Frequency-selective Response Research Articles

Smart meta-device powered by stray microwave energies: A green approach to shielding external interference and detection

Protecting sensitive electronic devices from external stray microwaves is essential for a range of practical applications. Current shielding devices such as filters and frequency-selective surfaces are vulnerable to harmful signals at in-band frequencies due to their mechanisms relying on filtering out-of-band signals through frequency-selective responses. This study presents a smart meta-device powered by stray microwave energies, designed to autonomously shield against external interference and detection without the need for an external power supply or human intervention. Such meta-devices integrate reconfigurable meta-atom arrays and sensing-powering modules to form a sensing-powering-feedback closed-loop system. This enables real-time sensing of high-power microwaves and the automatic transition from high transmission to shielding or absorbing external harmful microwave energy. A prototype was fabricated, characterized, and demonstrated as a smart high-power microwave shielding device and adaptive stealth radome, operating between 6.0 GHz to 6.7 GHz. Unlike other smart/intelligent meta-devices, the proposed meta-device operates autonomously without external power supplies or complex microwave networks. This signifies a paradigm shift in electromagnetic shielding technology and represents a breakthrough in the development of autonomous, sustainable, and maintenance-free microwave devices.

Effect of current probe location on response law of shielded multi-core wire

Abstract Bulk current injection (BCI) is a commonly used method in electromagnetic compatibility testing. In order to enhance the injection efficiency of the current probe, the interference process into the internal core wire is analyzed. Theoretical calculations indicate that the shielded multi-core wire exhibits a frequency selective response under BCI conditions. The-peak of the terminal load response voltage occurs at frequencies corresponding to the integral multiples of half the relative length of the cable. The position of the current probe along the cable only affects the amplitude of the distributed current on the shielding layer, but not its distribution pattern. The response of the internal core wire at the resonant frequencies is positively correlated with the distribution pattern of the distributed current on the shielding layer. The probe position has a significant effect on the magnitude of the wire-to-terminal response. Placing the current probe at both ends of the cable results in high injection efficiency and significant power savings.

Highly Sensitive Biomimetic Crack Pressure Sensor with Selective Frequency Response.

High-sensitivity sensors in practical applications face the issue of environmental noise interference, requiring additional noise reduction circuits or filtering algorithms to improve the signal-to-noise ratio (SNR). To address this issue, this study proposes a biomimetic crack pressure sensor with selective frequency response based on hydrogel dampers. The core of this research is to construct a biomimetic crack pressure sensor with selective frequency response using the high-pass filtering characteristics of gelatin-chitosan hydrogels. This design, inspired by the slit sensilla and stratum corneum structure of spider legs, delves into the material properties and principles of hydrogel dampers, exploring their application in biomimetic crack pressure sensors, including parameter selection, structural design, and performance optimization. By delving into the nuanced characteristics and working principles of hydrogel dampers, their integration in biomimetic crack pressure sensors is examined, focusing on aspects like parameter selection, structural engineering, and performance enhancement to selectively sieve out low-frequency noise and transmit target vibrational signals. Experimental results demonstrate that this innovative sensor, while suppressing low-frequency vibration signals, can selectively detect high-frequency signals with high sensitivity. At different vibration frequencies, the relative change in resistance exceeds 200%, and the sensor sensitivity is 7 × 104 kPa-1. Furthermore, this sensor was applied to human voice detection, and the corresponding results verified its frequency-selective performance evidently. This study not only proposes a new design for pressure sensors but also offers fresh insights into the application of biomimetic crack pressure sensors in intricate environments.

A Selective-Response Hypersensitive Bio-Inspired Strain Sensor Enabled by Hysteresis Effect and Parallel Through-Slits Structures

HighlightsA bio-inspired flexible strain sensor with hypersensitivity and highly selective frequency response is prepared by styrene–isoprene–styrene combined with monolayer graphene.Benefiting from the structural design inspired by nature and hysteresis of viscoelastic materials, bio-inspired structures, and original materials' properties complement each other.The frequency recognition resolution of bio-inspired flexible strain sensor reaches 0.2 Hz, making it ideal for human–computer interaction and mechanical equipment health inspection.

Integration Design of Compact Balun Filters and Duplexing Balun Filter With Arbitrary Frequency-Selective Response

Integration Design of Compact Balun Filters and Duplexing Balun Filter With Arbitrary Frequency-Selective Response

Maximum likelihood-based adaptive iteration algorithm design for joint CFO and channel estimation in MIMO-OFDM systems

In this paper, we present a joint time-variant carrier frequency offset (CFO) and frequency-selective channel response estimation scheme for multiple input-multiple output-orthogonal frequency-division multiplexing (MIMO-OFDM) systems for mobile users. The signal model of the MIMO-OFDM system is introduced, and the joint estimator is derived according to the maximum likelihood criterion. The proposed algorithm can be separated into three major parts. In the first part of the proposed algorithm, an initial CFO is estimated using derotation, and the result is used to apply a frequency-domain equalizer. In the second part, an iterative method is employed to locate the fine frequency peak for better CFO estimation. An adaptive process is used in the third part of the proposed algorithm to obtain the updated CFO estimation and track parameter time variations, including the time-varying CFOs and time-varying channels. The computational complexity of the proposed algorithm is considerably lower than that of the maximum likelihood-based grid search method. In a simulation, the mean squared error performance of the proposed algorithm was close to the Cramer-Rao lower bound. The simulation results indicate that the proposed novel joint estimation algorithm provides a bit error rate performance close to that in the perfect channel estimation condition. The results also suggest that the proposed method has reliable tracking performance in Jakes’ channel models.

Waveguide Components and Aperture Antennas With Frequency- and Time-Domain Selectivity Properties

Filtering modules are essential devices of modern microwave systems given their capability to improve the signal-to-noise ratio of the received signal or to eliminate the unwanted interferences. For discriminating between different components, a filter exhibits a frequency-selective response that, however, is not able to distinguish between different signals whose spectrum falls within the passband of the filter itself. In this regard, some electromagnetic structures exhibiting, at the same frequency, different responses depending on the waveform of the incoming waves have been recently proposed. In this communication, we extend the aforementioned approach to the case of a standard waveguide filtering module. In particular, by loading a bandpass filtering iris with a proper lumped-element circuit, we design a waveguide component able to distinguish between different pulsed waves, even at the same frequency, depending on their pulsewidth. Moreover, by using this filter for capping an open-ended rectangular waveguide, a radiating element with both frequency- and time-domain selectivity properties is presented. The structures discussed in this communication may pave the way to a new class of microwave systems that, being both frequency selective and time selective, are less sensitive to noise and interferences.

Passive Intermodulation in Simultaneous Transmit–Receive Systems: Modeling and Digital Cancellation Methods

This article presents novel solutions for suppressing passive intermodulation (PIM) distortion in frequency-division duplexing (FDD)-based radio transceivers, stemming from nonlinear radio frequency (RF) components and simultaneous transmission and reception, with special emphasis on modern carrier aggregation networks. With certain transmission band combinations in, e.g., long-term evolution (LTE) or the emerging 5G new radio (NR) mobile radio systems, the nonlinear distortion produced by the passive components of the transceiver RF front end can result in intermodulation (IM) distortion that falls within one of the configured reception bands. While the traditional solution to mitigate this problem is to reduce the transmit power, we take an alternative approach and seek to cancel such PIM in the transceiver digital front end by using the original transmit data as a reference. To generate as accurate cancellation signal as possible, we derive different advanced signal models for the observable IM distortion at own receiver band that incorporates also the power amplifier (PA) nonlinearities, together with the passive component nonlinearities and the frequency-selective responses of the duplex filters. The performance and the processing complexity of the devised digital cancellation and parameter estimation solutions are evaluated with real-life RF measurements, where an actual LTE-Advanced user equipment (UE)-type transceiver system is utilized. The obtained results show that the proposed cancellers are implementation feasible and can suppress the self-interference by over 20 dB, canceling the distortion nearly perfectly up to UE transmit powers of +24 dBm. The results also indicate that, in many cases, it is necessary to model the nonlinear distortion effects produced by the PAs, even if the individual component carriers are combined after the amplification stage.

Auditory and tactile frequency representations are co-embedded in modality-defined cortical sensory systems

Sensory information is represented and elaborated in hierarchical cortical systems that are thought to be dedicated to individual sensory modalities. This traditional view of sensory cortex organization has been challenged by recent evidence of multimodal responses in primary and association sensory areas. Although it is indisputable that sensory areas respond to multiple modalities, it remains unclear whether these multimodal responses reflect selective information processing for particular stimulus features. Here, we used fMRI adaptation to identify brain regions that are sensitive to the temporal frequency information contained in auditory, tactile, and audiotactile stimulus sequences. A number of brain regions distributed over the parietal and temporal lobes exhibited frequency-selective temporal response modulation for both auditory and tactile stimulus events, as indexed by repetition suppression effects. A smaller set of regions responded to crossmodal adaptation sequences in a frequency-dependent manner. Despite an extensive overlap of multimodal frequency-selective responses across the parietal and temporal lobes, representational similarity analysis revealed a cortical “regional landscape” that clearly reflected distinct somatosensory and auditory processing systems that converged on modality-invariant areas. These structured relationships between brain regions were also evident in spontaneous signal fluctuation patterns measured at rest. Our results reveal that multimodal processing in human cortex can be feature-specific and that multimodal frequency representations are embedded in the intrinsically hierarchical organization of cortical sensory systems.

3D conductive network wrapped CeO2-x Yolk@Shell hybrid microspheres for selective-frequency microwave absorption

Carbon nanotubes (CNTs) based materials have been popular as microwave absorbers owing to their tunable electrical conductivity. Here, a hierarchical CeO2-x yolk@shell microsphere encapsulated by the CeO2-x/CNTs hybrid three-dimensional (3D) conductive network (denoted as CMC) was successfully fabricated by a simple solvothermal method through a cooperative-assembly strategy owing to a synergistic effect of reactants. The well-designed hierarchical structure is beneficial to optimize the electrical conductivity for the dielectric-dominated microwave absorber, resulting in the enhanced electromagnetic impedance matching and energy dissipation ability. Importantly, the primary units made up of conductive CNTs and insulated CeO2-x nanoparticles simultaneously provide large room for resistivity adjustment and abundant of localized polarized sites. Moreover, the hierarchical dielectric structure is similar to a microwave trapping cage in micro-meter size can effectively dissipates microwave energy. Thus, the strong microwave absorption (−52.4 dB at 6.1 GHz) and selective-frequency response (shift from 5.2 GHz to 18 GHz) can be achieved through regulating the composite conductivity only by tailoring the input amount of oxidized CNTs. Considering the promising performance of the CMC yolk@shell@shell microspheres, this work is expected to enlighten the design and synthesis of the carbon-based materials in other related fields.

Responses in area hMT+ reflect tuning for both auditory frequency and motion after blindness early in life

Previous studies report that human middle temporal complex (hMT+) is sensitive to auditory motion in early-blind individuals. Here, we show that hMT+ also develops selectivity for auditory frequency after early blindness, and that this selectivity is maintained after sight recovery in adulthood. Frequency selectivity was assessed using both moving band-pass and stationary pure-tone stimuli. As expected, within primary auditory cortex, both moving and stationary stimuli successfully elicited frequency-selective responses, organized in a tonotopic map, for all subjects. In early-blind and sight-recovery subjects, we saw evidence for frequency selectivity within hMT+ for the auditory stimulus that contained motion. We did not find frequency-tuned responses within hMT+ when using the stationary stimulus in either early-blind or sight-recovery subjects. We saw no evidence for auditory frequency selectivity in hMT+ in sighted subjects using either stimulus. Thus, after early blindness, hMT+ can exhibit selectivity for auditory frequency. Remarkably, this auditory frequency tuning persists in two adult sight-recovery subjects, showing that, in these subjects, auditory frequency-tuned responses can coexist with visually driven responses in hMT+.

Planar Broadband Huygens’ Metasurfaces for Wave Manipulations

Electrically thin and effectively 2-D material composites, metasurfaces, have been widely exploited for the manipulation of electromagnetic waves. For many applications, it is desired to transform incident waves of a specific frequency range keeping the metasurface invisible at other frequencies. Such a frequency-selective response can be achieved based on subwavelength Huygens’ inclusions. However, their fabrication requires sophisticated processes due to the 3-D geometry. Here, we propose a planar Huygens’ meta-atom with the goal to open a way to realize broadband invisible metasurfaces with topologies suitable for the conventional printed circuit board fabrication technology. We synthesize and analyze, both numerically and experimentally, three different metasurfaces capable of polarization and amplitude transformations of incident waves.

Multisensory activation of ventral cochlear nucleus D-stellate cells modulates dorsal cochlear nucleus principal cell spatial coding.

Dorsal cochlear nucleus fusiform cells receive spectrally relevant auditory input for sound localization. Fusiform cells integrate auditory with other multisensory inputs. Here we elucidate how somatosensory and vestibular stimulation modify the fusiform cell spatial code through activation of an inhibitory interneuron: the ventral cochlear nucleus D-stellate cell. These results suggests that multisensory cues interact early in an ascending sensory pathway to serve an essential function. In the cochlear nucleus (CN), the first central site for coding sound location, numerous multisensory projections and their modulatory effects have been reported. However, multisensory influences on sound location processing in the CN remain unknown. The principal output neurons of the dorsal CN, fusiform cells, encode spatial information through frequency-selective responses to direction-dependent spectral features. Here, single-unit recordings from the guinea pig CN revealed transient alterations by somatosensory and vestibular stimulation in fusiform cell spatial coding. Changes in fusiform cell spectral sensitivity correlated with multisensory modulation of ventral CN D-stellate cell responses, which provide direct, wideband inhibition to fusiform cells. These results suggest that multisensory inputs contribute to spatial coding in DCN fusiform cells via an inhibitory interneuron, the D-stellate cell. This early multisensory integration circuit likely confers important consequences on perceptual organization downstream.

Color‐Sensitive Ultrafast Optical Modulation and Switching of Terahertz Plasmonic Devices

Abstract2D micro‐nanostructured metal films with hole arrays show promising features such as the extraordinary transmission of light. Such systems are interesting in the field of subwavelength photonics and nonlinear optics due to their high field confinement in addition to their inherent spectral scalability and frequency selective response. Several active schemes to control the extraordinary transmission are recently demonstrated. However, these dynamic devices do not reveal any obvious color‐dependent modulation of the resonant transmission behavior. Here, color‐sensitive ultrafast modulation of extraordinary resonant transmission of terahertz (THz) waves through 2D metallic hole arrays is demonstrated. Pumping the silicon beneath the metallic array with light of different colors and identical fluences exhibit significantly different ultrafast switching dynamics and modulation. The color‐dependent sensitivity and control of THz waves at an ultrafast timescale provide an extra degree of freedom that opens up new opportunities for future applications in active subwavelength optics, optoelectronics, and all‐optical switching of THz photonic devices.

A Depolarizing Chipless RF Label for Dielectric Permittivity Sensing

A novel depolarizing chipless radio-frequency identification tag is employed to estimate the variation of the dielectric properties of materials. The tag, comprising two 45° tilted dipoles printed on a thin substrate, is accommodated on the top of the target material, and it is interrogated wirelessly at radio frequency with a narrowband signal. Once interrogated, the tag radiates back a frequency-selective response in cross-polarization. The depolarizing property of the tag allows employing it on materials characterized by arbitrary large size, thus avoiding the conceling effect of the large radar echo of the platform. The properties of the target material are estimated by resorting to the amount of frequency shift detected on the backscattered signal. The operation principle of the tag is initially illustrated by using simulations, and then measurements are performed on various materials to show the reliability of the proposed procedure.

Fast Fading Channel Estimation by Kalman Filtering and CIR Support Tracking

Structured estimation of channel impulse response (CIR) is considered in orthogonal frequency division multiplexing (OFDM) systems for which the channel exhibits a sparse time-domain response. In particular, fast fading channels encountered in mobile wireless communications are envisaged. Such channels are characterized by time varying frequency selective response. This contribution exploits the much slower variation of propagation time delays, compared to propagation gains, to enhance CIR estimation. To this end, we propose a scheme that disjointly and successively tracks the delay-subspace, by Kalman filtering, then tracks the CIR structure. Contrarily to former subspace based channel response tracking, the channel order is unknown. The channel sparsity in the time-domain is accounted for by incorporating an adaptive CIR support tracking. This adaptive procedure combines the last and current OFDM blocks recovered CIR structures. To fine tune the CIR support, enhanced threshold-based CIR structure detection is applied on the recovered CIR estimate over its detected support. Finally, a structured LS estimation is processed. The proposed scheme outperforms sparsity-unaware Kalman tracking algorithm. It achieves similar performance than the best benchmark which is based on perfect CIR structure knowledge.

High selective frequency response in a complementary split ring resonator based transmission line

ABSTRACTIn this letter, a microstrip line based on complementary split ring resonator (CSRR) technology and coupled with square open‐loop resonator (OLR) is presented. The addition of OLR to the conventional CSRR‐loaded unit cell generates transmission zero in the upper stop band and improves the upper band rejection level without increasing the traversal dimensions. Consequently, a new high selective passband with increased bandwidth and a pair of transmission zeros at both lower and upper sides are formed due to the appropriate couplings among the resonators and the host microstrip line. Moreover, the geometry of the basic cell is quite simple without requiring complex configurations and via holes which is easy to fabricate and low in cost. For demonstration, a two‐stage transmission line prototype is fabricated and tested to experimentally validate the attractive out‐of‐band performance (>36 dB) for filter applications as predicted in theory. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:12–15, 2017

A process-tolerant out-of-band blocker rejection technique for SAW-less receivers

In this paper, an active filtering technique is presented which is capable of filtering the out-of-band blockers in wireless receivers. The concept is based on the feedforward cancellation technique where a blocker replica is subtracted at the output of the low-noise amplifier (LNA). In contrast to the previously reported feedforward cancellation methods, exact gain and phase matching are easily obtained in the proposed architecture to produce a highly selective narrowband frequency response at the output of the LNA with wide rejection bandwidth. For the proof of concept, the system is implemented in a 65nm CMOS technology. It occupies a total area of 0.8mm2 and the current consumption is 24mA from a 1.2V supply. The system post-layout simulations showed a blocker rejection of more than 33dB for blocker signals 100MHz away from the desired signal when the feedforward path is activated. The noise figure (NF) of the entire system is 3.8dB that degrades to 5.8dB when the feedforward path is activated.

Fabrication and Characterization of a Wavenumber-Spiral Frequency-Steerable Acoustic Transducer for Source Localization in Plate Structures

This paper reports on the fabrication and the experimental characterization of a wavenumber frequency-steerable acoustic transducer (WS-FSAT). Here, the transducer is employed for the localization of broadband acoustic events corresponding to the propagation of guided elastic waves in an isotropic plate. The WS-FSAT records the plate response and defines the source location through a time-frequency analysis of the received signal. This is achieved by exploiting the frequency selective response of the transducer which directly maps the dominant component of the received signal to the direction of arrival of the incoming wave. This feature is the result of the spatial filtering effect produced by the characteristic shape of the sensing surface, which is designed in the wavenumber domain. Experiments are performed on a prototype fabricated on a polyvinylidene fluoride substrate mounted on an aluminum test plate. Tests are conducted for various source locations, and with multiple sources activated simultaneously. The results highlight the robustness of the proposed device, its good sensitivity and angular resolution, as well as the low complexity of hardware and signal processing. This paper suggests the WS-FSAT as an attractive solution for the detection of broadband acoustic events, such as impacts on structural substrates, and its potential use as part of active structural health monitoring systems based on pitch-catch or pulse-echo operations.

Novel closely spaced planar dual‐band frequency‐selective surface

A novel, dual-band frequency-selective surface (FSS) is presented. The designed and fabricated FSS structure has closely spaced operation bands and high selective frequency response at each operation band. Two-dimensional compact U-shaped slot resonators are used in each FSS unit cell to obtain this ability. Also, each slot resonator is composed of straight slot section at the centre connected to two U-shaped arms at each end. The dimensions of the slot resonators are optimised using via full-wave electromagnetic simulations using CST Microwave Solver. The proposed FSS structure is independent of incident polarisation and angle because of 90° rotation of the unit cell. The validity of the proposed FSS is confirmed by both simulation and measurement results. In the experimental verification, a waveguide measurement setup is chosen. Both measurement and simulation results show that the FSS structure operates at the designed frequencies of 8.80 and 9.54 GHz, which are very close to each other.