Abstract: In the paper, an analysis of the state-of-the-art of active elements for analog signal processing is presented which support - in contrast to the conventional operational amplifiers - not only the voltage-mode but also the current- and mixed-mode operations. Several problems are addressed which are associated with the utilization of these elements in linear applications, particularly in frequency filters. A methodology is proposed which generates a number of fundamentally new active elements with their potential utilization in various areas of signal processing.

Abstract: A CMOS-based circuit for realization of high-performance current differencing transconductance amplifier (CDTA) is demonstrated. The proposed circuit offers the advantages of a wide frequency bandwidth and very small input terminal impedance. The results of circuit simulations and an application example are given to illustrate the advantages of the proposed circuit for precise high-frequency signal rectification. (C) 2007 Elsevier GmbH. All rights reserved.

Abstract: This article presents a simple current-mode quadrature oscillator using a single Current Differencing Transconductance Amplifier (CDTA) as the active element. The oscillation condition and oscillation frequency can be electronically controlled. The circuit structure is very simple, consisting of merely one CDTA, one resistor and two capacitors. The proposed circuit is suitable for IC architecture. The PSpice simulation and experimental results are shown, and the results agree well with the theoretical assumptions.

Abstract: This study proposes a current mode (CM) Kerwin-Huelsman-Newcomb (KHN) filter employing only two current differencing transconductance amplifiers (CDTA) and two grounded capacitors. It is concluded that the circuit described here offers a simpler and more economical alternative to other CM KEN filters reported previously in literature. (c) 2005 Elsevier GmbH. All rights reserved.

Abstract: A CDTA-based quadrature oscillator circuit is proposed. The circuit employs two current-mode allpass sections in a loop, and provides high-frequency sinusoidal oscillations in quadrature at high impedance output terminals of the CDTAs. The circuit has no floating capacitors, which is advantageous from the integrated circuit manufacturing point of view. Moreover, the oscillation frequency of this configuration can be made adjustable by using voltage controlled elements (MOSFETs), since the resistors in the circuit are either grounded or virtually grounded.

Abstract: A new description of linear periodically time-varying systems is proposed, To describe simultaneously the continuous and discrete time nature of the system, the generalized transfer function of the Laplace and discrete operator is introduced, This mixed description includes the transfer functions of continuous time and discrete time systems as Limiting cases, Special attention is paid to two-phase switched circuits and results of computer simulations are presented.

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Abstract: A universal current-mode biquad is described which is designed on the basis of well-known KHN (Kerwin, Huelsman, Newcomb) voltage-mode filter. In its most general form, the circuit consists of five operational transconductance amplifiers (OTAs) and of two grounded capacitors. The filter provides simultaneously three current outputs of HP, BP, and LP types into independent loads. The quality factor can be adjusted electronically independently on the natural frequency. Less rigorous rules of controlling the filter parameters can imply a reduction of number of OTAs to three or even two. The principle is verified by Spice simulation on the level of a CMOS structure of OTAs.

Abstract: The paper describes a conception of the so-called simulation manager that considerably extends the application range of the OrCAD PSpice simulation program. It enables operating this program in the so-called sequential mode, when the relatively independent tasks are run consecutively with a possibility of data exchange. A powerful programming language also enables iterative runs within the conditional loops, which can be utilized e.g. for optimization.

Abstract: A dual optical wireless test link works with two different optical wavelengths in spectral windows of 850 nm and 1550 nm. The transceivers of the link are placed on the highest peak of the Czech Central Mountains (Milesovka Mountain) so that the transmission path is oriented almost in a vertical direction. The installation site is situated in a locality with the harshest climate in the Czech Republic with extreme attenuation conditions. The almost vertically oriented path of the link allows analyzing the impact of the different atmospheric layers on the signal transmission. The monitoring of the received power and the archiving of the appropriate data are constantly provided. The relationship between the link attenuation and the atmospheric visibility has been investigated. The results of this experiment are presented.

Abstract: Avalanche photodiodes (APD) used in receivers for Free-Space Optical (FSO) communication operate in a relatively narrow range of avalanche multiplication. The optimal reverse voltage depends on temperature and is subject to manufacturing variance. These effects are usually compensated for in the open-loop mode, where a DC source is programmed with average datasheet temperature coefficients and individually adjusted to compensate for manufacturing variance. The paper describes the design of a receiver with closed-loop stabilization of APD parameters.

Abstract: The inability to provide direct small-signal AC analysis of circuits with periodical external switching belongs to well-known limitations of Spice-compatible programs. This paper refers to novel methods of modeling which enable the above analyses also in PSpice simulators with the aid of the so-called Simulation Manager.

Abstract: A generalization of the model of PWM switch according to Dijk et al. for averaged modeling of switched DC-DC converters is proposed This model now includes the on-resistance of the active switch, and the forward voltage drop and the differential resistance of diode-type passive switch. The way this model is compiled allows for the influence of parasitic ESRs of the capacitors in the circuit such that the resulting equations are equivalent to the averaged state space equations. The procedure of PSPICE modeling and the results of computer simulation of boost converter are shown.

Abstract: Direct AC analysis of idealized switched-capacitor circuits in Spice-compatible program is described in the paper. The capacitor is described by a special macro-circuit, whose mathematical model is based on modified z-domain charge equations and on the theory of equivalent signals.

Abstract: The paper deals with implementation details of a method for approximate symbolic analysis of linear circuits based on nontrivial transformations of voltage and current graphs. The method is based on eliminating the low-voltage branches from �high-voltage� loops and the low-current branches from �high-current� cuts. This goes beyond the simple edge deletion or contraction used in previous methods. The paper describes a graph transformation for decreasing the number of spanning trees and its computer implementation.

Abstract: The contribution deals with some open problems from the area of Transient and AC analyses of large continuous-time and switched circuits with the focus on switched capacitor circuits and switched DC-DC converters. The discussed weak points of SPICE-like and SPICE-compatible simulation programs consist in the absence of finding effectively the steady-state, in an inefficient integration algorithm for the transient analysis of switching phenomena, and in the absence of a direct A C analysis of switched circuits. The paper also describes some new approaches, which could be helpful in overcoming the above limitations.

Abstract: This paper deals with a modification of the CBCM method for floating nonlinear device characterization. Closed form analytical formulae for the estimation of CBCM errors due to parasitic charge injection are presented. A test chip implementing the method was designed and manufactured in the 0.35 mu m CMOS process.

Abstract: A universal current-mode biquad is described which represents an economical variant of well-known KHN (Kerwin, Huelsman, Newcomb) voltage-mode filter. The circuit consists of two multiple-output OTAs and of two grounded capacitors. Utilizing simple splitter of the input current and a pair of jumpers, all the basic 2(nd)-order transfer functions can be implemented. The principle is verified by Spice simulation on the level of a CMOS structure of OTAs.

Abstract: The paper deals with an analysis of visibility records collected from 210 European airports to obtain a realistic estimation of the availability of Free Space Optical (FSO) data links. Commercially available optical links usually operate in the 850mn waveband. Thus the influence of the atmosphere on the optical beam and on the visible light is similar. Long-term visibility records represent an invaluable source of data for the estimation of the quality of service of FSO links. The model used characterizes both the statistical properties of fade depths and the statistical properties of individual fade durations. Results are presented for Italy, France, and Germany.

Abstract: The paper deals with a modification of the CBCM method for nonlinear on-chip capacitance characterization. The proposed modification uses two DC swept sources to measure the whole nonlinear Q-v characteristic in both polarities without the necessity to switch the object being measured. A test-chip implementing the method was designed and manufactured in the 0.35 mu m CMOS process. It was used for MOSCAP characterization in the full operating voltage range.

Abstract: An effective Spice analysis of switched capacitor DC-DC converters is proposed. The model of switched circuit is made up in such a way that its transient analysis within a single switching phase results in direct evaluation of the steady-state features such as output voltage ripple, average currents and voltages, powers, and converter efficiency.

Abstract: The paper describes a method for finding steady states in switched DC-DC converters, which is based on the SPICE bias point computation rather than a time-consuming transient analysis. The switched converter is modeled by unified substitutive circuits whose DC analysis determines directly the limit values of steady-state circuit variables at time instants when switches change their states. From these values one can derive eg. the output voltage ripple or restore completely the steady-state signals.

Abstract: This paper deals with circuit topology transformations for approximate symbolic analysis of linearized circuits. The method is based on two-graph approach. The structure of current and voltage graphs is modified in order to decrease the number of common spanning trees, i.e. the number of symbolic terms.

Abstract: The estimation of FSO link availability is based on synthesis of two models: the steady model of given link and the statistical model of installation site. The steady model consists in the knowledge of FSO transceiver parameters so that we can compute the power budget for a given transceiver distance. The result is the dependence of link margin on the transceiver distance. The statistical model consists in the knowledge of statistical parameters of atmosphere at given installation site. The installation site is described by the exceedance probability of atmospheric attenuation. By means of synthesis of both models we can obtain estimation of link availability. In this contribution, estimation of FSO link availability in chosen Central - European localities is presented.

Abstract: The paper explains the basic ideas of how to model the dynamical properties of switched DC-DC converters by means of the so-called generalized transfer functions (GTFs). Switched DC-DC converters like bust or bust-buck types exhibit some properties of continuous- and discrete-time characters. These systems can be globally modeled by mixed s-z description, utilizing both the Laplace and z-transform operators. Then the resulting line-to-output and control-to-output transfer functions and the corresponding frequency responses can model some system behavior more faithfully than the classical s-domain zeros/poles location, including special effects above Nyquist�s frequency.

Abstract: An optimal pivoting strategy for the reduction algorithm transforming the general eigenvalue problem to the standard one is presented for both full- and sparse-matrix procedures. The algorithm increases the precision of the semisymbolic analysis especially for the large-scale microwave circuits. The accuracy is furthermore increased using longer numerical data. First, the long double precision is utilized. Further, the application of a suitable multiple-precision arithmetic library is suggested. Finally, using the longer numerical data to eliminate possible imprecision of the multiple eigenvalues; is evaluated. The algorithm is demonstrated by estimating the frequency of a distributed microwave oscillator, and by estimating the bandwidth of a distributed microwave amplifier.

Abstract: Research in analog integrated circuits has recently gone in the direction of low-voltage (LV), low-power (LP) design, especially in the environment of portable systems where a low supply voltage, given by a single-cell battery, is required. The current-conveyor (CCII) can be considered as the basic circuit block because all the active devices can be made by a suitable connection of one or two CCIIs. In this paper, a new principle of bulk-driven MOS transistors will be used to design a novel LV LP CCII. New possible applications can be expected in some basic analog functions such as filters, impedance simulators and converters, oscillators, etc. The designs were simulated in Cadence using a 0.7 mu m CMOS technology from AMI SEMICONDUCTOR. The novel bulk-driven current conveyor was designed to work with +/- 0.6 V supply voltage only.

Abstract: Several algebraic propositions from the theory of reciprocal equations are conceived in this contribution. Applying them to the z-domain system functions of discrete-time FIR filters with the symmetric and antisymmetric impulse responses yields basic conditions and rules that have to be fulfilled to reach in principle the linear phase of filters with prescribed amplitude frequency responses.

Abstract: A method of modeling PWM switches for symbolic analysis of pulse-width-modulated (PWM) dc-to-dc converters operating in the continuous conducting mode (CCM) is described for switches with both fixed and modulated duty ratio. The well-known average models of PWM switch according to Vorperian are implemented by means of modified nodal analysis into the SNAP program to analyze transfer and immittance functions, especially the line-to-output and control-to-output transfer functions in the symbolic and semisymbolic forms, and the corresponding frequency responses. An example of the analysis of boost converter is shown.

Abstract: Utilizing the signal flow graphs in teaching linear circuits is discussed in the paper. Some graph features are stated, which should make their inclusion in the existing curriculum easy. The graphs presented seem to be optimum for a quick �hand-and-paper� analysis of circuits containing ideal OpAmps, VCVSs and OTA elements.

Abstract: Flow graphs of the pulse-width-modulated (PWM) switch both with the fixed and modulated duty cycle are introduced in the paper. These graphs start from the classical Vorperian model of PWM switch in the continuous current mode (CCM). The flow graphs are intended for the �Hand-and-Paper� analysis of DC-to-DC converters, utilizing the,,averaged modeling�, especially - but not only - for the analysis of DC voltages, currents and powers. The method is illustrated on an example of buck-boost converter.

Abstract: Modified Mason-Coates� flow graphs are introduced in this paper. Utilizing a simple rule, the voltage graph node can be expanded by the current node, which corresponds to the current of a branch oriented out of the incident circuit node. Then an arbitrary type of controlled source can be easily described by means of a flow graph. Applying these procedures, we obtain flow graphs of such circuits that are not commonly utilized, e.g. current conveyors or current feedback amplifiers.

Abstract: The paper deals with the general questions relating to the utilisation of computers in teaching electrical engineering, electronics, circuit and signal theory. The generally applicable methods of computer program compilation and utilisation are evaluated on the basis of general didactic principles and on the authors� experiences. Fundamental didactic principles and their application to computer-aided education are discussed. Six fundamental didactic principles of Comenius presented here are the principles of goal-seeking; of systematicness and succession; of adequacy; of object teaching; of activity and creativity; and of emotionality. This set of principles can contribute to the decision-making process of today�s issues in pedagogy and engineering education.

Abstract: The paper discusses possible techniques of teaching the analysis of linear circuits. Three pedagogic aims are followed: student�s understanding of the function of the circuit analysed, ability to solve moderate circuits algorithmically, and the basic understanding of the function of professional circuit simulators as an assumption of their later effective utilisation. Injecting creative features into the algorithmic method can reduce the drawbacks encountered for students who have problems with creative thinking. Through symbiosis, each method fulfils its function in that the heuristic method induces physical thinking, while the algorithmic one offers a useful tool for practical solutions. The paper looks at the selection of the algorithmic method, with the inclusion of modified nodal analysis in electrical engineering curricula strongly advised. The paper also analyses the algorithmic matrix methods of analysis that have proven to be effective in teaching practice and are two special variants of modified nodal analysis. Ultimately, the choice of taught method of analysis depends on the pedagogical aims.

Abstract: This paper presents an idea of analog tilter design which is based on the discrete-time linear-phase prototype. After the inverse bilinear transformation, the designed linear-phase FIR filter is converted to the continuous-time filter. The phase properties of such structures are discussed.

Abstract: The algorithm of all z- and s- domain poles/zeros finding of real switched network with external switching is described in this contribution. This mixed s-z analysis enables realistic simulation of network behavior both in the classical and non-standard regimes as oversampling.

Abstract: It is shown in this contribution that besides the classical dynamics caused by reactance elements, each DC resistive circuit has its special hidden so-called virtual dynamics which contains information about the DC operating point stability. Virtual eigenvalues can be attached to each linear model of resistive circuit linearized around its DC operating point. Corresponding virtual trajectories describe motion of operating point in the virtual space after being deflected from the equilibrium state and they help to decide whether the operating point is stable.

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Abstract: Some paradox is described in this contribution consisting in the fact that in spite of evident circuit instability due to strong positive feedback, all poles of active dynamical circuit lie in the left-side complex plane. It is shown that the key problem is the incomplete circuit model, because the pole location can strongly depend on the fact if the parasitic reactances are included to the circuit model. This phenomenon is investigated together with so-called potential stability of DC operating point. They are interesting connections between the stability of dynamical circuit and corresponding operating point.

Abstract: The state variable method can be used for efficient simulation of block oriented systems. The modified method of state variables is described in this contribution which has been developed as the analogy of well-known modified nodal analysis method. Described procedure solves following problem: Consider two dynamical systems with known state equations. Systems will be interconnected by defined way and excited by input sources. Our task is to compile state equations of resulting system.

Abstract: The computer EEG data processing is modem trend in medical data processing. The computer processing is needful especially for longterm EEG data. Long-term EEG records of patients with diagnosis of epilepsy are registered during normal every day regime, where EEG record is contaminated not only by outside influence, but even by physiological activity of patient (e.g. breathing, moving of limbs, etc.). These activities from the point of clinical neurological interest are contamination and they have to be removed. This paper is dedicated to solving of removing artefacts of physiological origin in EEG data and gives complex solution of this problem from the adaptive segmentation of EEG signal over identification of each segment of signal to adaptive filtering.