Signal Transfer Function Research Articles

Novel Architecture of Feedforward Second-Order Multibit AD Modulator

This paper proposes novel feedforward architecture of the second-order multibit ΔΣAD modulator with single DAC-feedback topology. The ΔΣAD modulator realizes high resolution by oversampling and noise shaping techniques. However, its SNDR (Signal to Noise and Distortion Ratio) is limited by the dynamic range of the input signal and non-idealities of circuit building blocks, particularly by the harmonic distortion in amplifier circuits. A full feedforward ΔΣAD modulator structure has the signal transfer function of unity under ideal circumstances, which means that the signal swings through the loop filter become smaller compared with a feedbacked ΔΣAD modulator. Therefore, the harmonic distortion generated inside the loop filter can be significantly reduced in the feedforward structure because the effect of non-idealities in amplifiers can be suppressed when signal swing is small. Moreover, the reduction of the internal signal swings also relaxes output swing requirements for amplifiers with low supply voltage. However, in conventional feedforward ΔΣAD modulator, an analog adder is needed before quantizer, and especially in a multibit modulator, an additional amplifier is necessary to realize the summation of feedforward signals, which leads to extra chip area and power dissipation. In this paper, we propose a novel architecture of a feedforward ΔΣAD modulator which realizes the summation of feedforward signals without additional amplifier. The proposed architecture is functionally equivalent to the conventional one but with smaller chip area and lower power dissipation. We conducted MATLAB and SPICE simulations to validate the proposed architecture and modulator circuits.

Global exponential periodicity for BAM neural network with periodic coefficients and continuously distributed delays

By constructing a suitable Lyapunov function and using some analysis techniques, rather than employing the continuation theorem of coincidence degree theory as in other literature, a sufficient criterion is obtained to ensure the existence and global exponential stability of periodic solution for the bidirectional associative memory neural network with periodic coefficients and continuously distributed delays. The obtained result is less restrictive to the BAM neural network than the previously known criteria. And it can be applied to the BAM neural network in which signal transfer functions are neither bounded nor differentiable. In addition, an example and its numerical simulation are given to illustrate the effectiveness of the obtained result.

A new criterion to global exponential periodicity for discrete-time BAM neural network with infinite delays

The discrete-time bidirectional associative memory neural network with periodic coefficients and infinite delays is studied. And not by employing the continuation theorem of coincidence degree theory as other literatures, but by constructing suitable Liapunov function, using fixed point theorem and some analysis techniques, a sufficient criterion is obtained which ensures the existence and global exponential stability of periodic solution for the type of discrete-time BAM neural network. The obtained result is less restrictive to the BAM neural networks than previously known criteria. Furthermore, it can be applied to the BAM neural network which signal transfer functions are neither bounded nor differentiable. In addition, an example and its numerical simulation are given to illustrate the effectiveness of the obtained result.

COMBINE analysis of the specificity of binding of Ras proteins to their effectors

The small guanosine triphosphate (GTP)-binding proteins of the Ras family are involved in many cellular pathways leading to cell growth, differentiation, and apoptosis. Understanding the interaction of Ras with other proteins is of importance not only for studying signalling mechanisms but also, because of their medical relevance as targets, for anticancer therapy. To study their selectivity and specificity, which are essential to their signal transfer function, we performed COMparative BINding Energy (COMBINE) analysis for 122 different wild-type and mutant complexes between the Ras proteins, Ras and Rap, and their effectors, Raf and RalGDS. The COMBINE models highlighted the amino acid residues responsible for subtle differences in binding of the same effector to the two different Ras proteins, as well as more significant differences in the binding of the two different effectors (RalGDS and Raf) to Ras. The study revealed that E37, D38, and D57 in Ras are nonspecific hot spots at its effector interface, important for stabilization of both the RalGDS-Ras and Raf-Ras complexes. The electrostatic interaction between a GTP analogue and the effector, either Raf or RalGDS, also stabilizes these complexes. The Raf-Ras complexes are specifically stabilized by S39, Y40, and D54, and RalGDS-Ras complexes by E31 and D33. Binding of a small molecule in the vicinity of one of these groups of amino acid residues could increase discrimination between the Raf-Ras and RalGDS-Ras complexes. Despite the different size of the RalGDS-Ras and Raf-Ras complexes, we succeeded in building COMBINE models for one type of complex that were also predictive for the other type of protein complex. Further, using system-specific models trained with only five complexes selected according to the results of principal component analysis, we were able to predict binding affinities for the other mutants of the particular Ras-effector complex. As the COMBINE analysis method is able to explicitly reveal the amino acid residues that have most influence on binding affinity, it is a valuable aid for protein design.

Design of Sigma–Delta Modulators With Arbitrary Transfer Functions

This paper addresses the design of sigma-delta modulators with arbitrary signal and noise transfer functions using a genetic algorithm (GA)-based search method. The objective function is defined to include the difference D between the magnitude of the frequency responses of the designed transfer functions and the ideal one, the quantizer gain lambdacritical for which the poles of the modulator start moving out of the unit circle, and the spread of the coefficients S. Stability can be improved by reducing lambdacritical while a smaller S reduces the implementation complexity. A GA searches for poles/zeros of the transfer functions to minimize the objective function D+w1*lambdacritical+w2*S, where w1 and w2 are two weighing factors. Numerical results demonstrate the effectiveness of the proposed method

Continuous-time feedforward modulators with robust lowpass STF

Continuous-time sigma–delta modulators are often implemented using the feedforward structure. This type of architecture is prone to large out-of-band peaking and degraded anti-aliasing filtering owing to its sensitivity to process variation. Two alternative continuous-time delta–sigma modulators, which exhibit significantly lower signal transfer function (STF) sensitivity, are described.

Cascade modulator for low-voltage wideband applications

A new cascade ΣΔ modulator architecture with unity signal transfer function is presented, which avoids the need for digital filtering in the error cancellation logic. The combination of these two aspects makes it highly tolerant to noise leakages, very robust to nonlinearities of the circuitry and especially suited for low-voltage implementations at low oversampling. Behavioural simulations are presented that demonstrate the higher efficiency of the proposed topology compared to existing cascades intended for wideband applications.

Design of Interpolative Sigma Delta Modulators Via Semi-Infinite Programming

This correspondence considers the optimized design of interpolative sigma delta modulators (SDMs). The first optimization problem is to determine the denominator coefficients. The objective of the optimization problem is to minimize the passband energy of the denominator of the loop filter transfer function (excluding the dc poles) subject to the continuous constraint of this function defined in the frequency domain. The second optimization problem is to determine the numerator coefficients in which the cost function is to minimize the stopband ripple energy of the loop filter subject to the stability condition of the noise transfer function (NTF) and signal transfer function (STF). These two optimization problems are actually quadratic semi-infinite programming (SIP) problems. By employing the dual-parameterization method, global optimal solutions that satisfy the corresponding continuous constraints are guaranteed if the filter length is long enough. The advantages of this formulation are the guarantee of the stability of the transfer functions, applicability to design of rational infinite-impulse-response (IIR) filters without imposing specific filter structures, and the avoidance of iterative design of numerator and denominator coefficients. Our simulation results show that this design yields a significant improvement in the signal-to-noise ratio (SNR) and have a larger stability range, compared with the existing designs

A Reduced-Sample-Rate Sigma-Delta-Pipeline ADC Architecture for High-Speed High-Resolution Applications

A reduced-sample-rate (RSR) sigma-delta-pipeline (SDP) analog-to-digital converter architecture suitable for high-resolution and high-speed applications with low oversampling ratios (OSR) is presented. The proposed architecture employs a class of high-order noise transfer function (NTF) with a novel pole-zero locations. A design methodology is developed to reach the optimum NTF. The optimum NTF determines the location of the non-zero poles improving the stability of the loop and implementing the reduced-sample-rate structure, simultaneously. Unity gain signal transfer function to mitigate the analog circuit imperfections, simplified analog implementation with reduced number of operational transconductance amplifiers (OTAs), and novel, aggressive yet stable NTF with high out of band gain to achieve larger peak signal-to-noise ratio (SNR) are the main features of the proposed NTF and ADC architecture. To verify the usefulness of the proposed architecture, NTF, and design methodology, two different cases are investigated. Simulation results show that with a 4th-order modulator, designed making use of the proposed approach, the maximum SNDR of 115dB and 124.1 dB can be achieved with only OSR of 8, and 16 respectively.

Double-sampling single-loop /spl Sigma//spl Delta/ modulator topologies for broad-band applications

This paper presents novel double sampling high-order single loop sigma-delta modulator structures for wide-band applications. To alleviate the quantization noise folding into the inband frequency region, two previously reported techniques are used. The digital-to-analog converter's sampling paths are implemented with the single-capacitor approach and an additional zero is placed at the half of the sampling frequency of the modulator's noise transfer function (NTF). The detrimental effect of this additional zero on both the NTF and signal transfer function is also resolved through the proposed modulator architectures with a low additional circuit requirement.

High-level synthesis of /spl Delta//spl Sigma/ Modulator topologies optimized for complexity, sensitivity, and power consumption

This paper proposes a novel topology-synthesis methodology for single-loop single-bit /spl Delta//spl Sigma/ modulators. The goal is to explore all possible topologies and to obtain the optimal topology under various design considerations, such as hardware complexity, modulator sensitivity, and power consumption. A generic modulator architecture that incorporates all possible feedback and feedforward signal paths was defined and the symbolic noise transfer function (NTF) and signal transfer function (STF) for the generic topology were derived. The symbolic functions were then used to formulate the topology-exploration problem as a mixed-integer nonlinearly constrained programming (MINLP) problem that simultaneously generates and selects the optimal modulator topology with respect to the cost function. Experiments show the superiority of synthesized topologies as compared to traditional modulator topologies.

RACK1 Binds to a Signal Transfer Region of Gβγ and Inhibits Phospholipase C β2 Activation

RACK1 Binds to a Signal Transfer Region of Gβγ and Inhibits Phospholipase C β2 Activation

Hybrid Cascode Compensation for Two-Stage CMOS Opamps

This paper presents the analysis of hybrid cascode compensation scheme, merged Ahuja and improved Ahuja style compensation methods, which is used in two-stage CMOS operational transconductance amplifiers (OTAs). The open loop signal transfer function is derived to allow the accurate estimation of the poles and zeros. This analytical approach shows that the non-dominant poles and zeros of the hybrid cascode compensation are about 40 percent greater than those of the conventional cascode compensation. Circuit level simulation results are provided to show the accuracy of the calculated expressions and also the usefulness of the proposed cascode compensation technique.

Controlled behaviour of STF in CT modulators

One of the most interesting properties of continuous time /spl Sigma//spl Delta/ modulators is their implicit anti-aliasing filter. Unfortunately, peaking in their signal transfer function (STF) demands a 'no-overload' filter to precede the modulator. A technique and two new topologies, which can achieve an almost flat STF, are proposed. Meanwhile, the topologies still have the desired feature that the internal nodes contain few input related signals.

A 2/5mW CMOS .DELTA..SIGMA. modulator employed in an improved GSM/UMTS receiver structure

In this paper, the design of a reconfigurable low-power low-pass Switched Capacitor Delta-Sigma (SC ΔΣ) modulator for GSM/UMTS standards used in an optimum low-IF(LIF)/Zero-IF(ZIF) receiver architecture is described. A new approach for obtaining the optimum modulator coefficients is developed which results in relaxed specifications of the circuit components, aggressive noise transfer function (NTF) and a signal transfer function (STF) with the blocker-rejection property. The modulator employs a second/third-order single-stage dual-quantizer structure. It achieves 85.6dB/57dB SNDR and -0.2dBFS/-0.05dBFS overload factor at 0.2/2MHz bandwidth for GSM/UMTS standards and consumes only 2mW/5mW from a single 1.8V supply in a 0.18µm 1P6M CMOS process. The maximum spread of the modulator capacitors is 105.

Elliptic filtering in continuous-time sigma-delta modulator

An improved continuous-time sigma-delta (ΣΔ) architecture with elliptic signal transfer function is presented. It is demonstrated that the UTRA/FDD band 1 interferer filtering can be fully achieved within the sigma-delta loop.

Performance analysis of IEEE 802.11 MAC protocols in wireless LANs

AbstractIEEE 802.11 MAC protocol is the de facto standard for wireless local area networks (LANs), and has also been implemented in many network simulation packages for wireless multi‐hop ad hoc networks. However, it is well known that, as the number of active stations increases, the performance of IEEE 802.11 MAC in terms of delay and throughput degrades dramatically, especially when each station's load approaches its saturation state. To explore the inherent problems in this protocol, it is important to characterize the probability distribution of the packet service time at the MAC layer. In this paper, by modeling the exponential backoff process as a Markov chain, we can use the signal transfer function of the generalized state transition diagram to derive an approximate probability distribution of the MAC layer service time. We then present the discrete probability distribution for MAC layer packet service time, which is shown to accurately match the simulation data from network simulations. Based on the probability model for the MAC layer service time, we can analyze a few performance metrics of the wireless LAN and give better explanation to the performance degradation in delay and throughput at various traffic loads. Furthermore, we demonstrate that the exponential distribution is a good approximation model for the MAC layer service time for the queueing analysis, and the presented queueing models can accurately match the simulation data obtained from ns‐2 when the arrival process at MAC layer is Poissonian. Copyright © 2004 John Wiley & Sons, Ltd.

Lubberts effect in columnar phosphors

Noise transfer in granular x-ray imaging phosphor screens is not proportional to the square of the magnitude of the signal transfer when the transfer properties are considered for the entire screen thickness, unless appropriately weighted at each depth of interaction. This property, known as the Lubberts effect, has not yet been studied in columnar structured screens because of a lack of a generalized description of the depth-dependent light transport. In this paper, we investigate the signal and noise transfer characteristics of columnar phosphors used in digital mammography detectors using DETECT-II, an optical Monte Carlo light transport simulation code. We first validate our choice of optical parameters for the description of granular and columnar screens using published normalized modulation transfer (MTF) experimental data. Our calculations of MTF match empirically measured MTFs for a granular film/screen analog system, and for an indirect x-ray digital imaging system with CsI:Tl screen representative of digital mammography systems. Using the depth-dependent spread functions and collection efficiencies, we calculate the signal and noise transfer functions and the Lubberts fraction, which is the ratio of the signal transfer function to the noise transfer function, for different screen thicknesses of granular and columnar phosphors. We find that the Lubberts fraction of a 85 microm granular screen model corresponding to a Gd2O2S:Tb screen is similar to the fraction for a 100 microm columnar CsI:Tl screen.

Robust filtering under stochastic parametric uncertainties

This paper is concerned with a polynomial approach to robust deconvolution filtering of linear discrete-time systems with random modeling uncertainties. The modeling errors appear in the coefficients of the numerators and denominators of both the input signal and system transfer function models in the form of random variables with zero means and known upper bounds of the covariances. The robust filtering problem is to find an estimator that minimizes the maximum mean square estimation error over the random parameter uncertainties and input and measurement noises. The key to our solution is to quantify the effect of the random parameter uncertainties by introducing two fictitious noises for which a simple way is given to calculate their covariances. The optimal robust estimator is then computed by solving one spectral factorization and one polynomial equation as in the standard optimal estimator design using a polynomial approach. An example of signal detection in mobile communication is given to illustrate the effectiveness of our approach.

High-Order Multibit Modulators and Pseudo Data-Weighted-Averaging in Low-Oversampling>tex<$Delta Sigma$>/tex<ADCs for Broad-Band Applications

High-speed high-resolution /spl Delta//spl Sigma/ analog-to-digital converters (ADCs) for broad-band communication applications must be designed at a low oversampling ratio (OSR). However, lowering the OSR limits the efficiency of a /spl Delta//spl Sigma/ ADC in achieving a high-resolution A/D conversion. This paper presents several techniques that enable the OSR reduction in /spl Delta//spl Sigma/ ADCs without compromising the resolution. 1) Noise transfer function (NTF). In this paper, a single-stage multibit /spl Delta//spl Sigma/ modulator with a high-order finite-impulse-response NTF is proposed to achieve high signal-to-quantization-noise ratios at low OSRs. Its key features include: decreased circuit complexity, improved robustness to modulator coefficient variations, and reduced sensitivity to integrator nonlinearities. Its performance is validated through behavioral simulations and compared to traditional /spl Delta//spl Sigma/ modulator structures. 2) Signal transfer function (STF). This paper describes how the STF of a /spl Delta//spl Sigma/ modulator can be designed, independently of the NTF, in order to significantly reduce the harmonic distortion due to opamp nonidealities and to help lower the power dissipation. 3) Dynamic element matching (DEM) is also presented. Data weighted averaging (DWA) has prevailed as the most practical DEM technique to linearize the internal digital-to-analog converter (DAC) of a multibit /spl Delta//spl Sigma/ modulator, especially when the number of DAC elements is large. However, the occurrence of in-band signal-dependent tones, when using DWA at a low OSR, degrades the spurious-free dynamic range. This paper proposes a simple technique, called pseudo DWA, to solve the DWA tone problem without sacrificing the signal-to-noise ratio. Its implementation adds no extra delay in the /spl Delta//spl Sigma/ feedback loop and requires only minimal additional digital hardware. Existing schemes for DWA tone reduction are also compared.