Fundamental frequency

About: Fundamental frequency is a research topic. Over the lifetime, 8941 publications have been published within this topic receiving 131583 citations.

Performance limit of a passive vertical isolator using a negative stiffness mechanism

Abstract: A passive vibration isolator using a negative stiffness mechanism (NSM) is being considered for small precision instruments since it does not need any outer power supply and pressurized air, and its fundamental frequency can be lowered down to 0.5 Hz. Although the working principle of the NSM and its patents are well known, neither the isolation performance limit related to the lowest fundamental frequency nor its nonlinear behavior have been studied. This paper discusses the performance limit of the passive vertical isolator using the NSM and presents the design guidelines for the isolator based on that performance limit. First, a nonlinear dynamic model of the passive isolator is derived through solid approximations, and the fundamental frequency or performance limit is obtained using nonlinear analysis, which entirely explains the nonlinear behavior of the isolator. In addition, the approximate design equations of the isolator are derived to analyze its performance limit. Finally, an approximate expression of the lowest fundamental frequency of the isolator is derived using nonlinear analysis and design equations, which provide substantial design guidelines to improve isolator performance.

Semiconductor laser that generates second harmonic light with attached nonlinear crystal

Abstract: A semiconductor laser with a nonlinear crystal that generates second harmonic light at a frequency which is twice the fundamental frequency of the laser. The laser is configured in a vertical-cavity, surface-emitting structure or an edge-emitting structure. A nonlinear crystal is attached to a semiconductor optical amplifier by fusing with wafer bonding techniques or by epitaxially growing the nonlinear crystal on the amplifier. The amplifier and the nonlinear crystal are located inside a laser cavity that is defined between a pair of reflectors. One of the reflectors is located adjacent the nonlinear crystal and is highly reflective at the fundamental frequency but transmissive at twice the fundamental frequency. Light is generated at the fundamental frequency, doubled in frequency as it passes back and forth through the nonlinear crystal, and emitted through the reflector adjacent the nonlinear crystal. An optional region between the amplifier and the nonlinear crystal prevents reflections at the fundamental frequency or prevents light at twice the fundamental frequency from propagating from the nonlinear crystal into the amplifier.

Laser‐induced microwave sound by surface heating

Abstract: Microwave sound of discrete frequencies has been generated by the irradiation of a solid with the uniformly spaced, ultrashort optical pulses produced by a mode‐locked Nd:glass laser. The sound, rich in harmonic content, has its fundamental frequency fixed by the repetition frequency of the laser pulses. Believed to be thermally generated in the optical skin depth where the energy is absorbed, the sound has been detected at room temperature up to 2 GHz, the tenth harmonic of the fundamental repetition frequency. Such high harmonic content indicates that the acoustic pulses have rise times significantly less than 1 nsec and demonstrates the feasibility of utilizing ultrashort laser pulses to produce ultrashort acoustic pulses.

The Use of a Modified Prony Method to Track the Broken Rotor Bar Characteristic Frequencies and Amplitudes in Three-Phase Induction Motors

Abstract: The knowledge of the broken rotor bar characteristic frequencies and amplitudes has a great importance for all related diagnostic methods. The monitoring of motor faults requires a high resolution spectrum to separate different frequency components. The discrete Fourier transform (DFT) has been widely used to achieve these requirements. However, DFT can give meaningful information only for stationary harmonics which cannot be guaranteed in real cases. In addition, a long data sequence is necessary for DFT to get high frequency resolution. Nevertheless, the signals are time varying, and the steady-state conditions can be lost for a long time acquisition. As a solution for these problems, this paper proposes an efficient time-domain technique based on a modified Prony method for the estimation of the frequencies/amplitudes of broken rotor bar faults. Using this technique, the stator current is divided into short overlapped time windows, and each one is analyzed by the least squares Prony method. The proposed technique provides a linear time–frequency/amplitude representation with high frequency resolution and adjustable time resolution. It is shown that this technique allows tracking the frequencies and amplitudes of the sidebands around the fundamental frequency component with a very high accuracy. The efficiency of the proposed method is verified by simulation and experimental tests.

Radio frequency spectroscopy apparatus and method using multiple frequency waveforms

Abstract: An apparatus and method for measuring the concentration of a chemical substance in a test sample based on a technique of waveform distortion analysis is disclosed. The apparatus includes a waveform generator that generates a periodic electromagnetic signal having a plurality of frequencies simultaneously present. Preferably the signal has a fundamental frequency and simultaneously present harmonic frequencies which shape the waveform. The shaped periodic signal is transmitted through the test sample by a probe assembly which also receives the transmitted signal from the test sample. Propagation of the shaped signal through the test sample results in a change in the waveform shape or distortion by the chemical, and a detector circuit is provided that quantifies the change in shape of the signal to determine the concentration of the chemical in the test sample. The harmonics producing the waveform shape and the fundamental frequency of the signal are both selected so that the change in shape of the signal is particularly responsive to the presence of a selected chemical substance. The magnitude of the distortion or change in shape of the signal can be directly correlated to the concentration of the selected chemical substance.