Practical study of idle tones in 2nd-order bandpass ΣΔ modulators

This paper studies the tonal behaviour of the quantization noise in 2nd-order bandpass ΣΔ modulators. Closed-form expressions for the frequency of the idle tones are derived for different locations of the signal centre frequency. The analytical results are validated through experimental measurements taken from a 0.8 μm CMOS prototype realized using fully differential switched-current circuits.     

80-MHz bandpass /spl Delta//spl Sigma/ modulators for multimode digital IF receivers

Three fully differential bandpass (BP) /spl Delta//spl Sigma/ modulators are presented. Two double-delay resonators are implemented using only one operational amplifier. The prototype circuits operate at a sampling frequency of 80 MHz. The BP /spl Delta//spl Sigma/ modulators can be used in an intermediate-frequency (IF) receiver to combine frequency downconversion with analog-to-digital conversion by directly sampling an input signal from an IF of 60 MHz to a digital IF of 20 MHz. The measured peak signal-to-noise-plus-distortion ratios are 78 dB for 270 kHz (GSM), 75 dB for 1.25 MHz (IS-95), 69 dB for 1.762 MHz (DECT), and 48 dB for 3.84 MHz (WCDMA/CDMA2000) bandwidths. The circuits are implemented with a 0.35-/spl mu/m CMOS technology and consume 24-38 mW from a 3.0-V supply, depending on the architecture.     

IEC class 0.5 electronic watt–hour meter implemented with first-order sigma–delta converters

This paper presents a design of a precise electronic watt–hour meter implemented with first-order sigma–delta analog-to-digital converters. An efficient digital signal-processing circuit is introduced into the design where the whole signal processing is performed directly on an oversampled signal, eliminating the necessity for a decimation filter. It is shown that Gaussian noise, added to the input signal of sigma–delta modulators, decorrelates quantization error of first-order sigma–delta modulators and thus substantially reduces measurement error at low-signal amplitudes. The effect of added noise is analyzed by extensive behavioral simulations for various input signal amplitudes and for various input noise levels. The results show that IEC accuracy class 0.5 can be achieved with first-order sigma–delta converters and digital signal processing in the sigma–delta domain.     

Novel hybrid D/A structures for high-resolution SAR ADCs—analysis, modeling and realization

An intermittent-sleeping C-R hybrid D/A structure, a resistor-reusing R-C hybrid D/A structure and an R-C-R combined D/A structure are researched with theoretical analysis and Matlab modeling in this paper. The switching power and passive components' matching requirement of these D/A structures are discussed in detail. Three SAR ADCs with these three novel structures are realized based on SMIC 0.18 μm CMOS, SMIC 90 nm CMOS and UMC 90 nm CMOS, respectively. The theoretical analysis, modeling verification and circuit realization proves the applicability of these three D/A conversion networks to high-resolution SAR ADCs.     

Methodology and experimental system for measuring and displaying I–V characteristic curves of PV facilities

This paper describes a methodology and the developed system for measuring, capturing, and displaying I–V and P–V characteristic curves of photovoltaic (PV) modules or arrays based on single‐ended primary inductance converters (SEPIC) in parallel connection, operating in interleaved mode. The proposed methodology and the developed system allow the real time capture and displaying of the I–V and P–V curves of a PV panel or array, and show several advantages with regard to classical methods: simple structure, scalability, fast response, versatility, direct display, and low cost. The measuring of the characteristic curves of PV modules includes high speed of response and high fidelity, with low ripple. An experimental prototype based on four SEPIC converters in parallel connection has been implemented to validate the proposed methodology. This new methodology and experimental system has been registered in the Spanish Patent and Trademark Office with the number P200930198. Copyright © 2009 John Wiley & Sons, Ltd.     

A 0.13 μm CMOS adaptive sigma-delta modulator for triple-mode GSM/Bluetooth/UMTS applications

This paper describes the design and experimental characterization of a 0.13 μm CMOS switched-capacitor reconfigurable cascade ΣΔ modulator intended for multi-standard GSM/Bluetooth/UMTS hand-held devices. Both architectural- and circuital-level reconfiguration strategies are incorporated in the chip in order to adapt the effective resolution and the output rate to different standard specifications with optimized power dissipation. This is achieved by properly combining different reconfiguration modes that include the variation in the order of the loop filter (3rd- or 4th-order), the clock frequency (40 or 80 MHz), the internal quantization (1 or 2 bits), and the bias currents of the amplifiers. The selection of the modulator topology and the design of its building blocks are based on a top-down CAD methodology that combines simulation and statistical optimization at different levels of the modulator hierarchy. Experimental measurements show a correct operation of the prototype for the three standards, featuring dynamic ranges of 83.8/75.9/58.7 dB and peak signal-to-(noise+distortion) ratios of 78.7/71.3/53.7 dB at 400 ksps/2/8 Msps, respectively. The modulator power consumption is 23.9/24.5/44.5 mW, of which 9.7/10/24.8 mW are dissipated in the analog circuitry. The multi-mode ΣΔ prototype shows an overall performance that is competitive with the current state of the art.¹     

In Situ Bond Modulation of Graphitic Carbon Nitride to Construct p–n Homojunctions for Enhanced Photocatalytic Hydrogen Production

Graphitic carbon nitride (g‐C₃N₄) has recently emerged as an attractive photocatalyst for solar energy conversion. However, the photocatalytic activities of g‐C₃N₄ remain moderate because of the insufficient solar‐light absorption and the fast electron–hole recombination. Here, defect‐modified g‐C₃N₄ (DCN) photocatalysts, which are easily prepared under mild conditions and show much extended light absorption with band gaps decreased from 2.75 to 2.00 eV, are reported. More importantly, cyano terminal C?N groups, acting as electron acceptors, are introduced into the DCN sheet edge, which endows the DCN with both n‐ and p‐type conductivities, consequently giving rise to the generation of p–n homojunctions. This homojunction structure is demonstrated to be highly efficient in charge transfer and separation, and results in a fivefold enhanced photocatalytic H₂ evolution activity. The findings deepen the understanding on the defect‐related issues of g‐C₃N₄‐based materials. Additionally, the ability to build homojunction structures by the defect‐induced self‐functionalization presents a promising strategy to realize precise band engineering of g‐C₃N₄ and related polymer semiconductors for more efficient solar energy conversion applications.     

Adaptive modulation for space–frequency block coded OFDM systems

The growing popularity of both multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) systems has created the need for adaptive modulation to integrate temporal, spatial and spectral components together. In this article, an overview of some adaptive modulation schemes for OFDM is presented. Then a new scheme consisting of a combination of adaptive modulation, OFDM, high-order space-frequency block codes (SFBC), and antenna selection is presented. The proposed scheme exploits the benefits of space–frequency block codes, OFDM, adaptive modulation and antenna selection to provide high-quality transmission for broadband wireless communications. The spectral efficiency advantage of the proposed system is examined. It is shown that antenna selection with adaptive modulation can greatly improve the performance of the conventional SFBC–OFDM systems.     

A Novel Conductive Core–Shell Particle Based on Liquid Metal for Fabricating Real‐Time Self‐Repairing Flexible Circuits

As a critical part of flexible electronics, flexible circuits inevitably work in a dynamic state, which causes electrical deterioration of brittle conductive materials (i.e., Cu, Ag, ITO). Recently, gallium‐based liquid metal particles (LMPs) with electrical stability and self‐repairing have been studied to replace brittle materials owing to their low modulus and excellent conductivity. However, LMP‐coated Ga₂O₃ needs to activate by external sintering, which makes it more complicated to fabricate and gives it a larger short‐circuit risk. Core–shell structural particles (Ag@LMPs) that exhibit excellent initial conductivity(8.0 Ω sq^?1) without extra sintering are successfully prepared by coating nanosilver on the surface of LMPs through in situ chemical reduction. The critical stress at which rigid Ag shells rupture can be controlled by adjusting the Ag shell thickness so that LM cores with low moduli can release, achieving real‐time self‐repairing (within 200 ms) under external destruction. Furthermore, a flexible circuit utilizing Ag@LMPs is fabricated by screen printing, and exhibits outstanding stability and durability (R/R₀ < 1.65 after 10 000 bending cycles in a radius of 0.5 mm) because of the functional core–shell structure. The self‐repairable Ag@LMPs prepared in this study are a candidate filler for flexible circuit design through multiple processing methods.     

Vacuum Calcination Induced Conversion of Selenium/Carbon Wires to Tubes for High‐Performance Sodium–Selenium Batteries

A vacuum calcination approach is developed to fabricate selenium/carbon composites, which does not require intensive mixing and durable heating such as in commonly used melt‐infusion methods of loading selenium into carbon hosts. Starting from carbon‐coated selenium wires prepared via a wet‐chemical reaction, selenium/carbon tubes are fabricated by a straightforward calcination process. The calcination is conducted in a confined space to reduce the insulating carbon shell under vacuum, and selenium melts but remains a constituting part of the composite. Paired with sodium metal anode, the resultant selenium/carbon tubes deliver a high reversible capacity of 601 and 509 mA h g^?1 at 0.2 and 2 C normalized by the mass of selenium, which corresponds to energy and power densities of 860 and 667 Wh kg^?1 at 193 and 1770 W kg^?1, respectively. Such capacity and rate performance surpasses most typical cathode materials for lithium or sodium (ion) batteries, according to the comparative literature analysis. Moreover, the robust tubular‐like hollow structure of the selenium/carbon composites ensures for impressive capacity retention of more than 90% after 1000 cycles at 20 C.     

In Situ Formation of Copper‐Based Hosts Embedded within 3D N‐Doped Hierarchically Porous Carbon Networks for Ultralong Cycle Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are promising energy storage systems due to their large theoretical energy density of 2600 Wh kg^?1 and cost effectiveness. However, the severe shuttle effect of soluble lithium polysulfide intermediates (LiPSs) and sluggish redox kinetics during the cycling process cause low sulfur utilization, rapid capacity fading, and a low coulombic efficiency. Here, a 3D copper, nitrogen co‐doped hierarchically porous graphitic carbon network developed through a freeze‐drying method (denoted as 3D Cu@NC‐F) is prepared, and it possesses strong chemical absorption and electrocatalytic conversion activity for LiPSs as highly efficient sulfur host materials in Li–S batteries. The porous carbon network consisting of 2D cross‐linked ultrathin carbon nanosheets provides void space to accommodate volumetric expansion upon lithiation, while the Cu, N‐doping effect plays a critical role for the confinement of polysulfides through chemical bonding. In addition, after sulfuration of Cu@NC‐F network, the in situ grown copper sulfide (Cu_xS) embedded within Cu_xS@NC/S‐F composite catalyzes LiPSs conversion during reversible cycling, resulting in low polarization and fast redox reaction kinetics. At a current density of 0.1 C, the Cu_xS@NC/S‐F composites' electrode exhibits an initial capacity of 1432 mAh g^?1 and maintains 1169 mAh g^?1 after 120 cycles, with a coulombic efficiency of nearly 100%.     

Adaptive modulation with constrained variable power for space–time coded MIMO systems

Practically, the maximum transmission power of transmission systems is limited. This power constraint causes the variable power control derived from no maximum power limitation suffering from performance degradation. In this paper, a constrained variable‐power adaptive M‐ary quadrature amplitude modulation scheme for MIMO systems with space–time coding is developed. Convex optimization is used to derive the switching thresholds of the instantaneous signal‐to‐noise ratio for power control (PC) and adaptive modulation under the constraints of maximum power, average power, and target BER. In the derivation of the relation between modulation and power, the exact BER expression of binary phase shift keying modulation and a tight bound for higher order quadrature amplitude modulation are used to make the PC scheme fulfill the target BER even at low signal‐to‐noise ratio where the previous PC schemes fail to meet the target BER. Numerical results show that the derived control scheme under the power constraints can obtain the spectrum efficiency and BER performance close to the previous control scheme without power limitation. Moreover, it can satisfy the requirements of power limitation and target BER and can effectively avoid the excessive power consumption of previous PC scheme in poor channel condition. Copyright © 2012 John Wiley & Sons, Ltd.