Frequency Span Research Articles

Coherent satellites in multispectral regenerative frequency microcombs

Multispectral frequency combs provide new architectures for laser spectroscopy, clockwork, and high-capacity communications. Frequency microcombs have demonstrated remarkable impact in frequency metrology and synthesis, albeit with spectral bandwidth bounded by intrinsic second-order dispersion and consequently low-intensities at the spectral edges. Here we report coherent satellite clusters in multispectral regenerative frequency microcombs with enhanced intensities at the octave points and engineered frequency span. Beyond the conventional bandwidth of parametric oscillation, the regenerative satellites are facilitated by higher-order dispersion control, allowing for multiphase-matched parametric processes. The satellite span is deterministically controlled from 34 to 72 THz by pumped at C/L-bands, with coherence preserved with the central comb through the nonlinear parametric process. We further show the mirrored appearance of the satellite transition dynamics simultaneously with the central comb at each comb state. These multispectral satellites extend the scope of parametric-based frequency combs and provide a unique platform for clockwork, spectroscopy and communications.

Design of a CMOS 65‐nm inductor‐less VCO for ISM applications in the VHF band

SummaryThis paper presents a design of a CMOS cross‐coupled voltage‐controlled oscillator (VCO) using active inductors (AIs) for wide‐band applications and can also be applied to various wireless technologies standards. The compatibility of this design to different wireless standards highlights its potential to be implemented at the core of the communication front end in the Internet of Things (IoT). The proposed AI design employs a gyrator‐C topology as the basic structure to generate an inductance. The VCO uses a cross‐coupled oscillator structure with a pair of varactors to sweep the frequency. Two extra capacitors, between the AIs and the outputs of the VCO core tank, are employed to enhance the performance of the phase noise and make the VCO work similarly to a linear transconductance (LiT) oscillator. Both the AIs and the VCO are designed in the TSMC 65‐nm CMOS technology, and the performance is analyzed using postsimulation results, as well as through measurements. The fundamental frequency spans from 140 to 463 MHz. Thus, the relative tuning range of this design is approximately 107%. The optimal phase noise of the design is around −97 dBc/Hz at 1‐MHz offset. Furthermore, it achieves an excellent figure of merit (FOM) around −163 dBc/Hz with a direct current (DC) power consumption less than 3 mW. The proposed design shows an advantage in phase noise and power consumption in comparison with previous active inductor VCO and ring VCO designs, respectively. The final layout occupies only 0.4 × 0.62 mm2 including the pads. The proposed AI‐VCO shows a compact size, linear tuning, low power consumption, and good phase noise performance.

Metamaterial Cell-Based Superstrate towards Bandwidth and Gain Enhancement of Quad-Band CPW-Fed Antenna for Wireless Applications.

A multiband coplanar waveguide (CPW)-fed antenna loaded with metamaterial unit cell for GSM900, WLAN, LTE-A, and 5G Wi-Fi applications is presented in this paper. The proposed metamaterial structure is a combination of various symmetric split-ring resonators (SSRR) and its characteristics were investigated for two major axes directions at (x and y-axis) wave propagation through the material. For x-axis wave propagation, it indicates a wide range of negative refractive index in the frequency span of 2–8.5 GHz. For y-axis wave propagation, it shows more than 2 GHz bandwidth of near-zero refractive index (NZRI) property. Two categories of the proposed metamaterial plane were applied to enhance the bandwidth and gain. The measured reflection coefficient (S11) demonstrated significant bandwidths increase at the upper bands by 4.92–6.49 GHz and 3.251–4.324 GHz, considered as a rise of 71.4% and 168%, respectively, against the proposed antenna without using metamaterial. Besides being high bandwidth achieving, the proposed antenna radiates bi-directionally with 95% as the maximum radiation efficiency. Moreover, the maximum measured gain reaches 6.74 dBi by a 92.57% improvement compared with the antenna without using metamaterial. The simulation and measurement results of the proposed antenna show good agreement.

Persistence in the Realized Betas: Some Evidence for the Spanish Stock Market

This paper examines the stochastic behaviour of the realized betas within the one-factor CAPM for the six companies with the highest market capitalization included in the Spanish IBEX stock market index. Fractional integration methods are applied to estimate their degree of persistence at the daily, weekly and monthly frequency over the period 1 January 2000 – 15 November 2018 using 1, 3 and 5-year samples. On the whole, the results indicate that the realized betas are highly persistent and do not exhibit mean-reverting behaviour. However, the findings are rather sensitive to the choice of frequency and time span (number of observations).

Кросскорреляционный и когерентный анализ электрокортикограмм крыс, перенесших черепно-мозговую травму

Traumatic brain injury (TBI) is a leading cause of disability and mortality among young population both in Russian Federation and abroad. The development of adequate and reproducible models of TBI in laboratory animals as well as precise methods for assessment of neurological disorders degree will optimize investigation of new effective neuroprotective drugs. Aim of the study: to perform the comparative crosscorrelation and coherent analysis of ECOG signal in non-injured rats and rats with TBI rats. The open penetrating brain injury was modeled following the craniotomy over the motor cortex of the left hemisphere by controlled cortical impact injury method (CCI). The recording nichrome ECoG electrodes were implanted bilaterally in the primary and secondary motor cortex areas and also in the area of primary somatosensory cortex (over the hippocampus). ECoG recordings were performed on the 3rd and 7th day after the operation in condition of the home cage at resting state. Crosscorrelation analysis comprised the calculation of crosscorrelation coefficient, mean frequency and maximum span of crosscorrelation function (CCF). Also the calculated mean coherence power of delta-, tetha-, alpha- and beta-rhythms. Unilateral traumatic damage of motor cortex and near situated brain areas lead to disturbances of inter- and intrahemispheric connections, these changes are recorded not only in the damaged area, but also in the remote areas of the cortex on the 3rd and 7th day after the injury.

Hexagonal Shaped Near Zero Index (NZI) Metamaterial Based MIMO Antenna for Millimeter-Wave Application

A single-layered multiple-input multiple-output (MIMO) antenna working at 28 GHz loaded with a compact planar-patterned metamaterial (MTM) structures is presented in this paper for millimeter-wave application. A combination of a split square and hexagonal shaped unit cell is designed and investigated with a wide range of effective near-zero index (NZI) of permeability and permittivity, along with a refractive index (NZRI) property. The metamaterial characteristics were examined through the material wave propagation in two main directions at y and x-axis. For wave propagation at the y-axis, it demonstrates mu-near-zero (MNZ) with more than 6 GHz bandwidth, near-zero refractive index (NZRI), and epsilon-near-zero (ENZ) properties. However, it indicates a wide negative range of single mu metamaterial (MNG) from 27.6 to 28.9 GHz frequency span at x-axis wave propagation. A single antenna with $3\times 3$ metamaterial unit cells is proposed to operate at a frequency band (24 – 30) GHz. Furthermore, MIMO antenna with only 4 mm space between antenna elements provides high isolation of more than 24 dB. The measured results show that the MIMO antenna is satisfied with 6 GHz bandwidth, and maximum peak gain of 12.4 dBi. In addition to that, the proposed MIMO antenna loaded with MTM has also shown good performances with high diversity gain (DG > 9.99), envelope correlation coefficient (ECC) lower than 0.0013, channel capacity loss (CCL) $\times23$ mm.

A Method for Accurately Characterizing Single Overmoded Circular TM01-TE11 Mode Converter

In the case of characterizing waveguide mode converter, the widely applied back-to-back method can only extract two mode converters' joint S-parameters, the accuracy of the far-field radiation pattern method is limited. This paper experimentally demonstrates a method for accurately characterizing single circular TM 01 -TE 11 mode converter, which consists of extracting the asymmetric two-port TM 01 and TE 11 mode generator test fixtures' S-parameters and de-embedding them from the “TM 01 mode generator + TM 01 -TE 11 mode Converter + TE 11 mode generator” cascade measurement results. The peak problem occurring in measuring overmoded circular waveguide devices with VNA is analyzed and settled with an offset waveguide based peak-shifting tactic plus the Loess smoothing method. Fairly good agreement between the simulated and finally extracted complex S-parameter matrix of a TM 01 -TE 11 mode converter throughout the device's working frequency span validates the proposed method's merits over traditional methods.

Leak detection in a gas pipeline using spectral portrait of acoustic emission signals

This paper proposes a reliable method to detect leaks and recognize their various sizes in a gas pipeline based on the spectral portrait of acoustic emission (AE) signals. Through analyzing the relation of leak syndromes and AE signals in the frequency domain, this paper demonstrates that leak identification only by signatures obtained from measured individual signals might be unreliable due to strong attenuation in wave propagation along pipelines. The proposed method constructs a vector quantity in the frequency domain from two AE signals simultaneously acquired by two sensor channels, combined with a wave propagation model. This quantity characterizes the spectral portrait of AE signals emitted from a leak, its signatures therefore reflect leakage syndromes the same as leak signals, whereas those of individual sensor-measurements involve considerable distortions. Hence, the accuracy and reliability of leak detection and recognition are improved by the approach using the spectral portrait-related quantity. This paper also describes bank filters designed to encode the spectral envelope of AE signals instead of observing their total continuous frequency span. Finally, a trained multi-class support vector machine (SVM) classifier is responsible for the leak categorization, exploiting a set of the most discriminative encoded spectral envelope (ESE) features. The experimental results show that the proposed method outperforms conventional algorithms in pattern separability quantified by Kullback-Leibler distance and classification accuracy.

A Compact Source–Load Agnostic Flexible Rectenna Topology for IoT Devices

This article presents a new compact lightweight radio frequency (RF) energy harvesting system. The system relies on a dual-tapered transmission line-based matching network that stretches the rectification capability of an integrated Schottky diode. This topology is demonstrated on a 2.4 GHz rigid harvesting system with a resulting power conversion efficiency that reaches up to 58% over 0 dBm input RF power. Its performance is compared with a reference rectifier that relies on a typical open circuit shunt-stub matching network. The rectifier along with a miniaturized monopole antenna is then tailored for a flexible substrate with a resulting efficiency around 50% at 0 dBm input power. The rectifier exhibits an almost flat efficiency over the 2.3–2.5 GHz frequency span despite wide load variations. The system is characterized in multiple bent configurations featuring high and stable performance. It is demonstrated that for different bent states, the proposed flexible harvester does not display large variations in harvested power. Thus, the presented rectenna demonstrates the remarkable combination of compactness, flexibility, and stability. Equipped with these features, such rectifier can be plugged into a variety of sensors, even on wearable surfaces, which makes it ideal for Internet of Things (IoT) applications.

A 3–10-GHz CMOS Time-Domain Complex Dielectric Spectroscopy System Using a Contactless Sensor

This article presents an integrated time-domain (TD) microwave broadband dielectric spectroscopy (MBDS) system in the 3–10-GHz frequency span divided into nine subbands. The generated multitone input pulse with 1-GHz bandwidth is upconverted to RF frequencies in the transmitter using a broadband single-sideband (SSB) mixer to suppress the lower sideband of the excited signal and prevent the signal distortion at the configured dc-free direct conversion receiver with the center frequency of 500 MHz at the baseband. The upconverted pulse is passed through a contactless sensor in TD where the material under test (MUT) is placed in the middle of the two ultra-wideband (UWB) Vivaldi antennas coupled in their radiative near field. To identify both the real ( $\epsilon _{r}'$ ) and imaginary ( $\epsilon _{r}''$ ) parts of the MUT’s permittivity, the impact of the entire system on MUT characterization is alleviated by designing the system subblocks and the sensor with flat gain and constant group delay over the entire frequency range. The proposed MBDS system is fabricated in 65-nm CMOS and occupies an active area of 1.24 mm2 while consuming 64~69 mW from a 1-V supply. As a proof of concept, the complex dielectric permittivity of different pure organic chemical liquids has been detected within the desired frequency range with a customized quartz-glass cuvette with the MUT volume of ~ 5 mL by calibrating the sensor using the variations in the phase and the magnitude after FFT with respect to the reference material (air). The system results in an rms error of less than 0.2% and 0.4% for $\epsilon _{r}'$ and $\epsilon _{r}''$ , respectively, compared with theory. To the best of our knowledge, this is the first CMOS TD MBDS system with a homodyne RF transceiver architecture including an on-chip UWB excitation pulse generation and the contactless sensor.

Analysis of Turbine Rotor Fault Diagnosis Method

The role of steam turbine in generating set is very important. During the operation, the rotor state is monitored by high-frequency vibration sensor. With the rapid development of computer, sensor and communication technology, modern machinery and equipment is developing towards the direction of electromechanical integration, as a result of the monitoring equipment group of large scale and the required point, single point more high sampling frequency and data collection time span is long, so the monitoring data of exponential growth, mechanical equipment health monitoring field enter the era of “big data”. The traditional method of turbine rotor fault diagnosis has encountered some difficult technical problems and bottlenecks. This paper first introduces the fault mechanism of turbine rotor. Then, the fault diagnosis methods of turbine rotor proposed by many scholars this year are analyzed, the fault characteristics of turbine rotor are summarized and the challenges are discussed, and the direction of further research is prospected.

An all-dielectric 3D Luneburg lens constructed by common-vertex coaxial circular cones

An all-dielectric broadband 3D Luneburg lens has been designed and manufactured in our study, which is constructed by common-vertex coaxial circular cones. A prototype of the lens is fabricated using 3D printing technology. The far field of the, present lens is highly directive, incident waves are focused with highly concentration, and its operating frequency spans from 5 GHz to 20 GHz. It is also found that two highly directional beams are generated while two independent converged sources feed simultaneously at the edge of the lens. The intersection angle with the center of the lens is 90°, which indicates that the lens is an efficient double input-double output device.

Investigation of Dielectric Behaviors and a.c. Conduction of a-Se90Cd10–xSbx (2 ≤ x ≤ 8) Chalcogenide Glass

The dielectric relaxation and ac conduction play a vital role to understand the mechanism of conduction. The structure and defects of solid can be determined by dielectric relaxation whereas the nature of defect states and their density of states can be studied by a. c. conductivity. In the present work, we have prepared a novel Se90Cd10–xSbx (2 ≤ x ≤ 8) glass in bulk form by conventional melt quenching technique. The investigation of dielectric behavior and conduction of amorphous Se90Cd10–xSbx (2 ≤ x ≤ 8) of pellets has been studied in the frequency span 5 × 102–1 × 105 Hz at temperature 323 K. It is observed that the dielectric constants (ɛ′) dielectric loss (ɛ″), loss angle (tan (δ)) and AC conductivity (σac) show the frequency dependent behavior. It is also noticed that ɛ′, ɛ″, tan(δ) and σac decreases with increase of Sb concentration in Se90Cd10–xSbx (2 ≤ x ≤ 8) glassy alloys.

Dielectric Analysis in Amorphous Ge8Se60Te30Sb2 Chalcogenide Glass for Solid-State Batteries

Over the last few decades, study of dielectric relaxation and a. c. conductivity of chalcogenides glasses is fascinating field among various researchers to explore the new dimensions for their end user applications with enhanced properties. In this paper, dielectric parameters viz dielectric constant (ɛ1) and dielectric loss (ɛ2) have been made for bulk Ge8Se60Te30Sb2 chalcogenide glass in the frequency span 100 Hz to 1 MHz and temperature span 303 K to 328 K. Temperature and frequency dependence of ɛ1 and ɛ2 is reported and discussed in the investigated quaternary glassy alloy. The change in ɛ1 with frequency and temperature was described by orientational polarization while change in ɛ2 with frequency and temperature was described by conduction loss and single polaron hopping theory of charge carriers proposed by Elliot and Shimakawa for chalcogenide glasses.

Ultra-wideband 1–20 GHz Non-contact FMR Test System for TMR HGA

The read sensor of hard disk drive abnormality at the bar level is detected by a ferromagnetic resonant analyzer (FMRA) system requiring a galvanic contact to the sensor. After the read head is assembled to the slider, a quasistatic tester is used to check the slider functionality. At this stage, the FMR information of the read sensor cannot be easily accessed since there is no suitable measurement technique available. In this research, an innovative non-contact ferromagnetic resonant (FMR) measurement for the read head sensor quality control at slider level is reported for the first time. The proposed test system is designed for detecting the FMR from 1 to 20 GHz. The FMR is interpreted from the peak of read sensor resistance over the frequency span. This new measurement technique has been applied to a model of tunneling magnetoresistance (TMR) sensor and compared with the FMRA. The FMR frequency at 5.4 GHz can be detected by the proposed measurement system with only 1 MHz difference from the value obtained from conventional FMRA. The higher effective stiffness field of the read sensor is also observed from the lower FMR resistance in the shorter stripe height head.

Eight‐element‐based MIMO antenna with CP behaviour for modern wireless communication

In this work, an eight-element multiple-input multiple-output (MIMO) antenna with circular polarisation (CP) ability is proposed. The proposed antenna consists of eight off-centred fed modified rectangular open-loop resonating radiators. CP behaviour is realised by utilising a narrow slit on the loop and a modified ground plane. Measurement reveals that the proposed CP-MIMO antenna occupies an overlapping 10 dB return loss bandwidth and a 3 dB axial ratio bandwidth over the frequency span of 5-6 GHz with inter-element isolation > 8 dB. Essential diversity and multiplexing performances are estimated intuitively by means of the envelope correlation coefficient and ergodic channel capacity, which lie within their adequate range. To ensure its applicability in user terminal devices and confirm the use of other electronic circuitry, the antenna performances are also investigated in the presence of a conducting sheet which is commonly grounded with the antenna ground plane.

Research on an Electromagnetic Interference Test Method Based on Fast Fourier Transform and Dot Frequency Scanning for New Energy Vehicles under Dynamic Conditions

In recent years, electromagnetic interference (EMI) of new energy vehicles, including difference mode symmetric interference and common mode asymmetry interference, has attracted the attention of many scholars. So far, EMI tests for new energy vehicles under steady conditions cannot reflect the actual EMI of the running vehicle. The results of EMI test methods based on fast Fourier transform (FFT) under dynamic conditions have worse frequency resolutions, and frequency/amplitude accuracy has low precision. Therefore, this paper proposes an EMI test method based on FFT and dot frequency scanning (DFS) for new energy vehicles under dynamic conditions. The identification method for accelerating, sliding, and braking conditions is studied. A comprehensive EMI key evaluation index system for new energy vehicles is built, including characteristic points with maximum amplitude, area, ratio, and density coefficients for high-amplitude characteristic points. Among them, the maximum amplitude is an index to evaluate extreme values. The ratio of high-amplitude characteristic points is a comprehensive index to evaluate the overall region. The density coefficient is an index to evaluate the local region. Finally, this method is applied to three vehicles. With the same instruments, by reducing the FFT frequency span, the frequency resolution and frequency accuracy increase. The results indicate that the EMI of new energy vehicles can be tested under dynamic conditions with high accuracy according to the operable evaluation indexes.

Rattling-Induced Ultralow Thermal Conductivity Leading to Exceptional Thermoelectric Performance in AgIn5S8.

Rattling has emerged as one of the most significant phenomenon for notably reducing the thermal conductivity in complex crystal systems. In this work, using first-principles density functional theory, we found that rattlers can be hosted in simpler crystal systems such as AgIn5S8 and CuIn5S8. Rattlers Ag and Cu exhibit weak and anisotropic bonding with the neighboring In and S and reside in a very shallow anharmonic potential well. The phonon spectra of these compounds have multiple avoided crossing of optical and acoustic modes, which are a signature of rattling motion. This leads to ultralow thermal conductivity, which is inversely proportional to mass and frequency span of rattling modes. Even though Ag atoms contribute to the valence band states, the rattler modes of Ag do not scatter carriers significantly, leaving the electronic transport virtually unaffected. Moreover, AgIn5S8 possesses a combination of heavy and light valence bands resulting in a very high power factor. A combination of favorable thermal and electronic transport results in a very high figure of merit of 2.2 in p-doped AgIn5S8 at 1000 K. The proposed idea of having rattlers in simpler systems can be extended to a wider class of materials, which would accelerate the development of thermoelectric modules for waste energy harvesting.

Effects of bismuth on structural and dielectric properties of cobalt-cadmium spinel ferrites fabricated via micro-emulsion route

Spinel ferrites have a significant role in high-tech applications. In the present work nano-crystalline ferrites having general formula Co0.5Cd0.5BixFe2−xO4 with (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are synthesized via micro-emulsion route. Powder x-ray diffraction (XRD) studies discover the FCC spinel structure. Crystalline size is calculated in a range of 11 nm–15 nm. Lattice parameter calculations are reduced due to its substitution which leads to the exchange of large ionic radius of Fe3+ for small ionic radius of Bi3+. The x-ray density is analyzed to increase with doping. Fourier transform infrared spectroscopy (FTIR) is performed to analyze absorption band spectra. The two absorption bands are observed in a range of 400 cm−1–600 cm−1, and they are the characteristic feature of spinel structure. Thermo-gravimetric analysis (TGA) reveals the total weight loss of nearly 1.98%. Dielectric analysis is carried out by impedance analyzer in a frequency span from 1 MHz to 3 GHz by using the Maxwell Wagner model. Dielectric studies reveal the decrease of dielectric parameters. The alternating current (AC) conductivity exhibits a plane behavior in a low frequency range and it increases with the applied frequency increasing. This is attributed to the grain effects in a high frequency range or may be due to the reduction of porosity. Real and imaginary part of impedance show the decreasing trend which corresponds to the grain boundary action. The imaginary modulus shows the occurrence of peak that helps to understand the interfacial polarization. Cole-Cole graph shows a single semicircle which confirms that the conduction mechanism is due to the grain boundaries at low frequency. Dielectric studies reveal the applicability of these ferrites in high frequency equipment, microwave applications, high storage media, and semiconductor devices.

Beyond 100\u2009THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide

Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing electronic transitions of atoms and molecules. Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in ultraviolet regions. Here we demonstrate a simple route to chip-scale ultraviolet comb generators, simultaneously showing a gap-free frequency span of 128 terahertz and high conversion efficiency. This process relies on adiabatic quadratic frequency translation of a near-visible supercontinuum sourced by an ultrafast fiber laser. The simultaneous cubic and quadratic nonlinear processes are implemented in single-crystalline aluminum nitride thin films, where chirp-modulated taper waveguides are patterned to ensure a broad phase matching. The heterodyne characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain high coherence. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.