of GaN THz Gunn diodes

The performances of GaN-based Gunn diodes have been studied extensively for more than two decades, however, the diverging electron drift velocity characteristics employed in these studies merit a review of the potential of GaN Gunn diodes as THz sources. A self-consistent analytical-band Monte Carlo (MC) model capable of reproducing the electron drift velocity characteristics of GaN predicted theoretically by the first-principles full band MC model is used in this work to evaluate systematically the performance of GaN Gunn diodes in transit time mode. The optimal fundamental frequency of a sustainable current oscillation under a DC bias is determined as a function of the length of its transit region. The MC model predicts a GaN Gunn diode with a transit length of 500 nm capable of operating at frequencies up to 625 GHz with an estimated output power of 3.0 W. An MC model takes into account the effect of defects in order to replicate the much lower electron drift velocity characteristics derived from experimental work and predicts THz signal generation of 2.5 W at highest sustainable operating frequency of 326 GHz in a Gunn diode with a transit length of 700 nm.

Decayless Kink Oscillations Excited by Random Driving: Motion in Transitional Layer

In this article we study the plasma motion in the transitional layer of a coronal loop randomly driven at one of its footpoints in the thin-tube and thin-boundary-layer (TTTB) approximation. We introduce the average of the square of a random function with respect to time. This average can be considered as the square of the oscillation amplitude of this quantity. Then we calculate the oscillation amplitudes of the radial and azimuthal plasma displacement as well as the perturbation of the magnetic pressure. We find that the amplitudes of the plasma radial displacement and the magnetic-pressure perturbation do not change across the transitional layer. The amplitude of the plasma radial displacement is of the same order as the driver amplitude. The amplitude of the magnetic-pressure perturbation is of the order of the driver amplitude times the ratio of the loop radius to the loop length squared. The amplitude of the plasma azimuthal displacement is of the order of the driver amplitude times text{Re}^{1/6}, where Re is the Reynolds number. It has a peak at the position in the transitional layer where the local Alfvén frequency coincides with the fundamental frequency of the loop kink oscillation. The ratio of the amplitude near this position and far from it is of the order of ell, where ell is the ratio of thickness of the transitional layer to the loop radius. We calculate the dependence of the plasma azimuthal displacement on the radial distance in the transitional layer in a particular case where the density profile in this layer is linear.

SPECTRUM OF OWN FREQUENCIES OF A SPATIAL SURFACING GIRDER

Statement of the problem. The scheme of a statically definable girder of a spatial rectangular surfacing is discussed. The problem is to identify the formula for the dependence of the lower estimate of the first frequency of the natural oscillations of the structure by means of the Donkerley method on the number of panels. The truss has supports on the sides and consists of separate rod cells connected in pyramids. Results. Based on the analysis of the sequence of analytical solutions for the first frequency of girders with a different number of panels by induction, the coefficients in the desired formula are derived. The common members of the sequences of coefficients are found as solutions of homogeneous recurrent equations formed according to the results of the calculations using Maple operators. The resulting dependences are obtained in the form of polynomials by the number of panels. A comparison of the analytical solution with the numerical one is provided.Conclusions. An algorithm for deriving an analytical estimate of the fundamental frequency of oscillations of a spatial structure depending on the number of panels, mass, size, and elastic properties of the material is shown. The spectrum of oscillation frequencies of the structure is analyzed. The resulting dependences can be employed in seismic and structural optimization problems.

A Type-3 Modified SOGI-PLL With Grid Disturbance Rejection Capability for Single-Phase Grid-Tied Converters

This article presents a type-3 second-order generalized integrator phase-locked loop (SOGI-PLL) algorithm for single-phase system. Disturbances, such as dc offset and harmonics, are present in the practical grid voltages, due to occurrence of faults, sensors, and analog-to-digital converters. The type-3 SOGI-PLL exhibits fast dynamic performance with gain and phase-lead compensation. However, the general type-3 SOGI-PLL introduces fundamental frequency oscillations in the estimated quantities at the output, when grid voltage contains disturbances. The incorporation of filters for suppression of frequency oscillations, due to dc offset, reduces the bandwidth of PLL, which affects the dynamic response of PLL by introducing phase delay. Thus, a modified notch-filter SOGI (MNF-SOGI) based type-3 SOGI-PLL is proposed with frequency adaptive control to eliminate the phase error, without affecting the dynamic performance. A detailed analysis is presented on the effect of dc offset and scaling error. To enhance dynamic response, filtering, and stability designed guideline based on settling time is also discussed. The proposed type-3 MNF-SOGI-PLL has better grid parameter estimation and fast response with inherent dc offset rejection capability. The effectiveness of the proposed type-3 MNF-SOGI-PLL is evaluated under various grid disturbance conditions and experimentally verified.

Influence of the Properties of the Plate Surface on the Natural Oscillations of the Clamped Drop

We consider free oscillations of a clamped liquid drop. An incompressible fluid of different density surrounds the drop. In equilibrium, the drop has the form of a circular cylinder bounded axially by the parallel solid planes, the contact angle is right. These plates have different surface (wetting etc.) properties. The solution is represented as a Fourier series in eigenfunctions of the Laplace operator. The resulting system of complex equations for unknown amplitudes was solved numerically. The fundamental frequency of free oscillations can vanish in a certain interval of values of the Hocking parameter. The length of this interval depends on the ratio of the drop dimensions. Frequencies of other drop eigenmodes decrease monotonically with increasing Hocking parameter.

DC offset elimination method of Phase-locked loop based on novel generalized integrator

When the renewable energy power is connected to the grid, distortion voltage interference and grid voltage frequency fluctuation will occur, which will cause the inverter output current to be abnormal. So the grid-connected environment of weak grid puts forward higher requirements on the performance of phase-locked loop (PLL). The synchronous reference frame PLL(SRF-PLL) has become a usually adopted method in grid synchronization technique for its simple structure and strong stability. However, the conventional SRF-PLL has insufficient capability to suppress DC offset, which easily causes fundamental frequency and phase oscillations. Therefore, this paper presents a novel generalized integrator(NGI), and according to the presented NGI structure, a modified novel generalized integrator (MNGI) that can eliminate DC offset voltage is proposed. Then a novel PLL based on dual MNGI(DMNGI) is proposed. This method improves the capability of the phase-locked loop to suppress DC offset. At the same time, this paper designs the parameters of the PLL, and the rationality of the novel PLL is tested effectively through the experimental comparison with the other two methods.