Current impulse response of thin InP p-i-n diodes

The simulation of current impulse response using random response time model in avalanche photodiode (APD) is presented. A random response time model considers the randomness of times at which the primary and secondary carriers are generated in multiplication region. The dead-space effect is included in our model to demonstrate the impact on current impulse response of thin APDs. Current impulse response of homojunction InP p⁺-i-n⁺ diodes with the multiplication widths of 0.1 and 0.2 μm are calculated. Our results show that dead-space gives a slower decay rate of current impulse response in thin APD, which may degrade the bit-error-rate of the optical communication systems.     

Approximate realization of digital fractional forward operator using digital IIR filter

In this paper, a simple and an efficient approach for approximating the digital fractional forward operator z ^m (0?<?m?<?1) using digital infinite impulse response (IIR) filter is proposed. In this method, the coefficients of the closed form digital IIR filter derived for the approximation of the fractional forward operator, in a given frequency band, are based on approximation of fractional order systems. First, analog rational function approximation, in a given frequency band, of the fractional power zero (FPZ) is given. Then, the forward difference generating function is used to obtain a closed form IIR digital filter equivalent of the continuous FPZ, which approximates the digital fractional forward operator. Finally, illustrative examples have been presented to illustrate the effectiveness of the proposed design technique of the fractional forward operator z ^m approximation and its use in performing a fractional m-step prediction.     

Integrated CMOS GSM baseband channel selecting filters realized using switched capacitor finite impulse response technique

A problem of the design and optimization of analog channel selecting filters, which are needed in wireless communication systems, is considered and evaluated in this paper on an example of the baseband GSM (global system for mobile telephony) channel filter. Two versions of this filter, both designed by the authors using switched-capacitor finite impulse response (SC FIR) technique, are presented and compared to each other as well as to other concurrent designs. In order to fully and plausibly compare the both filter versions (the newer and the elder one), the authors decided to design and fabricate both filters using the same technology, i.e., the technology of the elder filter version, which is the two metal, two poly CYE CMOS 0.8 μm process. The conclusions, which have been drawn, are, however, general and to a large extent technology independent.Although both presented filters are switched-capacitor (SC) finite impulse response (FIR) systems [Dąbrowski A, Cetnarowicz D, Długosz R, Pawłowski P. Design and optimisation of integrated CMOS FIR SC channel filter for a GSM Receiver, European Conference on Circuit Theory and Design, Helsinki, 28–31 August 2001. p I.265; Długosz R, Dąbrowski A, Pawłowski P. Design and measurement results of the GSM SC FIR channel filter realized in CMOS 0.8 μm technology. In: 9th international conference mixed design of integrated circuits and systems, 2002. p. 607–12] they essentially differ to each other as they are based on two quite different SC FIR delay line structures. In the first filter version Gillingham delay elements [Gillingham P. Stray-free switched-capacitor unit delay circuit. Electron lett 1984;20(7):308–10] are used, while in the second version even and odd delay elements [Dąbrowski A. Multirate and multiphase switched-capacitors circuits, London: Chapman & Hall; 1997; Dąbrowski A, Menzi U, Moschytz GS. Design of switched-capacitor FIR filters with application to a low-power MFSK receiver. IEE Proceedings-G 1992;139(4):450] are alternately connected to form the delay line. In this way an interesting comparison of these two SC delay line concepts has been possible.It should also be stressed that the frequency responses of both presented filters have been designed using the same technique, i.e., the Kaiser window of order N = 31. The upper frequency is in both cases equal to 500 kHz and the frequency of the controlling clock generator is equal to 1 MHz.The filter with Gillingham delay elements dissipates 30 mW with the 3 V supply voltage and occupies 2.2 mm². On the contrary, the even–odd SC FIR filter dissipates 18 mW only with the 3 V supply voltage and occupies 2.4 mm². Moreover, the newer filter version has the stopband attenuation greater by about 10 dB than the previous version.     

A Design Example of a Fractional-Order Kerwin–Huelsman–Newcomb Biquad Filter with Two Fractional Capacitors of Different Order

Design, realization and performance studies of continuous-time fractional order Kerwin–Huelsman–Newcomb (KHN) biquad filters have been presented. The filters are constructed using two fractional order capacitors (FC) of orders α and β (0<α, β≤1). The frequency responses of the filters, obtained experimentally have been compared with simulated results using MATLAB/SIMULINK and also with PSpice (Cadence PSD 14.2), where the fractional order capacitor is approximated by a domino ladder circuit. It has been observed that fractional order filters can give better performance in certain aspects compared to integer order filters. The effects of the exponents (α and β) on bandwidth and stability of the realized filter have been examined. Sensitivity analysis of the realized fractional order filter has also been carried out to investigate the deviation of the performance due to the parameter variation.     

On the practical realization of higher-order filters with fractional stepping

We propose the use of a compact integer-order transfer function approximation of the fractional-order Laplacian operator s^α to realize fractional-step filters. Lowpass and bandpass filters of orders (n+α) and 2(n+α), where n is an integer and 0<α<1, can, respectively, be designed. A 5th-order lowpass filter with fractional steps from 0.1 to 0.9 (i.e. 5.1→5.9) is given as an example with its characteristics compared to 5th- and 6th-order Butterworth filters. Spice simulations and experimental results are shown.     

A microwave photonic filter based on multi-wavelength fiber laser and infinite impulse response

A microwave photonic filter (MPF) based on multi-wavelength fiber laser and infinite impulse response (IIR) is proposed. The filter uses a multi-wavelength fiber laser as the light source, two sections of polarization maintaining fiber (PMF) and three polarization controllers (PCs) as the laser frequency selection device. By adjusting the PC to change the effective length of the PMF, the laser can obtain three wavelength spacings, which are 0.44 nm, 0.78 nm and 1.08 nm, respectively. And the corresponding free spectral ranges (FSRs) are 8.46 GHz, 4.66 GHz and 3.44 GHz, respectively. Thus changing the wavelength spacing of the laser can make the FSR variable. An IIR filter is introduced based on a finite impulse response (FIR) filter. Then the 3-dB bandwidth of the MPF is reduced, and the main side-lobe suppression ratio (MSSR) is increased. By adjusting the gain of the radio frequency (RF) signal amplifier, the frequency response of the filter can be enhanced.     

Comparison between two approximation methods of state space fractional systems

A large number of real dynamic systems are better characterized using a non-integer order dynamic model based on fractional order differential equations. Nevertheless, their analytical solutions are rather complicated. In this paper we present two alternative methods of simulation of the fractional order dynamical system in state space. We particularly present a comparison between two approximation methods. The first one is based on the differentiation operator approximation and thus uses the usual form of the state space representation D^(α)(x)=Ax+Bu. The second method is based on the approximation of the integration operator and uses, in this case, the new representation w=AI^(α)(w)+Bu. The comparison between these two methods is made on the basis of initial and steady state approximation state and output errors which are characterized in this present paper. Numerical examples are also given to support this comparison.     

Adaptive two-pass rank order filter to remove impulse noise in highly corrupted images

In this paper, we present an adaptive two-pass rank order filter to remove impulse noise in highly corrupted images. When the noise ratio is high, rank order filters, such as the median filter for example, can produce unsatisfactory results. Better results can be obtained by applying the filter twice, which we call two-pass filtering. To further improve the performance, we develop an adaptive two-pass rank order filter. Between the passes of filtering, an adaptive process is used to detect irregularities in the spatial distribution of the estimated impulse noise. The adaptive process then selectively replaces some pixels changed by the first pass of filtering with their original observed pixel values. These pixels are then kept unchanged during the second filtering. In combination, the adaptive process and the second filter eliminate more impulse noise and restore some pixels that are mistakenly altered by the first filtering. As a final result, the reconstructed image maintains a higher degree of fidelity and has a smaller amount of noise. The idea of adaptive two-pass processing can be applied to many rank order filters, such as a center-weighted median filter (CWMF), adaptive CWMF, lower-upper-middle filter, and soft-decision rank-order-mean filter. Results from computer simulations are used to demonstrate the performance of this type of adaptation using a number of basic rank order filters.     

Image structure preserving denoising using generalized fractional time integrals

A generalization of the linear fractional integral equation u(t)=u₀+∂^−αAu(t), 1<α<2, which is written as a Volterra matrix-valued equation when applied as a pixel-by-pixel technique is proposed in this paper for image denoising (restoration, smoothing, etc.). Since the fractional integral equation interpolates a linear parabolic equation and a hyperbolic equation, the solution enjoys intermediate properties. The Volterra equation we propose is well-posed for all t>0, and allows us to handle the diffusion by means of a viscosity parameter instead of introducing nonlinearities in the equation as in the Perona-Malik and alike approaches. Several experiments showing the improvements achieved by our approach are provided.     

A novel approach for coefficient quantization of low-pass finite impulse response filter using differential evolution algorithm

Evolutionary computational techniques have been employed judiciously in various signal processing applications of late. In this paper, such an attempt has been made to design a low-pass linear-phase multiplier-less finite duration impulse response (FIR) filter using differential evolution (DE) algorithm. This particular evolutionary optimization technique has been explored to search the impulse response coefficients of the FIR filter in the form of sum of power of two (SPT) in order to avoid the multipliers during design process. The performance of the designed low-pass filter has been studied thoroughly in terms of its frequency characteristics and primitive requirement of fundamental hardware blocks. The superiority of our design has been ascertained over a number of existing techniques by various means. Finally, the proposed filter of different lengths has been implemented on a field programmable gate array (FPGA) chip for evaluating the competency of this work. The percentage improvement in hardware complexity produced by our design has also been computed and clearly listed in this paper for convenience.     

H optimal approximation for causal spline interpolation

In this paper, we give a causal solution to the problem of spline interpolation using H^∞ optimal approximation. Generally speaking, spline interpolation requires filtering the whole sampled data, the past and the future, to reconstruct the inter-sample values. This leads to non-causality of the filter, and this becomes a critical issue for real-time applications.Our objective here is to derive a causal system which approximates spline interpolation by H^∞ optimization for the filter. The advantage of H^∞ optimization is that it can address uncertainty in the input signals to be interpolated in design, and hence the optimized system has robustness property against signal uncertainty.We give a closed-form solution to the H^∞ optimization in the case of the cubic splines. For higher-order splines, the optimal filter can be effectively solved by a numerical computation. We also show that the optimal FIR (finite impulse response) filter can be designed by an LMI (linear matrix inequality), which can also be effectively solved numerically. A design example is presented to illustrate the result.     

Two-tap travelling-wave infinite impulse response filter with 12 dB peaking at 24 GHz

A two-tap infinite impulse response (IIR) filter using travelling-wave architecture is presented. The filter utilises poles as a means of frequency boosting, contrasting the conventional finite impulse response technique of utilising zeros and is the first ever implementation of an IIR filter using a double-loop multi-delay topology. Implemented in a 90 nm CMOS process, the filter achieves a 12.1 peak at 24 GHz when both filter taps are set to maximum peaking and consumes 55.2 mW from a 1.2 V supply.     

A new adaptive recursive RLS-based fast-array IIR filter for active noise and vibration control systems

Infinite impulse response filters have not been used extensively in active noise and vibration control applications. The problems are mainly due to the multimodal error surface and instability of adaptive IIR filters used in such applications. Considering these, in this paper a new adaptive recursive RLS-based fast-array IIR filter for active noise and vibration control applications is proposed. At first an RLS-based adaptive IIR filter with computational complexity of order O(n²) is derived, and a sufficient condition for its stability is proposed by applying passivity theorem on the equivalent feedback representation of this adaptive algorithm. In the second step, to reduce the computational complexity of the algorithm to the order of O(n) as well as to improve its numerical stability, a fast array implementation of this adaptive IIR filter is derived. This is accomplished by extending the existing results of fast-array implementation of adaptive FIR filters to adaptive IIR filters. Comparison of the performance of the fast-array algorithm with that of Erikson’s FuLMS and SHARF algorithms confirms that the proposed algorithm has faster convergence rate and ability to reach a lower minimum mean square error which is of great importance in active noise and vibration control applications.     

Optimal impulse response shortening for discrete multitonetransceivers

The authors have derived a new algorithm for the optimal shortening of a channel impulse response in discrete multitone (DMT) transceivers. This algorithm uses eigenvalues and eigenvectors to generate the coefficients of the shortening impulse response filter (SIRF). In comparison with the previous approach, this new algorithm can calculate the optimal settings of an SIRF with arbitrary length     

Enhanced performance of dye-sensitized solar cells with novel 2,6-diphenyl-4H-pyranylidene dyes

Novel 2,6-diphenyl-4H-pyranylidene derivatives were designed and synthesized as dyes for dye-sensitized solar cells (DSSC). Dyes 2a, b with a phenyl substituent showed high DSSC energy conversion efficiencies of 5.3% (J_sc = 10.3 mA/cm², V_oc = 0.72 V, FF = 0.72) and 4.7% (J_sc = 8.9 mA/cm², V_oc = 0.73 V, FF = 0.72) at 100 mW/cm² under simulated AM 1.5 G solar light conditions. These values are twice better than that of dye 1 without the phenyl substituent under the same conditions. Both the photocurrent density (J_sc) and open circuit voltage (V_oc) of DSSCs based on dyes 2a, b are increased compared with 1. It can be attributed to their twisted structures, absorption abilities and proper energy levels. This result shows that the tetraphenylpyranylidene is a promising electron-donor unit for high-efficiency DSSCs.     

Efficient computation of DFT commuting matrices by a closed-form infinite order approximation to the second differentiation matrix

In order to define the discrete fractional Fourier transform, Hermite Gauss-like eigenvectors are needed and one way of extracting these eigenvectors is to employ DFT commuting matrices. Recently, Pei et al. exploited the idea of obtaining higher order DFT-commuting matrices, which was introduced by Candan previously. The upper bound of O(h^2k) approximation to N×N commuting matrix is 2k+1≤N in Candan's work and Pei et al. improved the proximity by removing this upper bound at the expense of higher computational cost. In this paper, we derive an exact closed form expression of infinite-order Taylor series approximation to discrete second derivative operator and employ it in the definition of excellent DFT commuting matrices. We show that in the limit this Taylor series expansion converges to a trigonometric function of second-order differentiating matrix. The commuting matrices possess eigenvectors that are closer to the samples of Hermite-Gaussian eigenfunctions of DFT better than any other methods in the literature with no additional computational cost.     

Design of digital FIR variable fractional order integrator and differentiator

This paper presents an easy and simple method to design variable fractional order digital FIR integrators and differentiators based on fractional order systems. First, closed-form digital IIR fractional order integrators and differentiators have been obtained from the analog rational functions approximations, in a given frequency band, of the fractional order integrator s ^?m and differentiator s ^m (0?<?m?<?1) through the Tustin generating function. Then, closed-form digital FIR fractional order integrators and differentiators by truncation of the digital IIR ones have been derived. Next, polynomial interpolation has been used to design digital FIR variable fractional order integrators and differentiators that can be implemented by the Farrow structure. The main feature of variable fractional order operator is that its order can be changed without re-designing a new fractional order operator. Some examples have been presented through the paper to illustrate the performance and the effectiveness of the proposed design method. The results obtained have been discussed and have been compared to one of the most recent works in the literature using the same design parameters.     

Far infrared spectrum of CD3OH methanol from 40–220 CM−1

The Fourier transform far-infrared (FTFIR) spectrum of CD₃OH has been obtained from 40–220 cm^?1 at a resolution of 0.002 cm^?1, and partially analyzed. Numerousb-type branches have been assigned in the spectrum, ranging over torsional states fromn=0 to 3. The branches have been fitted toJ(J+1) power-series energy expansions in order to obtainJ-independent branch origins. These in turn have been fitted to the torsion-rotation Hamiltonian, and improved molecular constants have been obtained for the ground vibrational state.     

The transfer function and impulse response of photorefractivetwo-beam coupling

The transfer function and impulse response of photorefractive two-beam coupling are derived in the undepleted pump approximation. For sufficiently strong coupling ΓL, the impulse response features a broad delayed output pulse. In the limits of negligible and strong absorption αL, this coupling threshold reads ΓL_thr=4 and Γ_thr=2α, respectively. The time delay and pulse height are functions of the coupling ΓL, the photorefractive time constant τ, and the effective absorption αL. Experiments on a BaTiO₃ crystal measuring the absoluted square of its transfer function and the impulse response are used to determine the coupling and time constant