Narrow Frequency Band Research Articles

Narrowband Passive RF Tags for Frequency-Selective Harmonic Doppler Radar Tracking

A design for frequency-selective radio frequency (RF) harmonic tags for harmonic Doppler radar tracking is presented. Harmonic RF tags receive an incident microwave signal and generate a retransmitted signal at a harmonic of the incident frequency, typically the second harmonic. Since environmental backscatter occurs at the fundamental frequency, the second harmonic can be easier to detect. Typical harmonic tags operate over a reasonably wide fractional bandwidth, and thus to detect multiple tags within the same field of view, additional circuitry must generally be added to the tag to differentiate the responses. We propose a different approach based on frequency selectivity implemented by the tag antenna. Based on narrowband split-ring printed antennas, the tags accept incident signals over a specified narrow fractional bandwidth, after which the second harmonic is generated by a diode and retransmitted with a standard dipole antenna. The narrowband filtering occurs at the fundamental frequency antenna, and thus with tags designed at adjacent narrowband frequency channels, no additional circuitry is necessary to differentiate multiple tags. Fabricated tags achieved fractional bandwidths below 0.5% at carrier frequencies below 2.5 GHz while maintaining gains above 2 dBi. The construction of the tag is simple and low-cost, requiring only a single component (a diode) on a printed antenna structure. We demonstrate the feasibility of simultaneously measuring the time-varying harmonic Doppler signature of multiple tags in the field of view by experimentally verifying the frequency selectivity of tags in motion using a 2.4/4.8 GHz harmonic Doppler radar.

Vibroacoustic metamaterials as add-on solution for noise reduction in existing housing structures

Vibroacoustic metamaterials (VAMM) have the potential to reduce unwanted noise components in a very targeted and narrow-band frequency range. Many VAMM concepts are based on mechanical resonators that act as vibration absorbers in their natural frequency and thus absorb energy that would otherwise be radiated in the form of airborne sound and perceived as noise. Often, during the design phase, it is not yet possible to adequately assess which surfaces will be acoustically problematic and in which frequency ranges disturbing noise components are going to be. In addition, many VAMM concepts can only be manufactured using additive manufacturing (AM) processes, due to their complex geometry. However, AM parts often have strongly anisotropic material behavior, depending on the manufacturing process, which makes a prediction of the vibroacoustic behavior even more difficult. Direct integration into casing structures during the design phase is therefore not practical and economically feasible in most cases. This paper therefore investigates the extent to which retrofitted resonators can be effectively used in existing casing structures. For this purpose, FDM-printed bending resonators made of ABS and PLA are used, which have already been measured with regard to their natural frequencies in a previous publication. Different variants are attached to a demonstrator housing and the surface vibration velocity is measured using a 3D laser scanning vibrometer, and compared with the basic variant without resonators. The radiated airborne sound is also measured. Furthermore, a comparison is made with a dynamic FEM simulation in order to be able to evaluate its prediction quality, in particular with regard to the additively manufactured resonators.

Study of multi-frequency heating based on the nonlinear response characteristics of magnetron to improve uniformity

The majority of microwave ovens on the market are powered by magnetrons, which can only produce a narrow-band frequency that has a large peak, resulting in poor uniformity. A method is proposed to improve nonuniform heating based on nonlinear response characteristics of the magnetron to produce multi-frequency. First, a multi-physics model of microwave heating is established to realize the calculation of multi-frequency heating. Second, a multi-frequency heating experimental system based on the characteristics of the magnetron is built and verified with the calculated results. The results show that the calculated results are in good agreement with the experimental results. Finally, based on the multi-physics model, the influences of different parameters, such as frequency intervals, cavity size, location of the object to be heated and material properties, on microwave heating uniformity have also been discussed. The results show that the method adopted in this paper is feasible, and the heating uniformity of microwaves is improved.

Design of Depth-Focused Electromagnetic Transmitting Scheme Based on MFSPWM Method

Aiming at the frequency-domain electromagnetic method (FEM) detection system, a depth-focused transmitting scheme based on multi-frequency sinusoidal pulse width modulation (MFSPWM) method is proposed to solve the problems of low exploration efficiency in conventional single-frequency transmitting scheme and low vertical resolution in 2n multi-frequency pseudo-random (MFPR) transmitting scheme. Depending on the target depth range and required measurement accuracy, a depth-focused waveform is designed to focus the excitation field energy on a narrow frequency band. In this article, the principle of MFSPWM method is theoretically analyzed and the expression of depth-focused waveform is derived by Fourier analysis. Then, the parameter optimization of the main frequencies is performed for the desired spectrum. Further, simulations and experimental tests were carried out to verify the feasibility and effectiveness of the proposed scheme. The results show that the proposed scheme has better spectral features with multiple closely spaced frequencies and less undesired harmonics, which ensures high vertical resolution for the specific depth target. After parameter optimization, the peak-to-peak value of the current waveform was reduced by about 40% and the effective signal of the high-frequency component was increased by about 20%. The forward simulation and field test results show that the proposed scheme has higher detection accuracy and interference resistance for the specific depth target compared to the MFPR scheme.

Evaluation of Viral Inactivation on Dry Surface by High Peak Power Microwave (HPPM) Exposure.

Previous research has shown that virus infectivity can be dramatically reduced by radio frequency exposure in the gigahertz (GHz) frequency range. Given the worldwide SARS-CoV-2 pandemic, which has caused over 1 million deaths and has had a profound global economic impact, there is a need for a noninvasive technology that can reduce the transmission of virus among humans. RF is a potential wide area-of-effect viral decontamination technology that could be used in hospital rooms where patients are expelling virus, in grocery and convenience stores where local populations mix, and in first responder settings where rapid medical response spans many potentially infected locations within hours. In this study, we used bovine coronavirus (BCoV) as a surrogate of SARS-CoV-2 and exposed it to high peak power microwave (HPPM) pulses at four narrowband frequencies: 2.8, 5.6, 8.5, and 9.3 GHz. Exposures consisted of 2 µs pulses delivered at 500 Hz, with pulse counts varied by decades between 1 and 10,000. The peak field intensities (i.e. the instantaneous power density of each pulse) ranged between 0.6 and 6.5 MW/m2 , depending on the microwave frequency. The HPPM exposures were delivered to plastic coverslips containing BCoV dried on the surface. Hemagglutination (HA) and cytopathic effect analyses were performed 6 days after inoculation of host cells to assess viral infectivity. No change in viral infectivity was seen with increasing dose (pulse number) across the tested frequencies. Under all conditions tested, exposure did not reduce infectivity more than 1.0 log10. For the conditions studied, high peak power pulsed RF exposures in the 2-10 GHz range appear ineffective as a virucidal approach for hard surface decontamination. © 2023 Bioelectromagnetics Society.

Theory, design and characterization of metamaterial absorbers: a formal assessment

Metamaterial (MTM) absorbers and their design have been of prime interest in view of their capability to absorb electromagnetic waves of high frequencies. Different types of MTM absorbers have been reported in the last two decades. Keeping this in view an attempt was made to review the progress of MTM absorbers in terms of the theory behind them, designing and construction. This paper reviewed the basic theory and design regulations of a perfect MTM absorber at high, narrow and broad band frequencies. Also we reviewed tunable frequency and coherent absorbers. This exercise was done to focus on recent developments in metamaterial absorbers and present the tested results in a more precise way

Narrowband and wideband frequency modulation spectral characterization and bandwidth calculation

Frequency modulation (FM) is a class of analog modulation techniques that rely on varying the instantaneous frequency of a radio-frequency (RF) carrier in accordance with the baseband signal. The amplitude of the FM signal is constant, while its instantaneous frequency is time variant; therefore, the zero crossing rate (ZCR) of the carrier differs at a rate proportional to the Nyquist rate of the baseband signal. Unlike amplitude modulation (AM), FM is a nonlinear modulation technique; therefore, the calculation of the spectrum of an FM signal is expected to be a tedious mathematical problem.

Design and Experimental Evaluation of a Printed Monopole Antenna for GPR Applications

In this paper, a printed monopole antenna is designed and experimentally evaluated for Ground Penetrating Radar (GPR) applications. The antenna resonates at two narrow frequency bands centered at 0.5 GHz and 2 GHz. Four stages are used to achieve the final model, which is based on a rectangular patch, a simple microstrip feed line, and a reduced ground plane. The proposed antenna consists of a simple patch with notches and a quarter-wavelength feed line. The introduced notches help to reduce the conductor losses and decrease the weight of the design. The antenna prototype is fabricated on a low-cost FR-4 epoxy substrate with dimensions of 17.55 cm3, using the LPKF S103 laser printer, and then measured with the R&S®ZNB Vector Network Analyzer. The experimental results demonstrate that the operating bands range between 0.48–0.52 GHz (8%) and 1.94–2.04 GHz (5.02%). The antenna exhibits good radiation patterns with a reasonable gain of 1.5/2.6 dBi and a high radiation efficiency of 97/78% at the two resonating frequencies 0.5/2 GHz, respectively. To validate the usefulness of the designed antenna for GPR applications, a penetrability test is conducted through a concrete separator wall. The electric field probe HZ551 is used to receive the electromagnetic waves behind the obstacle, and the power received by the probe is measured with the GSP-730 spectrum analyzer. The obtained results confirm that the proposed model performs well in the UHF band at 0.5 GHz and 2 GHz, with a high level of penetrability.

Signal Processing Steady-State Surface NMR Data

Surface nuclear magnetic resonance (NMR) is a unique technique to study groundwater as it provides direct insights into water content and pore-scale properties. However, surface NMR suffers from an extremely weak signal that limits mapping speeds and can prohibit measurements in high ambient noise environments. It has recently been demonstrated that steady-state sequences can deliver orders of magnitude signal-to-noise ratio enhancement in comparison to standard methods. Most notably, the steady-state approach collapses the surface NMR signal an extremely narrow frequency band, which leads to vastly improved noise rejection. In this paper, we develop the signal processing chain for steady-state surface NMR data. Each step in the signal processing chain, transient culling, pulse windowing, de-spiking, powerline harmonics subtraction and spectral analysis are explained in detail. We demonstrate how error estimation can be efficiently performed using the power spectral density in the vicinity of the transmitting frequency. We also demonstrate how off-resonance excitation can be determined by locating the peak spectrum of surface NMR signal with an array of phase-corrected discrete Fourier transforms. The presented signal processing methods can be easily implemented and applied to extract signal features for quality control and inversion procedures. We give a field example where data collected with steady-state pulse sequences are processed and inverted.

Full Waveform Inversion of Viscoelastic Media Based on Gradient Preconditioning

In viscoelastic media, reverse time migration (RTM) requires accurate Q value for energy compensation and phase correction. Full waveform inversion (FWI) has the potential to provide accurate Q value. In the wide frequency band seismic data, the quality factor Q almost does not change with frequency, which is called constant Q model, but its computational complexity is large. In the narrow frequency band data, when the memory variable order L is assumed to be 1 in the standard linear solid (SLS) model, the attenuation simulation effect of the subsurface Q value is acceptable, and the calculation amount is small. In this paper, we derive the back propagation wave field equation and gradient formula of full waveform inversion of viscoelastic medium based on the SLS model with memory variable order L = 1, and analyzes the effectiveness and feasibility of this method. In viscoelastic medium, low velocity body will produce low frequency noise in seismic record, which will lead to heterogeneous gradient energy. This low frequency noise is similar to the low frequency noise produced near the source of shallow strata. This will reduce the accuracy of the inversion results. The gradient preconditioning method using the pseudo-Hessian operator can make the gradient energy more balanced. Therefore, we introduce the pseudo-Hessian operator in gradient preconditioning and derives the gradient preconditioning formula for the full waveform inversion in viscoelastic media, which solves the problem of the non-uniform gradient energy. Examples show that the method can suppress the non-uniform gradient energy in the full waveform inversion of viscoelastic media.

Long memory cointegration and dynamic connectedness of volatility in US dollar exchange rates, with FOREX portfolio investment strategy

<abstract> <p>Decisions of central banks on foreign exchange rates are based on the comovement of foreign exchange (FOREX) in mature markets such as US dollar rates to the British pound, euro, Chinese yuan, Japanese yen and Australian dollar. We investigate the long-run movement and dynamic quantile connectedness of volatility among pairs of these exchange rates. The updated residual-based fractional cointegration testing framework using narrow-band frequency domain least squares estimator is used to obtain the residual series for fractional cointegration. Quantile dynamic connectedness framework for volatility spillovers at different market conditions, depicted by quantiles, are used. We find evidence of long memory cointegration in seven pairs of exchange rates involving the previously mentioned currencies. These seven cases also correspond to a higher average index of quantile connectedness, with the effect of connectedness phasing out at higher quantiles and being more visible at lower quantiles. A portfolio investment strategy using optimal portfolio weights and hedge ratios for maintaining the accrued profit at the FOREX market is also presented.</p> </abstract>

AutoSpec: detection of narrowband frequency changes in time series

AutoSpec: detection of narrowband frequency changes in time series

AutoSpec: detection of narrowband frequency changes in time series

AutoSpec: detection of narrowband frequency changes in time series

A novel active tendon pendulum tuned mass damper and its application in transient vibration control

Enhancing the performance characteristics of Tuned Mass Dampers (TMDs), in particular extending their narrow frequency band of operation, has been the subject of many researches. In the adaptive type of TMDs, the device's properties can be adjusted gradually in response to a slight alteration in the excitation characteristics and physical properties of the main structure. A novel vibration absorbing system, namely Active Tendon Pendulum TMD (AT-PTMD), is proposed in the present study. The device consists of a simple pendulum whose horizontal movement is restricted by a tendon with an adjustable tensile force which is attached to the pendulum’s rod. The device is designed and formulated, and its efficiency is assessed numerically and experimentally. For this purpose, a 3-degree of freedom structure is considered. Scrutinizing the responses of the uncontrolled, passively controlled and actively controlled structure shows the appropriate performance of the AT-PTMD in comparison with the passive TMD system in transient vibration reduction, particularly in reducing the total vibrational energy of the structure's response. Considering the achieved results, the proposed AT-PTMD system is capable of reducing the maximum displacement and acceleration of the main structure up to 43% and 37%, respectively, which substantiates its excellence over the common passive vibration absorbing devices. The case study also proves that the utilized mechanism for adjusting the equivalent stiffness of AT-PTMD increases its operating frequency range by 105% compared to common TMDs. Unlike active structural control systems, the suggested approach will not induce structural instability in any operating mode.

Seismic source mapping by surface wave time reversal: application to the great 2004 Sumatra earthquake

SUMMARY Different approaches to map seismic rupture in space and time often lead to incoherent results for the same event. Building on earlier work by our team, we ‘time-reverse’ and ‘backpropagate’ seismic surface wave recordings to study the focusing of the time-reversed field at the seismic source. Currently used source-imaging methods relying on seismic recordings neglect the information carried by surface waves, and mostly focus on the P-wave arrival alone. Our new method combines seismic time reversal approach with a surface wave ray-tracing algorithm based on a generalized spherical-harmonic parametrization of surface wave phase velocity, accounting for azimuthal anisotropy. It is applied to surface wave signal filtered within narrow-frequency bands, so that the inherently 3-D problem of simulating surface wave propagation is separated into a suite of 2-D problems, each of relatively limited computational cost. We validate our method through a number of synthetic tests, then apply it to the great 2004 Sumatra–Andaman earthquake, characterized by the extremely large extent of the ruptured fault. Many studies have estimated its rupture characteristics from seismological data (e.g. Lomax, Ni et al., Guilbert et al., Ishii et al., Krüger & Ohrnberger, Jaffe et al.) and geodetic data (e.g. Banerjee et al., Catherine et al., Vigny et al., Hashimoto et al., Bletery et al.). Applying our technique to recordings from only 89 stations of the Global Seismographic Network (GSN) and bandpass filtering the corresponding surface wave signal around 80-to-120, 50-to-110 and 40-to-90 s, we reproduce the findings of earlier studies, including in particular the northward direction of rupture propagation, its approximate spatial extent and duration, and the locations of the areas where most energy appears to be released.

Interictal localization of the epileptogenic zone: Utilizing the observed resonance behavior in the spectral band of surrounding inhibition.

The gold standard for identification of the epileptogenic zone (EZ) continues to be the visual inspection of electrographic changes around seizures' onset by experienced electroencephalography (EEG) readers. Development of an epileptogenic focus localization tool that can delineate the EZ from analysis of interictal (seizure-free) periods is still an open question of great significance for improved diagnosis (e.g., presurgical evaluation) and treatment of epilepsy (e.g., surgical outcome). We developed an EZ interictal localization algorithm (EZILA) based on novel analysis of intracranial EEG (iEEG) using a univariate periodogram-type power measure, a straight-forward ranking approach, a robust dimensional reduction method and a clustering technique. Ten patients with temporal and extra temporal lobe epilepsies, and matching the inclusion criteria of having iEEG recordings at the epilepsy monitoring unit (EMU) and being Engel Class I ≥12 months post-surgery, were recruited in this study. In a nested k-fold cross validation statistical framework, EZILA assigned the highest score to iEEG channels within the EZ in all patients (10/10) during the first hour of the iEEG recordings and up to their first typical clinical seizure in the EMU (i.e., early interictal period). To further validate EZILA's performance, data from two new (Engel Class I) patients were analyzed in a double-blinded fashion; the EZILA successfully localized iEEG channels within the EZ from interictal iEEG in both patients. Out of the sampled brain regions, iEEG channels in the EZ were most frequently and maximally active in seizure-free (interictal) periods across patients in specific narrow gamma frequency band (∼60-80 Hz), which we have termed focal frequency band (FFB). These findings are consistent with the hypothesis that the EZ may interictally be regulated (controlled) by surrounding inhibitory neurons with resonance characteristics within this narrow gamma band.

Joint DOA tracking and spectral signature estimation of wideband sources using a multi-spectral TBD-CPHD filter

In this paper, a novel method for direction of arrival (DOA) tracking of multiple unknown time-varying number of wideband emitters is presented using a random finite set (RFS) framework. This method combines multiple narrowband covariance-based measurements to form a wideband cardinalized probability hypothesis density (CPHD) filter using the parallel combination (PC) Lemma generally used in multisensor fusion. By doing this, each narrowband frequency bin acts as a separate virtual spectral sensor. The covariance-based measurements, represented by complex Wishart random matrices, also allow the method to have a track-before-detect (TBD) property, which has shown to be advantageous in very low signal-to-noise ratios (SNR). Furthermore, the proposed method can simultaneously monitor the spectrum of the environment and extract the spectral characteristics of the active sources, i.e. can work as some sort of cognitive radio (CR). The suggested algorithm is implemented using an auxiliary particle filter. The simulation results and the comparison with the known state-of-the-art approaches confirm the superiority of the proposed method in negative SNR scenarios and low number of snapshots.

Wavelet packet transform applied to active noise control system for mixed noise in nonlinear environment

The noise emitted from rotating machinery usually presents a mixture of broadband and narrowband frequency components. Hybrid functional link artificial neural network (HFLANN) system consisting of narrowband ANC (NANC) subsystem, broadband ANC (BANC) subsystem and sinusoidal noise canceller (SNC) subsystem is effective to suppress the mixed noise. However, in the conventional HFLANN system, the attenuation performance of broadband subsystem cannot meet the requirement, the narrowband subsystem cannot adapt to the nonlinear environment. To address the problems, an improved HFLANN (IHFLANN) system is proposed in this paper. By applying the wavelet packet (WP) structure, the broadband noise is decomposed into different frequency bands, which improves the attenuation performance of broadband subsystem. The FLANN structure is applied to the NANC subsystem to improve the steady-state performance in nonlinear environment. Meanwhile, to reduce the computational complexity, we introduce a simplified IHFLANN (SIHFLANN) system. Simulation results demonstrate the proposed systems achieve better attenuation performance and the simplified system reduces computational complexity without sacrificing the attenuation performance.

Development of a new spectral method for fatigue damage assessment in bimodal and trimodal Gaussian random processes

Fatigue damage assessment for marine structures subjected to random environmental loadings is an important issue at the design stage. The bimodal and trimodal Gaussian fatigue damage problems have attracted great attentions from scholars in recent decades. In this paper, several existing spectral methods for fatigue damage assessment are first reviewed, then a new spectral method, named as the improved bands method, is developed based on the spectral decomposition approach. In the present method, the power spectrum of either bimodal or trimodal Gaussian process is split into narrow frequency bands, and the total fatigue damage can then be predicted by means of summing up the damages of each individual band via a linear combination rule. A parameter μ is introduced in order to modify the zero order spectral moments, so that the effect of interaction of high-frequency process superimposed on the low-frequency process can be reflected. Through comprehensive case studies, comparisons with time-domain rain-flow counting method and the other spectral methods are conducted to validate the accuracy and robustness of the proposed method. It is demonstrated that the present method can provide accurate and reasonable fatigue damage estimations for wide ranges of bimodal and trimodal cases for marine structures.

Performance analysis and enhancement of a PVDF-based P-P sound intensity probe

This study investigates the feasibility of a laminated polyvinylidene fluoride (PVDF) structure for realising a pressure–pressure (P-P) intensity probe, as well as the effects of various backing materials on the performance of the probe. The PVDF structure consists of two parallel PVDF films that serve roles as pressure sensors, and each of them is laminated between stiff plates. The sensitivity and directivity of these pressure sensors are examined numerically and experimentally. The physical properties and configuration of the PVDF sensing structure are optimised for the best sensing performance. For frequencies below 15 kHz, the averaged voltage outputs of the two PVDF films are proportional to the sound pressure of the incident plane sound wave. It has a flat frequency response and omni-directional directivity pattern. The difference between the outputs is proportional to the particle velocity of the incident sound, featured by a dipole directivity pattern and almost-linear dependence upon frequency. Such under-covered properties of the pressure- and velocity-generated voltages for the integrated sensing structure demonstrate its suitability for intensity estimation in the aforementioned frequency range. This study shows that the probe can determine the intensity of a plane wave field below an upper-frequency limit with pre-determined probe gains. The upper limit is determined by the finite-difference approximation error introduced by the pressure gradient method. Small errors exist in narrow frequency bands around resonances and are sensitive to the pressure-intensity index, which is the ratio of the mean square pressure to the sound intensity. These errors are caused by variations of probe gains due to the resonances and can be reduced by careful calibration around the resonance frequencies.