Linear Frequency Sweep Research Articles

High-spatial-resolution quasi-distributed acoustic sensing with phase noise suppression based on φ-OFDR

A quasi-distributed acoustic sensor with high-spatial-resolution based on phase-sensitive optical frequency domain reflectometry (φ-OFDR) was demonstrated. The source of noise that affects the accuracy of phase demodulation was analyzed. Two low noise linear frequency sweeps (LFSs) with different sweep ranges obtained by injection-locking method were used to achieve high-spatial-resolution and long-distance sensing. The phase noise caused by laser, intensity and fading noises was effectively suppressed by further combining a femtosecond-laser-inscribed weak reflector array (WRA) and a phase index method (PIM). Vibration signals located a long distance of over 1 km were successfully demodulated with a spatial resolution of 2 cm through the use of 12 GHz LFS and a WRA with an interval of 2 cm. In addition, the 5 mm ultra-high spatial resolution capability of the system had been demonstrated through the use of 52 GHz LFS and a WRA with an interval of 5 mm.

Simultaneous optical and RF linear frequency sweep generation using a Fourier domain mode-locked OEO with carrier-suppressed single sideband modulation

Simultaneous optical and RF linear frequency sweep generation using a Fourier domain mode-locked OEO with carrier-suppressed single sideband modulation

Incorporation of intermediate wheatgrass (Thinopyrum intermedium) into bread wheat doughs: Effects on pasting and rheological properties

AbstractBackground and ObjectivesIntermediate wheatgrass (IWG), a resilient perennial grain with adaptability to difficult climates, is emerging as a compelling candidate for sustainable food solutions in bakery product applications. This study evaluated the pasting and rheological properties of doughs made from bread wheat flour and blended bread wheat–IWG flours (IWG content: 15%, 30%, 45% and 60%), with the goal of exploring the potential of IWG flour as a nutritious and sustainable ingredient in cereal‐based products.FindingsThe RVA peak, trough, and final viscosity values significantly (p < .05) decreased, and the pasting temperature increased as the IWG substitution level increased. IWG‐containing flour blends had a lower retrogradation tendency compared to the control. As the IWG flour substitution level increased, both empirical (Mixograph, Kieffer dough and gluten extensibility, Glutograph) and fundamental (linear oscillatory frequency sweep and creep–recovery) rheological measurements indicated relatively weaker dough properties imparted by the addition of the IWG flour.ConclusionsThe outcomes of this research will shed light on the impacts of utilizing alternative grains, such as intermediate wheatgrass, in bakery applications. This will provide insights that could inform the development of nutritious and sustainable baked products to meet the demands of health‐conscious consumers.Significance and NoveltyThe dough weakening effect of the IWG flour was tolerated at the 15% IWG flour substitution level. At lower substitution levels, the IWG‐containing flour blends exhibited a lower retrogradation tendency compared to bread wheat flour, which could indicate lower staling of loaves of bread during bread storage without compromising the physical quality of bakery products. Finally, the present research fills a critical gap in the literature as it represents the first application of fundamental rheological methods at known and controlled strain and stresses to evaluate the physical properties of doughs prepared from IWG flour blends.

Modelling and numerical simulation of a concentrated mass-based branch vibration

To investigate the variation patterns of spectral characteristics of main branches in fruit trees due to uneven distribution of growth, pruning, and fruit quality, as well as to determine the optimal fruit vibration harvesting, a vibration differential model of tree branches was constructed using the concentrated mass method. A three-dimensional model of the fruit tree was built using finite element software. The spectral characteristics of each branch were obtained using the linear sweep frequency method. The method of changing the mass of each branch was employed to simulate the variation in branch mass. The frequency of occurrence, mean acceleration, and coefficient of variation of the inherent frequencies were used as evaluation indicators. The results revealed that when the vibration frequency of the fruit tree was at the maximum peak frequency in the tree trunk spectrum, the frequency of occurrence and mean acceleration were optimal. And the coefficient of variation of branch acceleration was less than 1, indicating a moderate dispersion. Additionally, reducing the branch mass resulted in an increase in the maximum peak frequency of the tree trunk spectrum. Therefore, the growth, pruning, and uneven distribution of the branches in fruit trees can alter the spectral characteristics. When harvesting fruit by vibration, selecting the maximum peak frequency in the trunk vibration spectrum as the excitation force frequency yields the best fruit detachment.

Nonlinear frequency modulation for Fourier transform ion mobility mass spectrometry improves experimental efficiency

Through optimization of terminal frequencies and effective sampling rates, we have developed nonlinear sawtooth-shaped frequency sweeps for efficient Fourier transform ion mobility mass spectrometry (FT-IM-MS) experiments. This is in contrast to conventional FT-IM-MS experiments where ion gates are modulated according to a linear frequency sweep. Linear frequency sweeps are effective but can be hindered by the amount of useful signal obtained using a single sweep over a large frequency range imposed by ion gating inefficiencies, particularly small ion packets, and gate depletion. These negative factors are direct consequences of the inherently low gate pulse widths of high-frequency ion gating events, placing an upper bound on FT-IM-MS performance. Here, we report alternative ion modulation strategies. Sawtooth frequency sweeps may be constructed for the purpose of either extending high-SNR transients or conducting efficient signal-averaging experiments for low-SNR transients. The data obtained using this approach show high-SNR signals for a set of low-mass tetraalkylammonium salts (<1000 m/z) where resolving powers in excess of 500 are achieved. Data for low-SNR obtained for multimeric protein complexes streptavidin (53 kDa) and GroEL (800 kDa) also reveal large increases in the signal-to-noise ratio for reconstructed arrival time distributions.

A wideband low frequency 3D printed electromagnetic energy harvester based on orthoplanar springs

Electromagnetic energy harvesters are commonly known for their high performances in terms of power output conversion, and they are suitable for low frequency environmental vibrations.This work reports the study, design, development and experimental validation of a new extremely compact, low frequency, electromagnetic energy harvester based on two stacked ortho-planar springs, which exploits a promising magnets disposition.The device is composed by two stacked ortho-planar springs connected externally by a rigid frame. The two internal moving parts are connected to each other by a central pivot where an array of magnets is fixed on in a peculiar disposition: a half has a magnetic field directed upward, while the other half directed downward. A copper coil is wound into a housing fixed on the external frame. It comes a central massive cursor that axially moves, exploiting the springs compliance, relatively to the external frame.The prototype was almost completely realized by using Filament Fused Fabrication (FFF) additive manufacturing process in Onyx material, carbon fibre reinforced nylon.In the experimental assessment an oscillating excitation was applied on the external frame in multiple linear sweep frequency tests with different amplitude signals. The experimental validation shows a large bandwidth, from 10 to 30 Hz, and consistent output voltage and power signals.

Ultra-linear broadband optical frequency sweep for a long-range and centimeter-spatial-resolution OFDR.

We demonstrated a long-range and centimeter-spatial-resolution optical frequency domain reflectometry (OFDR) system based on an ultra-linear broadband optical frequency sweep. The high nonlinear sweeping effect of the distributed feedback (DFB) diode laser was suppressed by a pre-distortion method, ensuring that the injection-locking process remained stable during fast tuning over a large span. An optical linear frequency sweep (LFS) with a sweep range and sweep rate of up to 60 GHz and 15 THz/s, respectively, was ultimately obtained by optimizing the injection-locking system. The high performance OFDR based on the proposed LFS achieved a sampling spatial resolution of 1.71 mm. Furthermore, distributed strain sensing was implemented with high-spatial resolutions of about 5 cm and 7 cm in the measurement range over 1 km and 2 km, respectively.

Choice and impact of frequency increment in direct digital synthesiser (DDS)‐based linear FMCW radar

AbstractFMCW radar is a popular technique for radar sensors. A Direct Digital Synthesiser (DDS) is commonly used to generate the required linear frequency sweep. DDS devices are widely‐available and easy to configure in terms of the key radar parameters of sweep bandwidth and pulse duration. They generate a step‐approximation to a linear frequency sweep for which the frequency increment parameter also needs to be defined. However, there is little information available in the literature to guide radar designers on the choice of this parameter. As a result, most designs are based on an empirical estimate of the DDS frequency increment. The aim is to provide an analytic basis and design rules for the choice of this parameter. Two principal types of FMCW radar (direct sampling and deramp) are studied to determine the effect on the range profile of the finite DDS frequency increment. It is shown that a set of spurs appears around each target response. Analytic expressions are derived to quantify the amplitude and distribution of these spurs from which simple design rules are presented to allow an informed choice of DDS frequency increment. These analytic results are convincingly validated using numerical simulations and experimental measurements.

Effect of Aqueous Extracts of Quercus resinosa on the Mechanical Behavior of Bigels

Quercus resinosa leaves are rich in polyphenol compounds, however, they are unstable to several chemical and physical factors that limit their activity. Several methods have been developed to solve such problems, among which bigels can be mentioned and obtained using hydrogels and oleogels. The mechanical characterization of this type of materials is by using rheological methods. Although the use of these methods is well documented, the Carreau-Yasuda model has been little used to evaluate the effect of polyphenols on the mechanical behavior of bigels. Therefore, bigels were obtained from hydrogels (guar gum/xanthan gum, 0.5/0.5% w/v) and oleogels (sesame oil/sorbitan monostearate 10% w/w). Micrographs, linear viscoelasticity range, frequency sweep, and single shear tests were performed. The data were analyzed using ANOVA and Tukey test (p &lt; 0.05); micrographs showed linear relationship between polyphenols concentration and droplet size. Liquid fraction of bigels showed a pseudoplastic behavior, while the parameters of Carreau-Yasuda model showed that the highest value of the complex viscosity at zero shear was at the lowest concentration of extract; the relaxation time presented the lowest value at higher concentrations of extracts. These results indicate that the presence of polyphenols modifyes the mechanical behavior of bigels.

Rapid Linear Frequency Swept Frequency-Modulated Continuous Wave Laser Source Using Iterative Pre-Distortion Algorithm

We present a simple iterative pre-distortion algorithm for achieving a rapid linear frequency sweep of semiconductor lasers. The algorithm achieves the desired frequency swept linearity with only four iterations. We derive a general formula for iterative pre-distortion by establishing the relationship between the laser output frequency and the drive current. The linear frequency-swept laser source obtained by this algorithm can be used in FMCW LiDAR systems. Experimentally, we investigated the algorithm using a 1550 nm distributed feedback (DFB) laser, achieving frequency swept excursion of 30.26 GHz, and frequency swept slope of 504 THz/s. We analyzed the linearity of the frequency swept results for the fourth iteration, achieving less than 5 MHz root mean square (RMS) value of frequency swept nonlinearity.

Neural processing and perception of Schroeder-phase harmonic tone complexes in the gerbil: Relating single-unit neurophysiology to behavior.

Schroeder-phase harmonic tone complexes have been used in physiological and psychophysical studies in several species to gain insight into cochlear function. Each pitch period of the Schroeder stimulus contains a linear frequency sweep; the duty cycle, sweep velocity, and direction are controlled by parameters of the phase spectrum. Here, responses to a range of Schroeder-phase harmonic tone complexes were studied both behaviorally and in neural recordings from the auditory nerve and inferior colliculus of Mongolian gerbils. Gerbils were able to discriminate Schroeder-phase harmonic tone complexes based on sweep direction, duty cycle, and/or velocity for fundamental frequencies up to 200 Hz. Temporal representation in neural responses based on the van Rossum spike-distance metric, with time constants of either 1ms or related to the stimulus' period, was compared with average discharge rates. Neural responses and behavioral performance were both expressed in terms of sensitivity, d', to allow direct comparisons. Our results suggest that in the auditory nerve, stimulus fine structure is represented by spike timing, whereas envelope is represented by rate. In the inferior colliculus, both temporal fine structure and envelope appear to be represented best by rate. However, correlations between neural d' values and behavioral sensitivity for sweep direction were strongest for both temporal metrics, for both auditory nerve and inferior colliculus. Furthermore, the high sensitivity observed in the inferior colliculus neural rate-based discrimination suggests that these neurons integrate across multiple inputs arising from the auditory periphery.

Physiological sensitivity to across-channel temporal patterns in response to Schroeder harmonic complexes, based on sequence rather than coincidence detection

The flat-envelope harmonic complexes devised by Schroeder (1970) contain a linear frequency sweep whose direction and speed depend on phase curvature C. Physiological studies have focused on responses at a given characteristic frequency (CF), which show only a subtle dependence on the sign and magnitude of C. The question arises whether CNS neurons are sensitive to the temporal pattern across nerve fibers differing in CF. Octopus cells in the cochlear nucleus receive convergent excitatory input from nerve fibers spanning a wide range of CFs and are thought to be monaural coincidence detectors. We studied their spike output and membrane responses to Schroeder complexes in anesthetized gerbils. Octopus cells were indeed tuned to a wide frequency range but showed discrete frequency “hotspots” of differing strength. Most cells showed marked sensitivity to C, with some neurons preferring upward and others downward frequency sweeps, which was broadly stable across SPL and fundamental frequency. The sensitivity to C arises not from coincidence detection, but from a sensitivity to the temporal sequence of activation of inputs tuned to different frequencies. We conclude that marked sensitivity to Schroeder phase exists at the first CNS processing stage and is based on temporal sequence detection across frequency channels.

The nonlinear rheology of complex yield stress foods

Many foods have a yield stress that allows them to retain a desired shape at rest, but transition into a viscous fluid when being served or consumed. The determination of the yield stress of the food dictates how the foods are formed and packaged, how they are served, and how they are perceived when being eaten. Oscillatory shearing provides an ideal test protocol to map the rheology across a range of time and flow strength scales. We couple oscillatory shearing and an iterative recovery procedure to show that the yielding process is a continuous transition for two common yield stress foods. We show that unrecoverable processes from oscillatory tests are equivalent to the steady shear flow behavior. We show that this yielding behavior can be well approximated by a recently published model that treats yield stress materials as continuous viscoelastic fluids with a rate-dependent relaxation time and has parameters that can be obtained from the linear viscoelastic oscillatory frequency sweep and the steady shear flow curve.

Sensitivity to Schroeder phase arising from sensitivity to temporal patterns across nerve fibers

The flat-envelope harmonic complexes devised by Schroeder (1970) have been extensively used in psychophysical studies. The stimuli contain a linear frequency sweep, whose direction and speed depend on phase curvature C. For a given characteristic frequency, basilar membrane and nerve responses show only a subtle dependence on the sign and magnitude of C. The question arises whether CNS neurons are sensitive to the temporal pattern across nerve fibers. We studied octopus cells in the cochlear nucleus, which receive a convergent excitatory input from nerve fibers and have been hypothesized to be monaural coincidence detectors. We studied both their spike output and nerve input using multiple recording techniques in anesthetized chinchillas. Octopus cells showed marked sensitivity to C, which was broadly stable across SPL and fundamental frequency. They were tuned to a wide frequency range, but with discrete frequency “hotspots” that generate major and minor inputs. The sensitivity to C arises not from coincidence detection, but from a sensitivity to the temporal sequence of activation of inputs tuned to different frequencies. We conclude that marked sensitivity to Schroeder phase is already present at the first CNS processing stage, based on temporal sequence detection across frequency channels rather than on coincidence detection.

Second-order phase correction of NMR spectra acquired using linear frequency-sweeps

NMR spectra acquired with experiments using frequency-sweeps such as the wide-band uniform-rate smooth truncation (WURST) spin-echo and Carr-Purcell-Meiboom-Gill (CPMG) sequences cannot be absorptively phased by using only conventional zeroth- and first-order phase correction. Implementation of phase correction up to the second-order is described for obtaining absorptive spectra, which have more desirable line shapes and noise properties than magnitude spectra. The relationship of the second-order phase to the parameters of frequency sweeps is derived. The second-order phasing in the frequency-domain is equivalent to a point spread in the time-domain signal. The application of second-order phase correction is demonstrated with a wideline 35Cl CPMG spikelet spectrum.

Highly efficient iteration algorithm for a linear frequency-sweep distributed feedback laser in frequency-modulated continuous wave lidar applications

We present an iteration algorithm for generating the linear frequency sweep of a distributed feedback laser at 1550 nm both theoretically and experimentally. This iteration algorithm is convergent and can reach a desired frequency-sweep linearity with only a few iterations. Experimentally, the residual nonlinearity 1 - r 2 reaches 5.19 × 1 0 − 8 , and the relative frequency error is 0.005% with only six iterations, which has already been comparable to the linearity of the linear chirp signal generated by the external modulation method.

Fatigue performance evaluation of recycled asphalt fine aggregate matrix based on dynamic shear rheometer test

Recycled asphalt mixture has been widely used in all grades of asphalt pavement in the world, but its unstable fatigue performance limits the addition of reclaimed asphalt pavement (RAP) materials. According to the characteristics of the two hot recycled methods commonly used in China, nine kinds of fine aggregate matrix (FAM) were produced in the laboratory. Based on the torsion bar fixture of dynamic shear rheometer (DSR), linear viscoelastic (LVE) test, strain sweep, frequency sweep and time sweep test were carried out on FAM. The results showed that aging time and the increase of RAP content would have a negative effect on the fatigue performance of FAM, and the effect of aging time was more significant. Both time sweep test and strain sweep test could predict the fatigue properties of FAM, but strain sweep test could save more time. Therefore, it was suggested that strain sweep test could be used as a prediction method of fatigue properties of FAM.

Optical linear frequency sweep based on a mode-spacing swept comb and multi-loop phase-locking for FMCW interferometry.

We report on the generation of a highly coherent broadband optical linear frequency sweep (LFS) using mode-spacing swept comb and multi-loop composite optical phase-locked loop (OPLL). We exploit a specially designed agile opto-electronic frequency comb as a sweeping reference, whose mode-spacing is capable of arbitrary frequency sweep while preserving a stable phase and power distribution per mode. By locking a continuous-wave (CW) laser to any of its modes using composite OPLL with a large loop bandwidth, it allows the extraction of the optical LFS at high-order modes in a coherent manner with a multiplied sweep range and rate. With such capability, only intermediate frequency LFS with smaller bandwidth is required to yield a broadband LFS while inheriting the coherence and precision from the comb. We achieve optical LFS of 60 GHz at 6 THz/s sweep rate with a nine-folded sweep bandwidth of the driving signal. Fourier transform-limited spatial resolution at more than 80 times of the intrinsic coherence length of the CW laser is demonstrated in an OFMCW interferometry, verifying the high coherence with more than 4 orders of magnitude improvement in spatial resolution. The characteristics in terms of agility, coherence, and precision are discussed together with the potential limitations. The proposed method is capable of generating arbitrary frequency-modulated optical waveforms with a multiplied bandwidth, showing attractive potential in future metrology applications.

Laser Linear Sweep Frequency Technique Based on Delay Self-Heterodyne Optical Phase Locking and Its Application

Laser Linear Sweep Frequency Technique Based on Delay Self-Heterodyne Optical Phase Locking and Its Application

Study on rheological properties of silica nanofluids modified asphalt binder

Due to the existence of the nanoparticles (NPs) agglomeration phenomena in nanometer silica (SiO2) modified asphalt binder, solvent-free SiO2 nanofluids (NFs) were applied in asphalt modification to solve this issue in this research. The main objective is to investigate the rheological properties of SiO2 NFs modified asphalt binder. Solvent-free SiO2 NFs were manufactured based on SiO2 nanoparticles (NPs) by surface functionalization, and subsequently utilized to modify base asphalt binder with various dosages of 1 wt%, 3 wt% and 5 wt%. Fourier transform infrared spectrometer (FTIR), X-ray diffractometer (XRD), scanning electron microscope (SEM) and thermogravimetric analyzer (TGA) were conducted to investigate the microstructure characteristics and thermal stability of SiO2 NFs. The results prove that the successfully manufactured SiO2 NFs are suitable as a modifier to blend with asphalt binder. In addition, the compatibility between SiO2 NFs and base asphalt binder was also observed by SEM, indicating that the dispersion of SiO2 NFs and the compatibility with asphalt binder are much better than SiO2 NPs. Moreover, the tests of rheological property such as temperature sweep (TS), multiple stress creep recovery (MSCR), linear amplitude sweep (LAS), frequency sweep (FS) and bending beam rheometer (BBR) were executed to analyze the rutting, fatigue and crack resistances of asphalt binder effected by the SiO2 NFs. A series of test results manifest that the addition of SiO2 NFs slightly reduces the rutting resistance at high temperatures, but is favorable to improving the fatigue life and low-temperature crack resistance of asphalt binder.