Spl Delta Research Articles

Modeling of Switched-Capacitor Delta–Sigma Modulators in SIMULINK

Precise behavioral modeling of switched-capacitor /spl Delta//spl Sigma/ modulators is presented. Considering noise (switches' and op-amps' thermal noise), clock jitter, nonidealities of integrators and op-amps including finite dc-gain (DCG) and unity gain bandwidth, slew-limiting, DCG nonlinearities and the input parasitic capacitance, quantizer hysteresis, switches' clock-feedthrough, and charge injection, exhaustive behavioral simulations that are close models of the transistor-level ones can be performed. The DCG nonlinearity of the integrators, which is not considered in many /spl Delta//spl Sigma/ modulators' modeling attempts, is analyzed, estimated, and modeled. It is shown that neglecting this parameter would lead to a significant underestimation of the modulators' behavior and increase the noise floor as well as the harmonic distortion at the output of the modulator. Evaluation and validation of the models were done via behavioral and transistor-level simulations for a second-order modulator using SIMULINK and HSPICE with a generic 0.35-/spl mu/m CMOS technology. The effects of the nonidealities and nonlinearities are clearly seen when compared to the ideal modulator in the behavioral and actual modulator in the circuit-level environment.

A fourth-order /spl Sigma//spl Delta/ interface for micromachined inertial sensors

This paper presents the design and implementation of a high-order /spl Sigma//spl Delta/ interface for micromachined inertial sensors, which employs an electronic filter in series with the mechanical sensor element to reject the excessive in-band quantization noise inherently present in state-of-the-art second-order solutions. A fourth-order prototype was fabricated in a standard 0.5-/spl mu/m CMOS process. The active circuit area measures 0.9 mm/sup 2/, and the interface consumes 13 mW from a 5-V supply and achieves resolution of 1/spl deg//s//spl radic/Hz with a gyroscope and 150/spl mu/g//spl radic/Hz with an accelerometer. Comparison between the measured and simulated behavior of the system shows that the contribution of the quantization error to the total noise is negligible.

CDMA System Sensitivity to<tex>$SigmaDelta$</tex>A/D Conversion

In this paper, the performance of a code-division multiple-access (CDMA) system using digital frequency conversion is analyzed and modeled. The joint effect of channel noise, sampling jitter, and analog-digital converter (ADC) errors is discussed and modeled assuming uniform converters. Variable resolution memoryless and /spl Sigma//spl Delta/ ADCs are considered, and the sensitivities of the resulting systems to channel noise, sampling jitter, and overloading errors are compared.

Noise-aided processing: revisiting ditheringin a /spl Sigma//spl Delta/ quantizer

In this paper, we show that a particular /spl Sigma//spl Delta/ quantizer, which is a strongly nonlinear dynamical system, can provide noise-enhanced processing effects in some sense. In particular, we will show that quantizing a sine, which means adding a small amount of noise in the input, can improve the quantization process. In particular, the output local signal-to-noise ratio (SNR) exhibits a maximum when plotted against the noise amplitude.

Air trench bends and splitters for dense optical integration in low index contrast

Low-loss air trench structures that permit reduced-size bends in low (/spl Delta/ = 0.7-7%) and medium (/spl Delta/ = 7-20%) index contrast waveguides are demonstrated. Local high index contrast at bends is achieved by introducing air trenches with cladding tapers that provide an adiabatic mode transition between low and high index contrast regions. We have fabricated and measured the performance of bends and T-splitters in silica low index contrast waveguides. Complimentary metal-oxide-semiconductor (CMOS) compatible processes are effective in processing, and measured losses are low and consistent with theoretical simulations.

A Novel Silicon Schottky Diode for NLTL Applications

The design and simulation of a novel silicon Schottky diode for nonlinear transmission line (NLTL) applications is discussed in this paper. The Schottky diode was fabricated on a novel silicon-on-silicide-on-insulator (SSOI) substrate for minimized series resistance. Ion implantation technology was used as a low-cost alternative to molecular beam epitaxy to approximate the delta (/spl delta/) doping profile, which results in strong nonlinear CV characteristics. The equivalent circuit model of the Schottky diode under reverse bias conditions was extracted from the S-parameter measurement performed on the diode. The measured CV characteristics show strong nonlinearity, the junction capacitance varies from 182 to 47.5 fF as the reverse bias voltage is varied from 0 to -5 V. A parasitic inductance of 40 pH was measured for the silicon Schottky diode, which is much smaller than a comparable sized GaAs Schottky diode. This small inductance is an advantage for the silicon Schottky diode offering improvement in the silicon NLTL performance.

Improved thermal, structural and electrical properties of epoxy for use at high voltages

In this paper, the epoxy elasticity factors were investigated by thermomechanical analysis (TMA), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA) and field emission scanning electron microscope (FESEM) to improve toughness and reduce brittleness of existing epoxy resin. Dumbbell shaped specimens were made and tested at rates of 0, 20 and 35 parts per hundred resins (phr). TMA and DMTA temperatures ranged from -20 to 200 /spl deg/C and TGA ranged from 0 to 600 /spl deg/C. The glass transition temperature (Tg) of elastic epoxy was measured by thermal analysis. Also investigated were the thermal expansion coefficient (/spl alpha/), the high-temperature characteristics, modulus and the loss factor (Tan /spl delta/). We analyzed the structure using FESEM and have found elastomer particles (elastic-factors) in the elastic epoxy matrix. We have made elastic epoxy by adding elastomer particles to existing epoxy. Generally, the toughness of elastic epoxy can be improved by changing the structure of existing epoxy of poor impact-strength. In addition, we measured the permittivity and Tan /spl delta/ for investigation of the electrical properties of elastic epoxy. The permittivity and Tan /spl delta/ depend on the elastomer composition. Namely, the permittivity and Tan /spl delta/ increase according to the elastomer contents. For experimental analysis results, 20 phr was considered an excellent specimen.

Characterizing the effects of the PLL jitter due to substrate noise in discrete-time delta-sigma modulators

This paper investigates the impact of clock jitter induced by substrate noise on the performance of the oversampling /spl Delta//spl Sigma/ modulators. First, a new stochastic model for substrate noise is proposed. This model is then utilized to study the clock jitter in clock generators incorporating phase-locked loops (PLLs). Next, the effect of the clock jitter on the performance of the /spl Delta//spl Sigma/ modulator is studied. It will be shown that substrate noise degrades the signal-to-noise ratio of the /spl Delta//spl Sigma/ modulator while the noise shaping does not have any effect on clock jitter induced by substrate noise. To verify the analysis experimentally, a circuit consisting of a second-order /spl Delta//spl Sigma/ modulator, a charge-pump PLL, and forty multistage digital tapered inverters driving 1-pF capacitors is designed in a 0.25-/spl mu/m standard CMOS process. Several experiments on the designed circuit demonstrate the high accuracy of the proposed analytical models.

Formation of Strongly Biaxial-Textured Ni Layer for YBCO Coated Conductor by Electrodeposition Process

A new method of strongly biaxial-textured Ni substrate for YBCO coated conductor has been developed by electrodeposition using cube-textured cathode such as Ni single crystal, RaBiTS-processed Ni or Ni-W tape. Electrodeposited Ni layers using Ni single crystal as a cathode were characterized with /spl theta/-scan and /spl phi/-scan methods, and it showed excellent out-of-plane (/spl Delta//spl theta/=1.6/spl deg/) and in-plane alignment (/spl Delta//spl phi/=3.6/spl deg/). In addition, electrodeposited Ni layers using RaBiTS tape as a cathode show good biaxial texture (/spl Delta//spl theta/=5.7/spl deg/,/spl Delta//spl phi/=9.6/spl deg/) and smooth surface. XRD characterization shows that biaxial texture of cathode was transferred to electrodeposit and texture of electrodeposited Ni layers changes with deposition time. Finally, a CeO/sub 2/ buffer layer has been deposited by MOD process to show the compatibility of substrate with further buffer layer processing. Our modified electrodeposition process shows good possibility of providing a simple and effective way to high quality cube-textured Ni substrates for YBCO coated conductor.

Continuous Deposition and Rapid Fabrication of Self-Epitaxial<tex>$rm CeO_2$</tex>Cap Layers by PLD Method on IBAD Buffers

We have fabricated long buffered substrate tapes by a reel-to-reel process. The architecture is a self-epitaxial PLD-CeO/sub 2/ cap layer on an IBAD-Gd/sub 2/Zr/sub 2/O/sub 7/ buffered Hastelloy-C 276 tape. At the same time, we have investigated the effect of the deposition temperature for self-epitaxial growth of the PLD-CeO/sub 2/ cap layer. We found that a deposition temperature of 500/spl deg/C for the self-epitaxial PLD-CeO/sub 2/ cap layer is the best in the range of 200/spl deg/C and 800/spl deg/C for O/sub 2/ pressure of 10-30 mTorr in terms of the in-plane grain alignment. At the deposition temperature of 500/spl deg/C in the reel-to-reel process, we obtained a PLD-CeO/sub 2/ cap layer with /spl Delta//spl phi/ values of about 5 degrees at a tape transfer speed of 6 m/h for IBAD-GZO tapes with /spl Delta//spl phi/ values of about 15 degrees in 100 m class long tapes. Furthermore, we also obtained a PLD-CeO/sub 2/ cap layer with better /spl Delta//spl phi/ values of 3.7-4.4 degrees at a tape transfer speed of 2 m/h for a 55 m long tape.

AC Loss Measurement With a Phase Difference Between Current and Applied Magnetic Field

Total AC losses in multi-filamentary, silver-sheathed Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (Bi-2223) tape were measured as functions of transport current, applied magnetic field and frequency by electromagnetic and calorimetric methods. Results of both methods agree well with each other. The calorimetric method was employed to investigate the effect of the phase difference between transport current and applied field, /spl Delta//spl phi/, on total AC loss. Total loss changes significantly with /spl Delta//spl phi/ when applied magnetic field is perpendicular to the surface of the sample. The effect of /spl Delta//spl phi/ on total loss in parallel magnetic applied field is smaller.

Single Sensor High-Temperature Superconducting Axial Gradiometer With Thick Film Pick-Up Loops

A high-temperature superconducting first-order axial gradiometer with thick film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// pick-up loops inductively coupled to a thin film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconducting quantum interference device magnetometer has been designed, fabricated and characterized. The gradiometer has a baseline of 79 mm, an estimated intrinsic system balance of 1:480 for uniform axial magnetic fields and an estimated axial magnetic field gradient sensitivity of 70 fT/(cm/spl radic/Hz) at 1 kHz, in magnetic shielding.

Thin-Film Technology for HTSC Josephson Devices

An overview is given on different thin film preparation methods to produce YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film Josephson devices on various substrates. Beside laser deposition and ion beam etching techniques the use of inhibit technologies for patterning is discussed for the preparation of large area superconducting structures. For the deposition of large area YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films a heater and dry etching concept is discussed. The quality of the films is compared for all these methods to optimize the crystalline quality, the superconducting properties and the long term stability of the samples. The technology of production of high sensitive and stable Josephson devices like single junctions, dc-SQUIDs and flux transformers is important with respect to applications in nondestructive evaluation and biomagnetism.

<tex>$rm YbBa_2-rm xrm Sr_rm xrm Cu_3rm O_7-delta$</tex>Coated Conductor Development by Sol-Gel Process

YbBa/sub 2-x/Sr/sub x/Cu/sub 3/O/sub 7-/spl delta// coated conductors were grown on Gd/sub 1.8/Ho/sub 0.2/O/sub 3/ buffer layered biaxially textured-Ni (100) substrates by the reel-to-reel sol-gel dip coating system. Sample with different Sr substitution ratio (x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.6) were heat treated using varying heat treatment parameters. The surface morphologies and microstructure of all samples were investigated by ESEM, EDS and XRD. The critical current measurements were performed using four-wire method with the 1 /spl mu/V/cm criterion. The critical current density, J/sub c/ was measured to be 10/sup 5/ A/cm/sup 2/ at 4.2 K and 10/sup 4/ A/cm/sup 2/ at 75 K in-self field for YbB/sub 1.9/Sr/sub 0.1/Cu/sub 3/O/sub 7-/spl delta// sample.

Study of Closely Spaced<tex>$rm YBa_2rm Cu_3rm O_7-delta$</tex>Josephson Junction Pairs

Using electron beam lithography and ion damage, high quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconductor-normal metal-superconductor in-plane Josephson junction pairs have been fabricated. These junctions operate at temperatures between 60 and 85 K and have spacing ranging between 150 nm and 800 nm. Central electrodes connecting to the area between the two junctions were made, allowing for simultaneous measurements of the individual junctions as well as the series. Josephson junction pairs with 150 nm spacing were found to have extraordinary properties under microwave radiation with different power. At low power the structure functioned as two independent Josephson junctions in series, while at high power it behaved as one single Josephson junction. A possible origin of this behavior could be that the applied microwave radiation induces weak link behavior in the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// between the two junctions.

<tex>$J_rm c(H)$</tex>Crossover in YBCO Thick Films and Bi2223/Ag Tapes With Columnar Defects

We report a study of critical current density J/sub c/ as a function of temperature T, magnetic field H, and film thickness d in a series of high quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films. A crossover in the thickness dependence of J/sub c/(H) curves was observed for T>50 K, where thinner film carries significantly lower J/sub c/(H) than thicker film at high fields, while the opposite was observed at low fields. This J/sub c/(H) crossover behavior seems to be a general feature for a superconducting system with a spectrum of pinning density, whose trend opposes to the effective volume faction for J/sub c/. This generalization is supported by examining J/sub c/(H) crossover behaviors observed earlier in the Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+/spl delta//(Bi2223)/Ag tapes irradiated with 5.8 GeV Pb ions, where enhanced J/sub c/(H) at high fields is attributed to the density increase in the columnar flux pinning centers.

Dependence of Surface Resistance in HTS Thin Films on a DC Magnetic Field

We investigated the behavior of the surface resistance (R/sub s/) of high-temperature superconductor (HTS) thin films exposed to a dc magnetic field (parallel to the c-axis). Measurements were done using the dielectric resonator method and superconducting magnet system. The DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (DyBCO) and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films (produced by THEVA GmbH) were prepared through a thermal co-evaporation method. The resonant frequency of the TE/sub 011/ mode in the resonator was approximately 22 GHz. A dc magnetic field of up to 5 T was applied vertically to two parallel superconducting thin films placed into a closed-type Cu cavity. R/sub s/ of the DyBCO and YBCO thin films increased as the applied dc magnetic field increased. The R/sub s/ ratio (defined as R/sub s/(5 T)/R/sub s/(0 T)) of the DyBCO and YBCO thin films at 4.5 K was approximately 5 and 50, respectively. These results indicate that the DyBCO thin film may be better for use in low-temperature nuclear magnetic resonance applications.

A Simple Measurement Technique for Critical Current Density by Using a Permanent Magnet

We developed a novel nondestructive and contactless method for measurements of critical current density (J/sub c/) in high-temperature superconductor (HTS) thin films by using a permanent magnet (Sm/sub 2/Co/sub 17/). This measurement technique is based on a repulsive force (F/sub r/) and attractive force (F/sub a/) between the magnet and HTS film caused by the shielding currents and pinning forces. The F/sub r/ and F/sub a/ were measured by using a high-resolution load sensor changing the distance (L) between the magnet and HTS film. An effective maximum repulsive force (F/sub m,eff/) could be determined from an extrapolated value of the F/sub r/ vs. L curve for L=0 mm. We investigated the relationship of the J/sub c/ to the F/sub m,eff/ in many HTS (YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and ErBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//) samples. The standard J/sub c/ of HTS films was measured by using an inductive measurement system produced from THEVA GmbH. We found that the J/sub c/ is almost proportional to the two-thirds power of the F/sub m,eff/ normalized by the film thickness. This result indicates that the simple permanent magnet technique can be easily estimate the J/sub c/ of the HTS films. This system has good reproducibility for the J/sub c/ measurements and is useful to provide a rapid, routine characterization of the superconductivity in the HTS materials.

Ultrawide-band tunable true-time delay lines using ferroelectric varactors

This paper reports on compact tunable true-time delay lines based on ferroelectric (Ba/sub 0.25/Sr/sub 0.75/TiO/sub 3/) varactors integrated on high-resistivity silicon. The delay lines are based on lumped elements, physically implemented as synthetic coplanar-strip lines. An approximate analytical design procedure, exactly valid for /spl omega//spl rarr/0, is proposed. The physical size of the fabricated delay lines is 2.0/spl times/0.33 mm/sup 2/, including bias pads. Measurements are performed from room temperature (RT) down to 80 K. The measurements reveal ultrawide-band characteristics for both group delay and insertion loss. At RT, the absolute group delay is /spl tau//sub /spl delta//(RT, 0 V) /spl ap/70 ps with tunability of 20% under 20-V dc bias, the insertion less than 3.5 dB, and the reflection loss better than 12 dB below 20 GHz. At 145 K, the absolute group delay is increased to /spl tau//sub /spl delta//(145 K, 0 V) /spl ap/100 ps with a tunability of 50% under 20-V applied bias. At 7 GHz, the insertion loss is 3 dB, resulting in figures-of-merit of 0.03 dB/ps and 50 ps/mm. The leakage current at RT is less than 0.1 /spl mu/A.

High-<tex>$T_c$</tex>Superconductors-Based Nanocomposites With Improved Intergrain Coupling and Enhanced Bulk Pinning

Heterogeneous sonochemical synthesis was used to modify superconducting properties of granular YBa/sub 2/Ca/sub 3/CuO/sub 7-/spl delta// and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/. Sonication of liquid-powder alkane slurries produces material with enhanced intergrain coupling and improved current-carrying capabilities. Co-sonication with metals and organometallics results in highly compact nanocomposites with increased magnetic irreversibility. Ultrasonic irradiation of YBa/sub 2/Ca/sub 3/CuO/sub 7-/spl delta// carried under partial oxygen atmosphere produces similar morphological effects and increases superconducting transition temperature due to effective surface saturation with oxygen. Detailed chemical and physical characterization of sonochemically prepared high-T/sub c/ nanocomposites is presented.