S. J. Darak · Christophe Moy · Jacques Palicot
Abstract Emerging paradigms such as smart cities and Internet of Things are expected to be an intrinsic part of next generation communication standards. To bring these paradigms to life, self-sustainable wireless sensor network (WSN) nodes capable of seamless and mainte- nance free operation at remote locations are desired. Recently, radio frequency energy harvesting (RFEH) circuits capable of harvesting RF power transmitted by base stations, TV towers and other ambient RF sources have been developed. Low power requirements and architectural compatibility between WSN nodes and RFEH circuits make RFEH a promising and fea- sible solution for WSN nodes. In this paper, a novel multi-stage decision-making policy (DMP) for RFEH enabled WSN nodes has been proposed. It offers an in- telligence, via online learning algorithm, for character- ization and selection of frequency bands based on their RF potential especially in the dynamic spectrum envi- ronment. Furthermore, proposed DMP supports multi- antenna multi-band harvesting capabilities of the RFEH circuits. The final contribution includes tunable RFEH duration that leads to significant improvement in the harvested energy and fewer number of frequencyband switchings (FBS). Derived theoretical performance bounds and simulation results validate the superiority of pro- posed DMP in terms of the harvested RF energy and
Annular array is expected to improve the volumetric imaging performance of such a bimodality system, thanks to its geometry. The circularly symmetrical response enables the volumetric imaging by rotating the B-mode image around the central axis. We demonstrated the avails of the dual frequencyband annular probe in the OA PE imaging by elaborating the adaptive synthetic aperture algorithm. The frequency-amplitude dependent distortions and the effect of light propagation are incorporated with the reconstruction algorithm to minimize the quantification errors in the accuracy of deliverable data regarding the physiology of the medium. The circular arrangement provides additionally a cavity for accommodating the light probe, enabling perpendicular illumination, shortening the optical path to the absorber and gaining the less skin reflection. This work was inspired by the recent manifestations of CMUT technology in both design and improvement in ultrasound and OA imaging (Choe et al., 2012; Kshirsagar et al., 2013). However, for CMUT technologies, FieldII requires further considerations associated with the physics of CMUT transduction (Baek et al., 2010). We will include this effect in the future studies.
Received: 18 November 2020; Accepted: 4 December 2020; Published: 6 December 2020 Abstract: This work presents lightweight epoxy foams loaded with very low weight percentages (≤0.5 wt.%) of carbon fibers (CFs) with different lengths (3 mm, 6 mm, and 12 mm) as broadband microwave absorbing materials for anechoic chamber application. The effect of CF length on microwave absorption, especially on the absorption frequencyband, is investigated for frequencies between 1 and 15 GHz. For the elaboration of composites, three different methods—spatula, shear mixing, and ultrasounds—are used for the dispersion of CFs. The observation of these CFs, after the dispersion step, shows a high fiber breakage rate when shear mixing is used, unlike when spatula or ultrasounds methods are used. On the other hand, the characterization of the elaborated composites highlights a correlation between the mixing methods, hence the fiber brakeage, and the measured reflection coefficient (reflection loss) of the composites. As a result, the minimum value of the reflection coefficient is shifted toward the high frequencies when the fiber breakage is observed, suggesting that short CFs absorb at high frequencies while long CFs absorb at low frequencies. Dielectric properties, extracted from the measurement in free space, of composites elaborated with different fiber lengths (3 mm, 6 mm, and 12 mm) confirm that short CFs (3 mm) show maximum losses at high frequencies (around 15 GHz) while long CFs (12 mm) show maximum dielectric losses at low frequencies (below 4 GHz). However, no significant variation is observed on the real part of the relative permittivity, as a function of fiber length, for these porous composites loaded with very low CF rates. A hybrid composite, with a mix of different CF lengths, is prepared and characterized. The simulation of the absorption performance of a pyramidal absorber, based on this hybrid composite, is compared to the one of pyramidal absorber based on composites loaded with a single length of carbon fibers. The pyramidal absorber-based hybrid composite predicts the best absorption performance, especially at the low frequencyband. The simulated reflection coefficient of this absorber is less than −12 dB in all the studied frequency range, and less than −40 dB for frequencies higher than 3 GHz. This result confirms the interest of using a mix of carbon fiber lengths to achieve a broadband microwave absorber.
Abstract. We address communications between Autonomous Underwa- ter Vehicles (AUVs), Underwater Sensors (USs) and remote operators. We assume the use of acoustic waves. Due to the Doppler effect, the com- munication frequency depends on the relative motion between the partic- ipants. We are interested in the Ultra Low Frequency (ULF) range, from 0.3 to 3 kHz. We relate the Doppler effect to the half-power bandwidth, versus distance. Numeric simulations are conducted. We show that the Doppler shift is significant with respect to the half-power bandwidth in the ULF band, for long distance communications.
These dipoles are supplied from a CW (Continuous Wave) frequency synthesizer through a -3 dB coupler (power divider) having a /2 phase shift. A picture of this -3 dB power-divider/phase-shifter is shown in Fig.5. Then, the RF signals, which are now in phase quadrature, are amplified by two 40 dB gain power amplifiers before being supplied to the antennas, through 2 coaxial cables of 5 m long. Finally, the gain and phase induced by the electronic parts (amplifiers, cables, connectors, …) between the output of the RF frequency synthesizer and the inputs of each antenna, have been measured using a vector network analyzer. Measurements are summarized in Table 1. Gains of the two transmission lines (TL1 and TL2) are very close to each other, with a difference of 0.8 dB. The phase difference is equal to 85.8°, which is slightly smaller than the desired 90°, despite the difference of lengths of the two output ports of the -3 dB coupler. But, we will see in the next part that it is good enough for our experiment.
Fig. 3. Noise limit and approximate entropy (ApEn) of test data and its phase-randomized surrogates. The test data was a nocturnal segment with 1024 beats from a representative subject; all surrogates were amplitude adjusted . Right ordinate is noise limit (red circles) and left ordinate is corresponding power spectral density (PSD, blue lines); lower abscissa is frequencyband and upper abscissa is beat number of time series (half-tone tracings). A. Noise limit of test data was 34.5±1.5% (mean±SEM) in 20 titrations. B. Noise limits vanished for conventional surrogate data with phase randomization in the entire frequencyband (shading under the PSD plot). C. Noise limit also vanished after phase randomization in the HF band (0.15-0.4 Hz) alone. D. In contrast, if phase randomization was restricted to the LF band and the remaining frequency spectrum other than the HF band, noise limit was reduced (22.9±4.2%) but not abolished. ApEn results (right upper corner) are marginally different between the various conditions.
Fig. 9. DTBA simulated and measured total radiation efficiency set at 616 MHz
V. C ONCLUSIONS
To conclude on the DTBA design, the current performance is promising because with small tuning ratio (Cmax/Cmin of 3) and despite a SCA ESR that reaches a 2.5 Ω value, a very miniaturized antenna may be compliant with any LTE FDD standard over a 600 – 1000 MHz range. It has been demonstrated the quite simple positioning of two resonances on the 600-960 MHz frequencyband with variable duplex spacing from 30 MHz to 60 MHz. More important, this highly coupled antenna structure seems to be less sensitive to ESR value compared to a typical tunable notch.
Twenty-six of the brightest pulsars were also observed us- ing the Superterp low-band antennas (LBAs) in the frequency range 15 −62 MHz. To mitigate the larger dispersive smearing of the profile in this band, 32 channels were synthesised for each of the 240 subbands. The sampling time was 491.52 μs. The integration time of these observations was increased to at least 2220 s to somewhat compensate for the lower sensitivity at this frequencyband (e.g. because of the higher sky temperature). For some sources, 17-minute HBA observations with the Superterp were insuﬃcient to achieve acceptable signal-to-noise (S/N) profiles. For these, longer integration times (or more stations) were needed. Hence, some of the pulsars presented here were later observed with 1 hr pointings as part of the LOFAR Tied-Array All-Sky Survey for pulsars and fast tran- sients (LOTAAS 3 : see also Coenen et al. 2014 ), which com- menced after the oﬃcial commissioning period, during Cycle 0 of LOFAR scientific operations, and is currently ongoing. LOTAAS combines multiple tied-array beams (219 total) per pointing to observe both a survey grid as well as known pulsars within the primary field-of-view.
The complete cell of Fig. 2(a) can be modelled by the circuit in Fig. 2(f), that is proposed as an intuitive extension of Fig. 2(d). Such a circuit is the result of the cascading and series interconnection of several T-networks. To build the complete circuit model, the first main approximation consists of the connection in cascade of the T-models associated with resonators operating in the same frequencyband. The second approximation is the series connection of the T-models associ- ated to resonators operating in different frequency bands. Such approximations are based in the multi-resonant modelling done in . More specifically, the set of two equal folded slots is represented by the upper block containing two cascaded identical T-networks (LC-tanks of lumped elements L 1 and C 1 ). Analogously, the group of three H-shaped resonators is represented by the lower block containing three cascaded T- networks (LC-tanks of lumped elements L 2 , C 2 , L 3 and C 3 , since the first and last slots are considered to be identical). The inductive coupling coefficients are captured by the in- ductances L f , L m , L g and L n . Finally, the inductances L ext and L int represent the presence of the waveguide aperture. Possible mutual coupling between folded and H-shaped slots is not expected to be significant within the structure bands of operation, and thus not included in the proposed circuit. This assumption is justified by the fact that such bands are mainly determined by the resonance character of a given type of perforation. In this case, and as it is explained in detail in  for the case of compound grating modelling, mutual coupling is almost negligible and its consideration increases
For future work, it is important to enhance the microwave performance of the VCSEL packaging in order to avoid the use of a test probe facility and to facilitate the laser integra- tion into the oscillator with no parasitic effect that degrades the direct modulation of the laser. The goal of achieving 5G frequencyband carriers is accomplished with no need of frequency multiplication. It is a technology road that needs to be promoted for the development of better spectrum man- agement and radio over fiber applications, such as remote carrier distribution.
DOI: 10.1103/PhysRevA.83.035806 PACS number(s): 42.70.Qs, 88.40.F−, 88.40.H−
Materials and structures that strongly discriminate electro- magnetic radiation based on one, or more of its properties (e.g. polarization, frequency) play an enabling role for a wide range of physical phenomena. For example, polarizers can selectively transmit light based on its polarization [ 1 ] over a wide range of frequencies; photonic crystals [ 2 ] (PhCs) can reflect light of certain frequencies irrespective of the angle of incidence and irrespective of the polarization. A material system that could transmit light based primarily on the angle of incidence might also enable a variety of novel physical phenomena. Light incident at a prescribed range of angles would be nearly perfectly transmitted, while light from other angles of incidence would be nearly perfectly reflected [Fig. 1(a) ]. Ideally, such an angular selectivity would apply independent of the incoming polarization and over a broad range of frequencies. Structures with such strong angular selectivity do not currently exist. For example, PhCs exhibit some angular discrimination of light, but this discrimination is always strongly dependent on frequency, as illustrated in Fig. 1(b) . In this Brief Report, we present a material system that opens the desired angular gaps, as shown in Fig. 1(c) . For example, using realistic constituent material parameters, we present numerical calculations demonstrating an angular photonic band gap (PBG) material system in which light close to normal incidence is nearly perfectly transmitted for a wide range of frequencies, independent of the polarization. In contrast, light of angles further from the normal (e.g., 22.5 ◦ –90 ◦ ) can be nearly perfectly reflected over >100% fractional frequencyband gap. The key to these interesting angular PBG material systems lies in exploring PhCs whose constituents have anisotropic permittivity and/or permeability.
𝑟 ′ = 𝑟 𝐸
𝐺0 0.5 (96)
For case (a), we find that compressive strain applied in the 100 direction results in an increase in current while tensile strain results in a decrease in current, which is seen in Figure 22a. The results stem from the change in band gap with strain of the x and y-lobes as tunneling to the z-lobe is inhibited due to its large longitudinal tunneling mass. Since uniaxial stress is applied along the 100 crystal direction, more strain exists along the x-direction compared to the y- direction due to Poisson’s ratio, and the x-lobe dominates the behavior of the change in current at low stresses.
Automatic frequency control and frequency indicators Mounce, G.R.
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Most recent SAR sensors use wide band signals to achieve metric range resolution. One can also take advantage of wide band to split it into sub-bands and generate several lower-resolution images, centered on slightly different frequencies, from a single acquisition .
This process, named Multi Chromatic Analysis (MCA) corresponds to performing a spectral analysis of SAR images. From this spectral analysis, three potential applications are shown:
A B S T R A C T
Ultrasonic fatigue testing at 20 kHz was developed to accelerate fatigue tests and explore the very high cycle fatigue range. However, the use of ultrasonic fatigue systems raises the open question of the impact of frequency on the material fatigue response. In this paper, the impact of the loading frequency on the fatigue response of face-centered cubic polycrystalline copper is investigated. Firstly, experiments with conventional fatigue devices inducing loading frequencies lower than 100 Hz were carried out and compared with previously published re- sults obtained with an ultrasonic fatigue machine at 20 kHz. At a stress amplitude of 100 MPa, the number of cycles required for failure was found to be more than 200 times greater for tests at 20 kHz than for tests at frequencies below 100 Hz. Secondly, for both types of fatigue tests, the conditions in stress amplitude and number of cycles needed for the emergence of the early slip markings were investigated. The early slip marking S–N curve was found to have the same sensitivity as the failure S–N curve to the frequency effect. Thirdly, the different possible reasons responsible for this frequency effect are discussed. It is concluded that the physical mechanisms of slip marking formation is preserved while their kinetics is sensitive to the loading frequency. Finally, two simplified models were considered and for the first time, the frequency effect on the fatigue response of copper is quantitatively correlated with the occurrence of time-dependent dislocations cross slip and vacancies production/diffusion involved in the persistent slip band formation for face-centered cubic structure. The ob- tained results could be extended to a full class of materials where slip band formation plays a key role in the crack initiation process while suggesting no frequency effect for materials with a lower degree of plasticity develop- ment at crack initiation (High Strength Steels for example).
Before going into further detail, we analyze a typical example. Let us consider two truncated sinusoids, x 1 (t) and x 2 (t). The two signals have the same amplitude and one has twice the frequency of the other: f 2 = 2f 1 . A geometrical approach, associated with the study of the curves of the two signals, shows that the length of the curve connected to x 2 (t) seems to be the double that of x 1 (t) (see Fig. 2).
Monolithic lasers on silicon are ideal for large scale electronic-photonic integration (1). Ge-on-Si is an interesting candidate due to its compatibility with silicon complementary metal oxide semiconductor (CMOS) process and its pseudo-direct band gap behavior in the near infrared regime for optical communications. We have proposed through theoretical analysis that epitaxial Ge-on-Si can be band-engineered to behave like a direct gap material to achieve optical gain and lasing by using tensile strain and n-type doping to compensate the energy difference between the direct and indirect conduction valleys (2). In this paper we present lasing from the direct gap transition of band-engineered Ge-on-Si edge emitting waveguide devices at room temperature. The emission wavelength range of 1590-1610 nm is in good agreement with the optical gain spectrum reported previously (3). We also present theoretical modeling and room-temperature electroluminescence results on n + Si/n + Ge/p + Si double heterojunction structures for
We have investigated the dynamics of band-edge illumination in films of HgTe QDs with absorption in the MIR. We are able to see a current modulation under illumination with very high dynamics, and we claim that our devices are capable of MIR detection with a large bandwidth (above 10 MHz). We believe that those fast dynamics are the result of the ambipolar behavior of our material, as well as reduced role played by the traps under low power illumination. As a result, photoresponse in our system mainly results from recombination-limited processes and mostly reflects instantaneous carrier density changes. We presented evidences for multi-exciton generation detected through photocurrent measurements in our devices, with a threshold around 3 E G and an efficiency of 3 to 4 (compared to efficiency in the MIR) at 6 E G . This MEG effect seems to be robust on a large frequency range, reinforcing our claim for high-bandwidth device. To conclude, we investigated the effect of gate on photoconduction and showed that photoresponse can be switched from positive to negative under hole or electron injection, respectively. We explain this effect with a simple model where carrier density is set by the light under hole injection, and by the gate under electron injection. Mobility reduction under illumination (because of the introduction of recombination centers) then explains why negative photoconduction arises.