CMOS wideband analogue delay stage

A new second-order all-pass filter with maximum achievable delay-bandwidth-product (DBW) is presented. The proposed circuit will be used as a wideband delay element in impulse radio ultra-wideband transceivers. Benefiting from a simple architecture, the proposed circuit achieves a 60 ps delay across a 10 GHz bandwidth, which is the largest delay ever reported over such a wide bandwidth. In addition, the most noticeable advantage of this delay circuit is the small variation of group delay across a wide frequency range, which means negligibly small phase distortion introduced by the circuit     

COGNITIVE RADIOS FOR DYNAMIC SPECTRUM ACCESS - The Throughput Potential of Cognitive Radio: A Theoretical Perspective

Cognitive radios are promising solutions to the problem of overcrowded spectrum. In this article we explore the throughput potential of cognitive communication. Different interpretations of cognitive radio that underlay, overlay, and interweave the transmissions of the cognitive user with those of licensed users are described. Considering opportunistic communication as a baseline, we investigate the throughput improvements offered by the overlay methods. Channel selection techniques for opportunistic access such as frequency hopping, frequency tracking, and frequency coding are presented. The trade-off between regulation and autonomy inherent in the design and performance of cognitive networks is examined through a simple example, which shows that the optimal amount of licensing is equal to the duty cycle of the traffic arrivals     

Basic science in the Islamic Republic of Iran

The population of Iran has nearly doubled in less than 25 years, while the number of university students has increased more than 10 times and 720 Ph. D. degrees have been awarded in basic science in the past 10 years. Despite the great difficulties that the Iranian scientists have been facing for more than two decades (as a consequence of a social revolution, 8 years of a destructive war imposed by Iraq, excessive brain drain, discriminatory practices by some international journals in publishing the Iranian articles, and unfair sanctions imposed by the industrialized countries) Iran's science is still thriving and the current number of yearly scientific publications exceeds 1500. When normalized with respect to the number of researchers and the research budget, the Iranian scientists seem to outperform most of their counterparts in the advanced industrialized nations. Main reason: total engagement in truncated research activities (basic or applied) leading solely to pure publications; lack of infrastructure for developmental research activities leading to new technologies. The average impact factor of the papers in various fields of basic science seems quite satisfactory considering the difficult conditions the Iranian scientists are working under. Should the research budgets and conditions improve and the unfair sanctions currently imposed by the world politics be eliminated, a far better contribution to the world science can be expected. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the Role of Both Polypropylene Fibers and Silica Nanoparticles on the Viscoelastic Behavior of Silicone Rubber Nanocomposites

In the present study, silica nanoparticles and polypropylene fibers were added to silicone rubber matrix and their effect was investigated on the viscoelastic behavior of silicone rubber composites by dynamic mechanical analysis and compression set test. Bonding of the reinforcements with silicone rubber matrix was studied by Fourier transform infrared spectroscopy (FTIR). The results proved that the role of silica particles on the elastic modulus of silicone rubber was more effective than that of polypropylene at room temperature because of higher stiffness and also partial interaction of polypropylene with matrix.     

Mathematical Modeling of Phase-Controlled Converters Considering Discontinuous Conduction Modes

Despite possessing outstanding features, phase-controlled converters are also known for their control performance deterioration whenever their output currents become discontinuous. This paper aims to provide a mathematical framework to overcome converter input/output characteristic nonlinearities, arising from discontinuous current conduction regime. A secondary purpose is to expand the idea of Predictive Current Control (PCC) for phase-controlled converters, taking into account multiple discontinuous current conduction modes. To this end, starting with a detailed converter circuit analysis, equations describing boundaries between converter operating modes are derived and depicted as a graph. The resulted graph is utilized to devise an algorithm to identify mode of operation and to calculate firing angle corresponding to “desired average output voltage”. Experiments in conjunction with simulation studies are conducted to evaluate the assumptions made, the equations derived, and the algorithm devised. Based on the proposed algorithm, two improvements on converter current control are achieved. Firstly; the transfer characteristic of the converter is linearized. Secondly; the PCC strategy, which has attracted much attention recently, is implemented in Single Phase Full Controlled (SPFC) converter. Moreover, the comprehensive treatment of all operating mode boundaries presented in this paper helps deepen physical insight into the SPFC converter operation.     

Estimating water content of natural gas: A radial basis function neural network method

Association of water with natural gas streams can highly affect processing and transmission of natural gas. Therefore, the water content of the natural gas must be known in order to determine best possible processing and transmission conditions. This study aims to develop a simple predictive approach to predict water content of sweet gas in wide pressure and temperature ranges, using a radial basis function artificial neural network. The proposed model shows lower deviation from experimental data compared to existing empirical correlation. R-squared and mean relative error values are 0.998% and 4.07%, respectively.     

CMOS second-harmonic quadrature voltage controlled oscillator using substrate for coupling

In this work, a new circuit configuration for second-harmonic quadrature voltage controlled oscillator (QVCO) with CMOS technology is proposed. The proposed QVCO is made by coupling two identical cross-connected VCOs. The coupling elements (two resistors and two capacitors) do not increase the power consumption of the core VCOs and do not disturb the resonant frequency of the tank circuit in the core VCOs and also, according to the simulations the coupling elements do not adversely affect the phase noise. The role of the substrate terminal of cross-connected MOSFETs of the core oscillators as common mode nodes is demonstrated. Using this node for coupling makes it possible to eliminate the tail transistors in the core oscillators and therefore the circuit can operate with supply voltages as low as 0.5 V. Using the same method to couple more than two core oscillators, results in a multiphase VCO.