Compact SC frequency tuning circuit for continuous-time Gm–C filters

A novel automatic frequency tuning circuit for continuous-time filters is presented. Based on the switched-capacitor technique, the circuit offers an advantage in terms of simplicity resulting from the use of only two matched current sources, an operational amplifier with relaxed specifications and a transconductor that is a replica of the filter transconductors. Despite the simplicity of the scheme, the accuracy of the system is less than 1 % of frequency error. The circuit has been designed in a 0.5 μm CMOS technology with a 3.3 V power supply and simulation results confirm the suitability of the proposed approach.     

Spectral study and classification of worldwide locations considering several multijunction solar cell technologies

Multi‐junction solar cells are widely used in high‐concentration photovoltaic systems (HCPV) attaining the highest efficiencies in photovoltaic energy generation. This technology is more dependent on the spectral variations of the impinging Direct Normal Irradiance (DNI) than conventional photovoltaics based on silicon solar cells and consequently demands a deeper knowledge of the solar resource characteristics. This article explores the capabilities of spectral indexes, namely, spectral matching ratios (SMR), to spectrally characterize the annual irradiation reaching a particular location on the Earth and to provide the necessary information for the spectral optimization of a MJ solar cell in that location as a starting point for CPV module spectral tuning. Additionally, the relationship between such indexes and the atmosphere parameters, such as the aerosol optical depth (AOD), precipitable water (PW), and air mass (AM), is discussed using radiative transfer models such as SMARTS to generate the spectrally resolved DNI. The network of ground‐based sun and sky‐scanning radiometers AErosol RObotic NETwork (AERONET) is exploited to obtain the atmosphere parameters for a selected bunch of 34 sites worldwide. Finally, the SMR indexes are obtained for every location, and a comparative analysis is carried out for four architectures of triple junction solar cells, covering both lattice match and metamorphic technologies. The differences found among cell technologies are much less significant than among locations. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of small-signal AC voltages on C–V characterization and parameter extraction of SiO2 thin films

The effect of different small-signal ac voltage amplitudes on C–V curves characterized by thin SiO₂ based p-type MOS capacitor with aluminum gate is reported. When the small-signal ac voltage is comparable to the gate bias, the thickness of SiO₂ thin films extracted from the accumulation capacitance is found to be independent of small-signal ac voltage amplitudes, but the flat band voltage shift and interface state density associated with the variation of depletion layer capacitance are dependent on small-signal ac voltage amplitudes. They all increase with the small-signal ac voltage amplitudes. The experimental results reveal that the optimum small-signal ac voltage should be less than 100 mV. The mechanisms involving the depletion layer changes with small-signal ac voltages in SiO₂ thin films are also discussed in this paper.     

Computer Simulations of VANETs Using Realistic City Topologies

Researchers in vehicular ad hoc networks (VANETs) commonly use simulation to test new algorithms and techniques. This is the case because of the high cost and labor involved in deploying and testing vehicles in real outdoor scenarios. However, when determining the factors that should be taken into account in these simulations, some factors such as realistic road topologies and presence of obstacles are rarely addressed. In this paper, we first evaluate the packet error rate (PER) through actual measurements in an outdoor road scenario, and deduce a close model of the PER for VANETs. Secondly, we introduce a topology-based visibility scheme such that road dimension and geometry can be accounted for, in addition to line-of-sight. We then combine these factors to determine when warning messages (i.e., messages that warn drivers of danger and hazards) are successfully received in a VANET. Through extensive simulations using different road topologies, city maps, and visibility schemes, we show these factors can impact warning message dissemination time and packet delivery rate.     

Compact Modules for Wireless Communication Systems in the E-Band (71–76 GHz)

The millimeter-wave spectrum above 70 GHz provides a cost-effective solution to increase the wireless communications data rates by increasing the carrier wave frequencies. We report on the development of two key components of a wireless transmission system, a high-speed photodiode (HS-PD) and a Schottky Barrier Diode (SBD). Both components operate uncooled, a key issue in the development of compact modules. On the transmitter side, an improved design of the HS-PD allows it to deliver an output RF power exceeding 0 dBm (1 mW). On the receiver side, we present the design process and achieved results on the development of a compact direct envelope detection receiver based on a quasi-optical SDB module. Different resonant (meander dipole) and broadband (Log-Spiral and Log-Periodic) planar antenna solutions are designed, matching the antenna and Schottky diode impedances at high frequency. Impedance matching at baseband is also provided by means of an impedance transition to a 50 Ohm output. From this comparison, we demonstrate the excellent performance of the broadband antennas over the entire E-band by setting up a short-range wireless link transmitting a 1 Gbps data signal.     

Crystal Size,Morphology, and Growth Mechanism in Bio‐Inspired Apatite Nanocrystals

Bio‐inspired apatite nanoparticles precipitated in the presence of citrate ions at increasing maturation times are characterized in terms of structure, size, morphology, and composition through advanced X‐ray total scattering techniques. The origin of the platy crystal morphology, breaking the hexagonal symmetry, and the role of citrate ions is explored. By cross‐coupling the size and shape information of crystal domains with those obtained by atomic force microscopy on multidomain nanoparticles, a plausible mechanism underlying the amorphous‐to‐crystal transformation is reconstructed. In the present study, citrate plays the distinct roles of inducing the platy morphology of the amorphous precursor and controlling the thickness of the Ca‐deficient apatite nanocrystals. These findings can open new scenarios also in bone mineralization, where citrate might have a broader role to play than has been thought to date.     

Structural Changes in the BODIPY Dye PM567 Enhancing the Laser Action in Liquid and Solid Media

In the search for more efficient and photostable solid‐state dye lasers, newly synthesized analogs of the borondipyrromethene (BODIPY) dye PM567, bearing the polymerizable methacryloyloxypropyl group at position 2 (PMoMA) or at positions 2 and 6 (PDiMA), have been studied in the form of solid copolymers with methyl methacrylate (MMA). The parent dye PM567, as well as the model analogs bearing the acetoxypropyl group in the same positions, PMoAc and PDiAc, respectively, have been also studied both in liquid solvents and in solid poly(MMA) (PMMA) solution. Although in liquid solution PMoAc and PDiAc have the same photophysical properties as PM567, PDiAc exhibited a photostability up to 10 times higher than that of PM567 in ethanol under 310 nm‐irradiation. The possible stabilization factors of PDiAc have been analyzed and discussed on the basis of the redox potentials, the ability for singlet molecular oxygen [O₂(¹Δ_g)] generation, the reactivity with O₂(¹Δ_g), and quantum mechanical calculations. Both PMoAc and PDiAc, pumped transversally at 532 nm, lased in liquid solution with a high (up to 58 %), near solvent‐independent efficiency. This enhanced photostabilization has been also observed in solid polymeric and copolymeric media. While the solid solution of the model dye PDiAc in PMMA showed a lasing efficiency of 33 %, with a decrease in the laser output of ca. 50 % after 60 000 pump pulses (10 Hz repetition rate) in the same position of the sample, the solid copolymer with the double bonded chromophore, COP(PDiMA‐MMA), showed lasing efficiencies of up to 37 %, and no sign of degradation in the laser output after 100 000 similar pump pulses. Even under the more demanding repetition rate of 30 Hz, the laser emission from this material remained at 67 % of its initial laser output after 400 000 pump pulses, which is the highest laser photostability achieved to date for solid‐state lasers based on organic polymeric materials doped with laser dyes. This result indicates that the double covalent linkage of the BODIPY chromophore to a PMMA polymeric matrix is even more efficient than the simple linkage, for its photostabilization under laser operation.     

Mapping of Discrete Cosine Transforms onto Distributed Hardware Architectures

We present an algorithmically-aware, high-level partitioning methodology for discrete cosine transforms (DCT) targeted to distributed hardware architectures. The methodology relies on the exploration of alternate DCT formulations as part of the partition optimization process. To the best of our knowledge, no previously proposed DCT algorithm exists that is capable of consistently producing alternate regular formulations for an n-size DCT. Hence, a new Cooley-Tukey-like DCT factorization algorithm was developed to allow exploration of alternate formulations as part of the partitioning optimization process. The use of our factorization mechanism along with a greedy strategy to explore the space of equivalent DCT formulations yielded partitioning solutions with as much as 18% reduction in latency and 83% reduction in run-time as compared to previously proposed regular DCT formulations.

Analysis of Heuristic Graph Partitioning Methods for the Assignment of Packet Control Units in GERAN

Over the last few years, graph partitioning has been recognized as a suitable technique for optimizing cellular network structure. For example, in a recent paper, the authors proposed a classical graph partitioning algorithm to optimize the assignment of cells to Packet Control Units (PCUs) in GSM-EDGE Radio Access Network. Based on this approach, the quality of packet data services in a live environment was increased by reducing the number of cell re-selections between different PCUs. To learn more about the potential of graph partitioning in cellular networks, in this paper, a more sophisticated, yet computationally efficient, partitioning algorithm is proposed for the same problem. The new method combines multi-level refinement and adaptive multi-start techniques with algorithms to ensure the connectivity between cells under the same PCU. Performance assessment is based on an extensive set of graphs constructed with data taken from a live network. During the tests, the new method is compared with classical graph partitioning approaches. Results show that the proposed method outperforms classical approaches in terms of solution quality at the expense of a slight increase in computing time, while providing solutions that are easier to check by the network operator.