Broadband Signals Research Articles

A Broadband Fixed-Beam Leaky-Wave Antenna

In point-to-point communication, antennas are often required to radiate a broadband signal in a specific direction. Traditional leaky-wave antennas (LWAs) have the features of low cost and high gain, but also the frequency-beam-scanning property, which is not suitable for this demand. In this paper an LWA with broadband fixed-beam property is designed in millimeter-wave band. The antenna is implemented on a ridge gap waveguide, and a series of microstrip branches are employed as phase delay lines. The working principle of the antenna is elaborated, and the main beam can be designed in any direction (including broadside and inclined angles) by selecting proper lengths for the microstrip branches. Side lobe levels (SLLs) can be reduced by using slots with gradually changed lengths to excite the microstrip branches. Three antennas with main beam directions in 0° and ±25° are designed and measured as verifications. In the experiment, the antennas show stable radiation properties (including beam angles and realized gains) over the band from 25 GHz to 39 GHz (a bandwidth of 43%). This LWA shows potentials in millimeter-wave broadband applications.

Flexible channel selection method based on optical combs for broadband signals receiving.

A flexible channel selection method based on optical combs is proposed for reconfigurable optical channels in this paper. Optical-frequency combs with a large frequency interval are used to modulate broadband radio frequency (RF) signals, and an on-chip reconfigurable optical filter [Proc. of SPIE, 11763, 1176370 (2021).10.1117/12.2587403] is used to perform periodic carrier separation of wideband and narrowband signals and channel selection. In addition, flexible channel selection is achieved by presetting the parameters of a fast-response programmable wavelength-selective optical switch and filter device. Channel selection only relies on the combs through the Vernier effect of the combs and the passbands for different periods and does not require the use an additional switch matrix. Finally, flexible switching between and selection of specific channels for 13 GHz and 19 GHz broadband RF signals are experimentally verified.

Promoting spintronic terahertz radiation via Tamm plasmon coupling

Spectral fingerprint and terahertz (THz) field-induced carrier dynamics demands the exploration of broadband and intense THz signal sources. Spintronic THz emitters (STEs), with high stability, a low cost, and an ultrabroad bandwidth, have been a hot topic in the field of THz sources. One of the main barriers to their practical application is lack of an STE with strong radiation intensity. Here, through the combination of optical physics and ultrafast photonics, the Tamm plasmon coupling (TPC) facilitating THz radiation is realized between spin THz thin films and photonic crystal structures. Simulation results show that the spectral absorptance can be increased from 36.8% to 94.3% for spin THz thin films with TPC. This coupling with narrowband resonance not only improves the optical-to-spin conversion efficiency, but also guarantees THz transmission with a negligible loss (∼4%) for the photonic crystal structure. According to the simulation, we prepared this structure successfully and experimentally realized a 264% THz radiation enhancement. Furthermore, the spin THz thin films with TPC exhibited invariant absorptivity under different polarization modes of the pump beam and weakening confinement on an obliquely incident pump laser. This approach is easy to implement and offers possibilities to overcome compatibility issues between the optical structure design and low energy consumption for ultrafast THz opto-spintronics and other similar devices.

Simulating electrified powertrain noise and vibration measurement signals for machine learning based fault diagnosis applications

Machine learning classification is a common method for vehicle noise and vibration (N&V) fault diagnosis which helps improve vehicle safety, comfort, and reduce maintenance cost. To improve the accuracy of classification, the model requires sufficient training data which is often expensive and time-consuming to acquire. A possible solution to resolve this limitation is to extract representative features from measurement signals and generate realistic simulatied signals based on a smaller set of high-quality N&V measurement signals. A method to simulate N&V measurement signals of rotating machineries such as electrified powertrains is proposed. In its feature extraction process, tonal and broadband components of measurement data are separated. After the separation, time-varying tonal amplitudes, broadband spectra, and various related statistical features are extracted. In the simulation process, tonal and broadband signals were simulated using the extracted features with random variations added to each feature. In the current work N&V measurement signals of rotating machinery are simulated with relatively low speed fluctuation and results are presented.

Transmission-Based Vertebrae Strength Probe Development: Far Field Probe Property Extraction and Integrated Machine Vision Distance Validation Experiments

We are developing a transmission-based probe for point-of-care assessment of vertebrae strength needed for fabricating the instrumentation used in supporting the spinal column during spinal fusion surgery. The device is based on a transmission probe whereby thin coaxial probes are inserted into the small canals through the pedicles and into the vertebrae, and a broad band signal is transmitted from one probe to the other across the bone tissue. Simultaneously, a machine vision scheme has been developed to measure the separation distance between the probe tips while they are inserted into the vertebrae. The latter technique includes a small camera mounted to the handle of one probe and associated fiducials printed on the other. Machine vision techniques make it possible to track the location of the fiducial-based probe tip and compare it to the fixed coordinate location of the camera-based probe tip. The combination of the two methods allows for straightforward calculation of tissue characteristics by exploiting the antenna far field approximation. Validation tests of the two concepts are presented as a precursor to clinical prototype development.

Detection range of airborne objects by broadband active-passive radar systems

In the interests of searching and observing aerial objects with low levels of background contrast of secondary electromagnetic radiation and signals of on-board radio-electronic means, it is required to use active-passive radar systems with identification of the belonging of unmasking signs identified by heterogeneous information sensors to one object. When constructing procedures for joint processing of information signals, the issues of determining the range of active and passive search (observation) channels become important when implementing measures to reduce the reflective properties and structural and energy secrecy of the operation of electronic equipment of aircraft. To establish the boundaries of the area controlled by the active-passive radar system, it is necessary to use energy equations for transmitting and receiving broadband signals with a priori indefinite arrival time in a radio channel with scattering and a direct radio channel. The equations are found by integrating spectral equations for inharmonic wave processes propagating in free space in the band of cyclic operating frequencies when approximating the partial directional coefficients of antennas and partial effective scattering areas of objects by power functions of cyclic frequencies. The goal of the article is identification of patterns of influence of object reflective properties and characteristics of airborne radio sources on the range of active-passive radar systems. Using energy equations of transmission-reception of non-harmonic wave processes in a radio channel with scattering and direct radio channel, analytical expressions for determining probabilistic characteristics of signal detection with a priori indefinite arrival time, the possibilities of detecting air objects by broadband active-passive radar systems have been investigated. Ratios of ranges of detection of objects have been found based on the results of receiving and processing of information signals of radar stations and intrinsic radiation of airborne radio-electronic equipment. The revealed regularities of the influence of the object reflective properties and the characteristics of airborne radio sources on the range of radar stations and passive radar complexes allow us to make a rational choice of parameters of active-passive radar system designs in order to achieve the required indicators of the effectiveness of air space control. The energy equations of transmission and reception of broadband signals in radio channels form the basis for the development of proposals to ensure the covert use of aircraft.

Source localization based on steered frequency-wavenumber analysis for sparse array.

When using a sparse array, locating the target signal of a high-frequency component is difficult. Although forecasting the direction in a sparse situation is challenging, the frequency-wavenumber (f-k) spectrum can simultaneously determine the direction and frequency of the analyzed signal. The striation of the f-k spectrum shifts along the wavenumber axis in a sparse situation, which reduces the spatial resolution required to determine the target's direction using the f-k spectrum. In this study, f-k spectra of a high-frequency signal were used for near-field source localization. Snapping shrimp sounds (5-24 kHz) from SAVEX15 (a shallow-water acoustic variability experiment conducted in May 2015) were used as the data source, and a simulation was used to evaluate the proposed method. Beam steering was performed before creating the f-k spectrum to improve spatial resolution. We found that the spatial resolution was improved, and the location of the sound source could be determined when a signal with beam steering was utilized. The shrimp sound from SAVEX15, a near-field broadband signal, was used to determine the shrimp's location (range, 38 m; depth, 100 m) and the tilt of the vertical line array. These results suggest that the proposed analysis helps to accurately estimate the location of sound source.

Instantaneous Frequency Measurement Using Photonics-Assisted Broadband Signal Generation and Processing

A photonics-based instantaneous microwave frequency measurement with broadband signal generation and processing is proposed and demonstrated. In this scheme, a broadband triangular linear frequency-modulated (LFM) signal is first optically generated, and photonic mixing of the LFM signal with a signal under test (SUT) is implemented before extracting the frequency information in the oscilloscope. In the experiment, both accurate single-frequency and multifrequency measurements are successfully realized using low-speed electronic devices. The experimental results confirm a measurement accuracy of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&lt;$</tex-math> </inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 50 MHz within a frequency range from 1 to 39 GHz.

Acoustic travel-time variability observed on a 150-km radius tomographic array in the Canada Basin during 2016-2017.

The Arctic Ocean is undergoing dramatic changes in response to increasing atmospheric concentrations of greenhouse gases. The 2016-2017 Canada Basin Acoustic Propagation Experiment was conducted to assess the effects of the changes in the sea ice and ocean structure in the Beaufort Gyre on low-frequency underwater acoustic propagation and ambient sound. An ocean acoustic tomography array with a radius of 150 km that consisted of six acoustic transceivers and a long vertical receiving array measured the impulse responses of the ocean at a variety of ranges every four hours using broadband signals centered at about 250 Hz. The peak-to-peak low-frequency travel-time variability of the early, resolved ray arrivals that turn deep in the ocean was only a few tens of milliseconds, roughly an order of magnitude smaller than observed in previous tomographic experiments at similar ranges, reflecting the small spatial scale and relative sparseness of mesoscale eddies in the Canada Basin. The high-frequency travel-time fluctuations were approximately 2 ms root-mean-square, roughly comparable to the expected measurement uncertainty, reflecting the low internal-wave energy level. The travel-time spectra show increasing energy at lower frequencies and enhanced semidiurnal variability, presumably due to some combination of the semidiurnal tides and inertial variability.

Time reversal of broadband microwave signal based on frequency conversion of multiple subbands.

Time reversal of broadband microwave signals based on frequency conversion of multiple subbands is proposed and experimentally demonstrated. The broadband input spectrum is cut into a number of narrowband subbands, and the center frequency of each subband is reassigned by multi-heterodyne measurement. The input spectrum is inversed, while the time reversal of the temporal waveform is also realized. The equivalence between time reversal and the spectral inversion of the proposed system is verified by mathematical derivation and numerical simulation. Meanwhile, spectral inversion and time reversal of a broadband signal with instantaneous bandwidth larger than 2 GHz are experimentally demonstrated. Our solution shows good potential for integration where no dispersion element is employed in the system. Moreover, this solution for an instantaneous bandwidth larger than 2 GHz is competitive in the processing of broadband microwave signals.

448 Analysis of Bipolar Re-Referencing on Spectral Composition and Spike Detection in Human Electrocorticography

INTRODUCTION: Electrocorticography (ECoG) enables the localization of epileptic seizure foci prior to surgical resection. Improving the signal-to-noise ratio is paramount, and can be accomplished with bipolar re-referencing. METHODS: Intracranial recordings for drug-resistant epilepsy were performed at UCSF using high-density ECoG grid (4 mm inter-electrode pitch), strip (10 mm), and penetrating depth (5 mm) electrodes. Raw voltages for each participant (n = 11) were systematically re-referenced (bipolar subtraction) at increasing scales (4 mm-50 mm) and a multi-taper power spectrum (2-200 Hz) was generated around spike-free baseline windows. Cluster based permutation tests (p &lt; 0.05) were used to compare spectral composition features after z-scoring log power features by frequency and binning by bipolar distance. High-density and low-density (4mm vs 8mm) re-referencing schemes were used for spike localization by applying a line length transform to highlight epileptiform features in peri-spike windows and comparing to baseline windows using cluster-based permutation tests. RESULTS: Bipolar pair distances &lt;10 mm decreased low-frequency (&lt;30 Hz) power, whereas both low and high frequency (30-200 Hz) power increased by longer bipolar distances. These findings were consistent across different components (grids, strips, depths). The detection of high gamma (50-200 Hz) neural activity in the STG related to an audio stimulus was enhanced at bipolar distances of &lt;45 mm but attenuated at higher distances. Similar spike detection (duration and number of channels involved) was seen between high- vs. low-density bipolar re-referencing. CONCLUSIONS: Re-referencing at &lt;10 mm bipolar distance preferentially attenuated low-frequency signal while standard clinical and larger distances systematically augmented broadband (2-200 Hz) signals. Shorter distances, especially high-density, augmented spatial delineation of neural activity when compared to more widely spaced arrays, however, standard clinical arrays capture the general spatio-temporal extent of spike waveforms.

Broadband sound source hologram formation and application in oceanic waveguides

The physical and mathematical basics of holographic interferometry of broadband signal processing in oceanic waveguides are presented. The application of holographic processing for localization of low-noise sources is considered.

Empirical Parameterization of Broadband VLF Attenuation in the Earth‐Ionosphere Waveguide

AbstractThe Earth‐ionosphere waveguide (EIW) determines the propagation of Very Low Frequency (VLF; ∼3−30 kHz) waves. Characterizing the waveguide is a longstanding challenge due to its large spatial scale and the complex variability of the lower ionosphere. Here we apply a novel linear basis function regression technique to characterize attenuation in the EIW using broadband measurements of lightning‐generated radio waves. The process begins with defining a basis function set, which ideally encompasses a feature set that can predict the variability seen in VLF attenuation properties. With this basis set defined, a system of linear equations is then constructed using sensor pair observations to eliminate the dependency on source amplitude in each observation. Using this formalism, an empirical attenuation model for broadband signals from lightning is constructed and the dependence on attenuation properties with the boundary conditions is explored. The empirically derived results show attenuation rates over ice that are 12 dB/Mm higher compared to paths over saltwater. Well‐known east/west attenuation rate asymmetry stemming from anisotropic reflection coefficients of the ionosphere is also demonstrated and investigated. For example, under a daytime ionosphere, westward‐propagating waves suffer up to 2.8 dB/Mm greater attenuation compared to eastward‐propagating waves. The trained model is used for propagation corrections in a global lightning locating system (LLS), but this technique can be expanded to further study VLF attenuation rates by employing different sets of basis functions.

Estimation of a moving broadband sound target motion velocity based on spectrum standard deviation

Traditional Doppler velocity estimation method usually requires radiant noise in a high energy line spectrum, which is often less effective for a target without an obvious line spectrum. Considering a moving target radiating a broadband signal whose spectrum has a regular undulating structure and the presence of Doppler shifts will make the structure of the spectrum more complex, we propose a method of Doppler-warping transform combined with the spectrum standard deviation to estimate the velocity of the target. The received signal after the Doppler-warping transform by the correct motion parameters will restore the undulating structure of the sound source signal spectrum, resulting in a significant increase in standard deviation. The Doppler-warping transform result is related to the motion parameters. Therefore, parameters, such as the velocity of a moving target, can be obtained by this method. Both the simulation and the car and motorboat experiment results suggest the effectiveness of this method.

Ultrahigh Electromagnetic Wave Transmitting Polyphenylene Sulfide Microcellular Foams Based on Molecular Structure Design for 5G Communication

In the scenario of fifth-generation communication, especially millimeter wave (mmW) transmission, microelectronic material is required to have better signal stability, that is, a lower dielectric constant (Dk) and dielectric loss (Df). This work takes full advantage of the low dielectric molecular structure characteristics in polyphenylene sulfide (PPS) and the nonresidual low dielectric characteristics of supercritical CO2 (scCO2) foaming to enhance the low dielectric performance. The distinct structure of a highly stable thioether bond and highly reactive hydrogen in the benzene ring in the PPS macromolecule enables the oxo-bridging between PPS molecular chains and hence increases PPS melt strength. By further adjusting the regularity in PPS molecular chains to broaden the melting range of PPS crystals, the PPS microcellular foam of large expansion ratio (16.5-fold) with uniform cells is obtained. It has an ultralow Dk of 1.14 and Df of 0.0005 at 3 GHz. In the Ka band of 26–40 GHz, the broadband signal transmission has nearly no loss (∼99%). Furthermore, it has a low density (<0.08 g/cm3), is hydrophobic (contact angle of 123.4°), and reaches V-0 level flame retardancy. This ultrahigh mmW transmission material with excellent comprehensive performance provides an alternative for solving the problems of mmW propagation in the existing dielectric materials.

Mathematical model of spatial and temporal processing of broadband signals in satellite radio systems of broadband access and radio navigation

To effectively receive a useful broadband signal, it is necessary to compensate for distortions caused by the complex frequency-dependent coefficient of the receiving and transmitting antenna, multiplicative interference and noise from the heterogeneity of the propagation medium, additive interference and noise. Using only correlation processing under certain conditions will not allow receiving a useful broadband signal with the required quality. The article develops a mathematical model of spatio-temporal processing of broadband signals in satellite broadband access systems, showing that spatio-temporal processing should be carried out in two stages. At the first stage, space-time processing is carried out in the channel adaptively to the antenna array based on the formation of a complex frequency-dependent vector of weighting coefficients, which should change with a change in the signal-interference situation in real time, taking into account the change in the direction of radiation sources. At the second stage, correlation processing of the broadband signal is performed based on the use of long-length code sequences. This model is the basis for the development of methods for the formation and correlation (temporal) processing of broadband signals and methods of spatio-temporal processing of broadband signals under conditions of intentional and unintentional exposure. Based on the presented mathematical model of spatio-temporal processing of broadband signals, the main directions of increasing the noise immunity of radio-electronic systems are determined.

A Broadband Analog Predistortion Linearizer Based on GaAs MMIC for Ka-Band TWTAs

In this article, a Ka-band broadband analog predistortion (APD) microwave monolithic integrated circuit (MMIC) with independent tunability based on a 0.15 μm GaAs pHEMT process is proposed, which can be cascaded in front of traveling wave tube amplifiers (TWTAs) to improve their linearity. The influence of different diode sizes on the parameters of Schottky diodes is analyzed and used to design the gain and phase nonlinear branches. The broadband APD MMIC is realized based on a dual-branch vector synthesis design and nonlinear frequency adjust module (NFAM). The independent tunability and broadband characteristics of the APD MMIC are verified by simulated and measured results with an error of less than 5%. Furthermore, a Ka-band 60 W TWTA is linearized by the APD MMIC, and the gain and phase compressions are reduced from 8 dB and 50° to within 3 dB and 12°, respectively. The third-order intermodulation (C/IM3) is greater than 28 dBc and noise power ratio (NPR) is greater than 15.7 dBc at 3 dB output power backoff (OPBO) over the operating band of 25.1~27.5 GHz, indicating that the APD MMIC can improve the nonlinearity of TWTA effectively under broadband signals.

Photonic Phase-Stabled Transmission of Broadband Signals Based on Passive Compensation

A photonic phase-stabled transmission of broadband signal based on passive frequency mixing compensation is proposed. The mechanism of jitter compensation of aimed broadband signal transmission relies on an adaptive cancellation by mixing two signals under approximately the same external time jittering. A theoretical model is built and the frequency dependent jitter suppression is analyzed and verified. Compared with previous methods, the proposed photonic phase-stabled transmission of broadband signal can withstand faster jittering frequency up to 1 kHz. The experimental results show that the proposed method can significantly reduce the phase drift of the broadband signal of 400 MHz bandwidth. The measured overlapping Allan deviation at the center frequency of 7.2 GHz shows a stability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.53\times 10 ^{-14}$ </tex-math></inline-formula> at 100 s average time.

Quantifying repeatability, reproducibility, and bias in sound power level measurement methodologies via a round-robin study

This paper presents results from an interlaboratory study to examine the repeatability, reproducibility, and bias of three different sound power measurement methods used in the heating, ventilation, and air-conditioning industry: free field method, diffuse field method, and intensity method. A loudspeaker playing both a broadband signal and a broadband signal with four discrete tones has been measured in participating laboratories according to the standardized test methods that are commonly used in each facility. Results across 1/3 and 1/1 octave bands are then compared between signals, between methods, between laboratories, and across all laboratories using the same method. Additional discussion regarding potential sources of variation between facilities is presented. Ultimately, in accordance with ISO 5725, repeatability, reproducibility, laboratory bias, and test method bias are estimated. To reduce uncertainty of these estimates for some methods, additional laboratory participants are needed. [Work supported by the Air-Conditioning, Heating, and Refrigeration Institute.]

Quantifying impacts of hearing protection devices on sound localization in azimuth and elevation: Refinement of acoustic predictors

Modern hearing protection devices (HPDs) mitigate the risk of noise-induced hearing loss when used as intended, but negative auditory perceptual side-effects continue to limit usability in critical settings. Dozens of studies have shown that HPDs lead to significant errors in sound source localization. Several studies, including previous studies by our group, have linked such errors to distortions of the spectra of transmitted signals. Here we provide an update on a multi-site effort to develop acoustic predictors and summary metrics to quantify HPD impacts on sound source localization. At two independent study sites, human listeners localized brief broadband signals with open ears and during use of passive and active earplug and earmuff-style devices. Sources spanning 360° in azimuth and −30° to + 60° in elevation enabled measurement of two-dimensional localization error across a broad range of source locations. An initial set of parallel acoustic measurements was obtained using a standardized acoustic test fixture. Present efforts are focused on evaluating prospective advantages of (1) incorporating binaural acoustic data in the prediction of errors and (2) accounting for individual HPD-impacted acoustic variability. The long-term goal of this work is to support application-specific selection of existing HPDs and inform the design of new HPDs.