Linear Frequency Sweep Research Articles

An Experimental and Modeling Study on Vibro-Acoustic Response of Double-Walled Steel Cylindrical Shells

Based on precise transfer matrix method (PTMM), the analytical model of the double-walled steel cylindrical shell was setup by taking into account of the annular plate and interlayer water. Under the linear sweep frequency excitation or the fixed frequency excitation, an experimental model of the double-walled steel cylindrical shell has been designed, which is performed to gain the natural frequencies, forced vibration in air and water, underwater acoustic radiation. The analytical model was established to calculate natural frequencies, vibration acceleration level and sound pressure level and compare with the relevant experimental results. The compared results show that analytical results coincide with the experimental value and prove that the analytical model established by PTMM is reliable and credible. Meanwhile, the forced vibration behaviour of measuring positions at inner and outer shells was investigated both theoretically and experimentally. Effects of different types of external excitations on the vibration and sound radiation of the double-walled cylindrical shell are discussed. As to acoustic radiation, the acoustic excitation plays a leading role in the low-frequency range. The force excitation is a major contributor in middle-high frequency range conversely.

Nonlinearity-compensation-free optical frequency domain reflectometry based on electrically-controlled optical frequency sweep

A nonlinearity-compensation-free optical frequency domain reflectometry (OFDR) scheme is proposed and experimentally demonstrated based on the electrically-controlled optical frequency sweep. In the proposed scheme, the linear frequency sweep light is generated by propagating an ultra-narrow-linewidth continuous-wave (CW) light through an electro-optic frequency shifter which consists of a dual-parallel Mach-Zehnder modulator (DPMZM) and an electronic 90° hybrid, where the electro-optic frequency shifter is driven by a linear frequency modulated signal generated by a direct digital synthesizer (DDS). Experimental results show that the spatial resolution and signal-to-noise ratio (SNR) of the proposed OFDR scheme without the nonlinear phase compensation are comparable to those of OFDR employing a commercial tunable laser source (TLS), an auxiliary interferometer, and a software-based nonlinear phase compensation method. The proposed OFDR scheme is helpful to reduce the complexity of the optical structure and eliminate the difficulty of developing the nonlinear phase compensation algorithm.

Microwave reflectometer for density profile and turbulence measurements on the COMPASS tokamak.

The fast microwave reflectometer system on the COMPASS tokamak consists of an O-mode polarized K-band (18 GHz-26 GHz), Ka-band (26 GHz-40 GHz), and a part of U-band (40 GHz-54 GHz). The plasma density profile from the edge plasma area is measured using a fast sweeping rate up of to 6 µs of the probing wave. The processing of the reflected signal is realized by the heterodyne detection configuration based on the I/Q modulator. Two different methods of dynamic calibration of the required linear sweep frequency, together with static frequency and dispersion calibration, were used. The electron density profile was reconstructed by a spectrogram-based method with four sweeps on average. The system has the capability to measure the mid-plane low-field side electron density profile in the density range from 4 × 1018 m-3 to 3.6 × 1019 m-3. Experimental results obtained on COMPASS discharges are presented to demonstrate the performance of the diagnostics.

Real-time detection and correction of frequency sweeping non-linearities of FMCW reflectometry microwave sources

FMCW reflectometry density profile measurements will be used to complement or replace magnetic measurements for plasma position control in future plasma fusion reactors, such as ITER and DEMO. Although FMCW reflectometry is a well established diagnostic technique, on present experimental devices with short discharges (typically of a few seconds), on-line measurement quality validation and correction is not a common practice. One of the technique's main requirements is to ensure that the microwave sources always produce a linear frequency sweep within the probing frequency range. Failing to do so leads to inaccurate density profile reconstruction and, hence, to erroneous plasma position feedback to the tokamak control loops. Traditionally, static calibrations are used to build the voltage ramps driving the reflectometer's VCO (Voltage Controlled Oscillator) sources. By using extra signals like frequency markers or calibrated delay lines, the experimental reflectometry measurements can be further calibrated, off-line, to minimize frequency sweep linearity related distortions. We herein propose a technique to monitor and re-calibrate in real-time the linearity of the frequency sweeps of FMCW reflectometers. It can be applied in the long duration discharges of future devices either to correct thermal or electronic drifts of the probing frequency sources or to generate alarm signals when measurement anomalies or fundamental diagnostic malfunctions are detected. Experimental data is presented showing the benefits of the proposed technique.

Ultracold bosonic atom–molecule conversion dynamics with a linearly chirped Gaussian pulse

We investigate the conversion dynamics of ultracold bosonic atoms to molecules in an external field with a Gaussian temporal shape and a linear frequency sweep. Under the mean-field theory, we study the atom–molecule conversion efficiency and the dynamics in the conversion process. By using the Fock-state basis, we further research the quantum many-body dynamics, wherein the many-body effect on the conversion is discussed. We find that the atom–molecule conversion dynamics depend on the particle interaction, peak coupled strength, sweep rate, and total particle number. Notably, some analytical results are derived in the regime without particle interactions and in the sudden limit, respectively. Our theory indicates that the ultracold atom–molecule conversion can be achieved stably and efficiently in the Gaussian external field model.

Ultra-long range optical frequency domain reflectometry using a coherence-enhanced highly linear frequency-swept fiber laser source.

We report on an ultra-long range optical frequency domain reflectometry (OFDR) using a coherence-enhanced highly linear frequency-swept fiber laser source based on an optoelectronic phase-locked loop (OPLL). The frequency-swept fiber laser is locked to an all-fiber-based Mach-Zehnder interferometer (MZI) to suppress sweep nonlinearity and enhance the laser coherence, leading to a high coherence linear frequency sweep of 1 GHz in the duration time of 25 ms. This enables the OFDR to realize an ultra-long range measurement with a high spatial resolution. As a result, we obtain a 10 cm transform-limited spatial resolution at a 20 km fiber within 25 ms measurement time, and a 72 cm spatial resolution over an entire 200 km fiber link within 5 ms measurement time. The proposed reflectometry provides a high-performance solution with both high spatial resolution and ultra-long measurement range for field real-time fiber network monitoring and sensing applications.

Respiration resolved imaging with continuous stable state 2D acquisition using linear frequency SWEEP.

PurposeTo investigate the potential of continuous radiofrequency (RF) shifting (SWEEP) as a technique for creating densely sampled data while maintaining a stable signal state for dynamic imaging.MethodsWe present a method where a continuous stable state of magnetization is swept smoothly across the anatomy of interest, creating an efficient approach to dense multiple 2D slice imaging. This is achieved by introducing a linear frequency offset to successive RF pulses shifting the excited slice by a fraction of the slice thickness with each successive repeat times (TR). Simulations and in vivo imaging were performed to assess how this affects the measured signal. Free breathing, respiration resolved 4D volumes in fetal/placental imaging is explored as potential application of this method.ResultsThe SWEEP method maintained a stable signal state over a full acquisition reducing artifacts from unstable magnetization. Simulations demonstrated that the effects of SWEEP on slice profiles was of the same order as that produced by physiological motion observed with conventional methods. Respiration resolved 4D data acquired with this method shows reduced respiration artifacts and resilience to non‐rigid and non‐cyclic motion.ConclusionsThe SWEEP method is presented as a technique for improved acquisition efficiency of densely sampled short‐TR 2D sequences. Using conventional slice excitation the number of RF pulses required to enter a true steady state is excessively high when using short‐TR 2D acquisitions, SWEEP circumvents this limitation by creating a stable signal state that is preserved between slices.

Laser frequency sweep linearization by iterative learning pre-distortion for FMCW LiDAR.

We report on a laser frequency sweep linearization method by iterative learning pre-distortion for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) systems. A pre-distorted laser drive voltage waveform that results in a linear frequency sweep is obtained by an iterative learning controller, and then applied to the FMCW LiDAR system. We have also derived a fundamental figure of merit for the maximum residual nonlinearity needed to achieve the transform-limited range resolution. This method is experimentally tested using a commercial vertical cavity surface-emitting laser (VCSEL) and a distributed feedback (DFB) laser, achieving less than 0.005% relative residual nonlinearity of frequency sweep. With the proposed method, high-performance FMCW LiDAR systems can be realized without expensive linear lasers, complex linearization setups, or heavy post-processing.

Algorithm for Data Processing In Radar Measurement Systems with Linear Transmitter Frequency Sweep

An algorithm based on a parametric model of the reflected signal for data processing in radar measurement systems with linear transmitter frequency sweep is proposed. Results of simulations using this algorithm for determining the distance of two dynamically varying targets against a noise background are shown.

Analytical Method for Stroboscopically Sampling General Periodic Functions With Arbitrary Frequency Sweep Rates

Parameter sweeps are commonly used to explore the behavior of dynamical systems. This paper derives exact solutions for the instances in time to stroboscopically sample the response of a dynamical system subject to varying input excitations. This work will enable more accurate bifurcation diagrams and Poincaré sections in parameter regimes where numerical approaches may lead to incorrect behavior characterization. The simplest case of a linear frequency sweep is first considered before generalizing the results to include more complex functions with nonlinear sweep rates and arbitrary phase shifts.

Numerically optimized modulations for adiabatic pulses in surface nuclear magnetic resonance

Adiabatic pulses, which provide an effective means of generating a large-amplitude nuclear magnetic resonance (NMR) signal in the presence of a heterogeneous magnetic field, have the potential to greatly improve the signal-to-noise ratio of the surface NMR experiment. To ensure efficient implementation of adiabatic pulses into the surface NMR framework, a numerically optimized modulation (NOM) approach is used to design adiabatic pulses specifically intended for application in surface NMR. The scenario in which the frequency response of the tuned transmit coil is used to modulate the current amplitude is considered. The performance of a NOM pulse is contrasted against two alternative adiabatic pulses (described by a linear frequency sweep and a hyperbolic tangent sweep) that are currently implemented with the existing hardware. The NOM approach provides equivalent excitation as the chirp and hyperbolic tangent pulse while shortening pulse durations and reducing power consumption. Furthermore, the NOM approach also provides sharp resolution and large signal amplitudes. Considerations for the design of the NOM adiabatic pulse for surface NMR are given, as well as a discussion about their implementation into the surface NMR experimental framework.

Exact quantification by the nonparametric fast Pad\xe9 transform using only shape estimation of high-order derivatives of envelopes

This study deals exclusively with total shape spectra (envelopes). Yet, the goal is to exactly reconstruct all the components of the given envelope by relying only upon non-parametric signal processors (shape estimators). To this end, the so-called derivative envelope spectra are investigated. A derivative spectrum is the result of the application of the differentiation transform left( {hbox {d}/hbox {d}nu } right) ^{m} to the given conventional spectrum. Here, non-negative integer m is the order of differentiation and nu is the real linear sweep frequency. For the customary envelope (the hbox {zero}{mathrm{th}}-order derivative, m=0), we use the non-parametric fast Padé transform to generate the derivative fast Padé transform (dFPT). Explicit computations are carried out by successively increasing the differentiation order m from low through intermediate to high values of derivatives of complex envelopes. The dFPT can disentangle the spectrally crowded regions by splitting apart any multiplet of closely packed peaks. Hidden resonances, even those that are very weak, can be not only visualized, but also exactly quantified by the dFPT, despite performing shape estimations alone. Most importantly, while the envelopes in the derivative fast Fourier transform exhibit huge noise amplification with increasing m, the same-order of the differentiation transform in the dFPT acts as an effective noise suppressor. The results of the dFPT are illustrated for the envelopes with overlapping peaks stemming from synthesized noise-free and noise-contaminated time signals associated with encoding by in vitro proton magnetic resonance spectroscopy (MRS) of breast cancer tissue. This new methodology is anticipated to significantly enhance resolution as well as signal-to-noise ratio and the overall performance of single-voxel MRS in clinical diagnostics. It is also expected to be of special benefit for volumetric coverage of the scanned tissue by magnetic resonance spectroscopic imaging.

Experimental Study of Vibration Isolation Characteristics of a Geometric Anti-Spring Isolator

In order to realize low-frequency vibration isolation, a novel geometric anti-spring isolator consisting of several cantilever blade springs are developed in this paper. The optimal design parameters of the geometric anti-spring isolator for different nonlinear geometric parameters are theoretically obtained. The transmissibility characteristic of the geometric anti-spring isolator is investigated through mathematical simulation. A geometric anti-spring isolator with a nonlinear geometric parameter of 0.92 is designed and its vibration isolation performance and nonlinearity characteristic is experimentally studied. The experiment results show that the designed isolator has good low-frequency vibration isolation performance, of which the initial isolation frequency is less than 3.6 Hz when the load weight is 21 kg. The jump phenomena of the response of the isolator under linear frequency sweep excitation are observed, and this result demonstrates that the geometric anti-spring isolator has a complex nonlinearity characteristics with the increment of excitation amplitude. This research work provides a theoretical and experimental basis for the application of the nonlinear geometric anti-spring low-frequency passive vibration isolation technology in engineering practice.

Linear and Nonlinear Rheology Combined with Dielectric Spectroscopy of Hybrid Polymer Nanocomposites for Semiconductive Applications.

The linear and nonlinear oscillatory shear, extensional and combined rheology-dielectric spectroscopy of hybrid polymer nanocomposites for semiconductive applications were investigated in this study. The main focus was the influence of processing conditions on percolated poly(ethylene-butyl acrylate) (EBA) nanocomposite hybrids containing graphite nanoplatelets (GnP) and carbon black (CB). The rheological response of the samples was interpreted in terms of dispersion properties, filler distortion from processing, filler percolation, as well as the filler orientation and distribution dynamics inside the matrix. Evidence of the influence of dispersion properties was found in linear viscoelastic dynamic frequency sweeps, while the percolation of the nanocomposites was detected in nonlinearities developed in dynamic strain sweeps. Using extensional rheology, hybrid samples with better dispersion properties lead to a more pronounced strain hardening behavior, while samples with a higher volume percentage of fillers caused a drastic reduction in strain hardening. The rheo-dielectric time-dependent response showed that in the case of nanocomposites containing only GnP, the orientation dynamics leads to non-conductive samples. However, in the case of hybrids, the orientation of the GnP could be offset by the dispersing of the CB to bridge the nanoplatelets. The results were interpreted in the framework of a dual PE-BA model, where the fillers would be concentrated mainly in the BA regions. Furthermore, better dispersed hybrids obtained using mixing screws at the expense of filler distortion via extrusion processing history were emphasized through the rheo-dielectric tests.

Design of Q-band FMCW reflectometry for electron density profile measurement on the Joint TEXT tokamak

The Q-band (33–50 GHz) fast sweep frequency modulated continuous wave (FMCW) reflectometry has been recently developed for electron density profile measurement on the Joint TEXT tokamak. It operates in ordinary mode (O-mode) with a 20 μs sweeping period, covering the density range from 1 × 1019 m−3 to 3 × 1019 m−3. On the bench test, a Yttrium Iron Garnet (YIG) filter is used for the dynamic calibration of the voltage controlled oscillator (VCO) to obtain a linear frequency sweep. Besides, the use of a power combiner helps to improve the side-band suppression level of the single side-band modulator (SSBM). The reconstructed density profiles are presented, which demonstrate the capability of the reflectometry.

Spectral control of dispersion‐tuned wavelength‐swept laser

Spectral profile control method of a dispersion-tuned fibre laser is demonstrated. A dispersion-tuned fibre laser is a fast and widely tunable wavelength-swept laser without mechanical tunable filters. The lasing centre wavelength is tuned by changing the active mode-locking modulation frequency to that of the corresponding operating wavelength matching the fundamental repetition rate of the cavity. The sweep spectra profile can be controlled by modulation frequency sweep. By applying a pre-calculated modulation frequency sweep, the flatness of the wavelength-swept spectral profile can be improved. We applied the dispersion-tuned wavelength-swept laser to a spectral-domain optical coherence tomography system as a wideband light source and successfully obtained OCT images. The image resolution is enhanced by applying a nonlinear modulation frequency sweep to the laser instead of linear modulation frequency sweep. The potential of the proposed dispersion-tuned wavelength-swept laser as a spectrally flexible laser source is demonstrated.

Stability assessment of non-aqueous polymer dispersions through viscous flow and linear viscoelastic rheological tests

This work studies the microstructure and stability against aggregation of non-aqueous polymer dispersions. Styrene-co-acrylonitrile particles were sterically stabilized by block copolymer dispersants composed of one block adsorbed on the particle surface and the counter-part soluble in the continuous medium (a liquid polyether). A comprehensive rheological characterization (by steady and transient flow tests and linear viscoelastic measurements) and a microstructural study (by SEM microscopy, particle size distribution, etc.) were conducted on systems with different particle volume fraction, dispersant concentration and styrene to acrylonitrile ratio. A trend to particle aggregation, related to either low particle surface dispersant coverage or weakly dispersant adsorption onto the particle surface, has been identified through viscous flow tests, mainly assessing shear-thickening phenomena related to flow-induced particle aggregation, and by linear viscoelastic frequency sweep measurements. This work shows the use of rheological techniques as a powerful tool for stability testing and dispersant design of these sterically stabilized dispersions.

Rheological properties of waterborne polyurethane paints

The rheological properties of two specific waterborne polyurethane (PU) paints were studied by both macrorheological and microrheological methods. During the macrorheological measurement on a rotary rheometer, evaporation of solvent cannot be totally excluded, which has an influence on the reliability of rheological results. So, the linear oscillatory frequency sweep results (storage and loss modulus versus frequency) and steady shear results (viscosity versus shear rate) got from the rotary rheometer measurement are only used for qualitative analysis. As the evaporation of solvent can be neglected during microrheological measurements on a diffusing wave spectroscope (DWS), the results of storage modulus (G′) and loss modulus (G″) versus frequency are more credible than the results obtained from the rotary rheometer measurement. Thus, the results of G′ and G″ versus frequency from DWS measurements are used for quantitative analysis in this work. The G′ for both of the waterborne PU paints are larger than G″ at low frequency and that is opposite at high frequency in the experimental angular frequency range. The values of modulus at same frequency and viscosity at low shear rate for the two PU paints have apparent difference, which determines the difference of their application.

Optimized ultrasonic attenuation measures for non-homogeneous materials

In this paper the study of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials is addressed. To determine the attenuation accurately over a wide frequency range, it is necessary to have suitable excitation techniques. Three kinds of transmitted signals have been analysed, grouped according to their bandwidth: narrowband and broadband signals. The mathematical formulation has revealed the relation between the distribution of energy in their spectra and their immunity to noise. Sinusoidal and burst signals have higher signal-to-noise ratios (SNRs) but need many measurements to cover their frequency range. However, linear swept-frequency signals (chirp) improve the effective bandwidth covering a wide frequency range with a single measurement and equivalent accuracy, at the expense of a lower SNR. In the case of highly attenuating materials, it is proposed to use different configurations of chirp signals, enabling injecting more energy, and therefore, improving the sensitivity of the technique without a high time cost. Thus, if the attenuation of the material and the sensitivity of the measuring equipment allows the use of broadband signals, the combination of this kind of signal and suitable signal processing results in an optimal estimate of frequency-dependent attenuation with a minimum measurement time.

Neural representation of dynamic frequency is degraded in older adults

Older adults, even with clinically normal hearing sensitivity, often report difficulty understanding speech in the presence of background noise. Part of this difficulty may be related to age-related degradations in the neural representation of speech sounds, such as formant transitions. Frequency-following responses (FFRs), which are dependent on phase-locked neural activity, were elicited using sounds consisting of linear frequency sweeps, which may be viewed as simple models of formant transitions. Eighteen adults (ten younger, 22–24 years old, and nine older, 51–67 years old) were tested. FFRs were elicited by tonal sweeps in six conditions. Two directions of frequency change, rising or falling, were used for each of three rates of frequency change. Stimulus-to-response cross correlations revealed that older adults had significantly poorer representation of the tonal sweeps, and that FFRs became poorer for faster rates of change. An additional FFR signal-to-noise ratio analysis based on time windows revealed that across the FFR waveforms and rates of frequency change, older adults had smaller (poorer) signal-to-noise ratios. These results indicate that older adults, even with clinically-normal hearing sensitivity, have degraded phase-locked neural representations of dynamic frequency.