Showing papers on "Stopband published in 1995"

Phase-shifted Bragg-grating filters with improved transmission characteristics

Abstract: Standard Bragg filters (dielectric waveguides with a grating overlay) show a stopband only in reflection. Filters with quarter-wave-shifted gratings have an extremely narrow transmission peak the center of the stopband but its shape (a sharp triangle with a broad bottom) is not suitable for system design. Using standard coupled-mode theory together with transfer-matrix formalism, we show that by introducing several phase-shift regions and properly choosing their locations and magnitudes the transmission spectrum can be tailored into a nearly rectangular shape. For InP-InGaAsP based Bragg-grating filters estimates are made of achievable channel spacings in wavelength-division multiplexing systems.

Large electromagnetic stop bands in metallodielectric photonic crystals

Abstract: An electromagnetic stop band spanning about one octave is observed in a periodic structure consisting of a face‐centered‐cubic Bravais lattice of metal spheres supported by a low‐loss dielectric. The low‐frequency edge of the stop band is consistent with the Mie condition for the metallic spheres, and the center frequency of the stop band is a function of the lattice constant and the support dielectric material. For example, a sample having a cubic lattice constant of 1.65 cm, 3/8 in. diam chrome spheres at each atomic core, and Teflon support dielectric displayed a lower band edge of approximately 5.2 GHz, an upper band edge of about 12.8 GHz, and a maximum rejection at the center of the band of roughly 20 dB.

Periodic Green's functions analysis of SAW and leaky SAW propagation in a periodic system of electrodes on a piezoelectric crystal

Abstract: A method of periodic Green's functions with a propagation factor exp(i/spl beta/x), unknown in advance, is used to calculate dispersion curves and attenuation coefficients for Rayleigh- and leaky- waves propagating in a periodic system of thin electrodes on a piezoelectric surface. To describe the charge distribution on the electrodes both a step approximation and Chebyshev polynomials are used, the last being more adequate in most cases. Numerically determined values of the Green's function are used and interpolated either linearly or using a modified variant of Ingebrigtsen's formula. Such basic parameters as stopband width, stopband center frequency, wave velocity and attenuation in the stopband are found. These parameters can be used in the coupling-of-modes (COM) analysis and design of SAW devices. The analysis includes bulk wave radiation and scattering. The dependence of the corresponding attenuation coefficient on frequency is determined. Results obtained allow the determination directly and properly of the COM parameters and the design of SAW devices having large number of electrodes most precisely and rapidly. Numerical results for Rayleigh waves on YZ-LiNbO/sub 3/ and leaky waves on 36/spl deg/YX-LiTaO/sub 3/ substrates are presented. >

Plane wave propagation in finite 2-2 composites

Abstract: A common practice in the study of wave propagation in stratified structures is to use the Floquet (or Bloch) condition to derive the dispersion relation, leading to the passband and stopband structures. However, the Floquet condition is valid only for an infinite system while a real system always has finite dimensions. We report a study on wave propagation in a finite 2‐2 composite by using the transfer (T) matrix technique. Through introducing a new definition for the dispersion relation using the T matrix, the passbands and stopbands are calculated for a finite system without the Floquet condition. The formation of stopbands and passbands with the increase of composite size can now be clearly seen. The spatial profile of the vibration pattern inside a finite composite can also be calculated using this technique, which reveals strong edge effects. The effects of randomization on the wave localization in a 2‐2 composite are also studied.

Ultra-wideband photonic band gap crystal having selectable and controllable bad gaps and methods for achieving photonic band gaps

Abstract: The present invention provides multidimensional stacked photonic band gap crystal structures improving the performance of current planar monolithic antennas and RF filters by forbidding radiation from coupling into the substrate thereby significantly enhancing radiation efficiency and bandwidth. This invention comprises a number of sub-crystals with each having at least two lattices disposed within a host material, each lattice having a plurality of dielectric pieces arranged and spaced from each other in a predetermined manner, the sub-crystals being stacked in a crystal structure to provide a photonic band gap forbidding electromagnetic radiation propagating over a specially designed frequency band gap, or stopband. Both two dimensional and multidimensional crystals are disclosed. The preferred embodiment is a three-dimensional photonic band gap crystal comprising two or more sub-crystals, with each sub-crystal having a diamond-patterned lattice constructed from a plurality of dielectric zigzag pieces orthogonally interconnected, disposed within a host material.

Surface acoustic wave filter and mobile communication system using same

Abstract: A composite surface acoustic wave filter includes an interdigitated interdigital surface acoustic wave filter having input and output terminals. Two one-port surface acoustic wave resonators are electrically connected in series to the interdigitated interdigital surface acoustic wave filter respectively at the input and output terminals, the one-port surface acoustic wave resonators having antiresonant frequencies in a stop band that is close to and higher than the passband of the interdigitated interdigital surface acoustic wave filter. Two other one-port surface acoustic wave resonators are electrically connected parallel to the interdigitated interdigital surface acoustic wave filter, the one-port surface acoustic wave resonators having resonant frequencies in a stop band that is close to and lower than the passband of the interdigitated interdigital surface acoustic wave filter.

A filter with an adjustable shunt zero

Abstract: A filter (102) with an adjustable shunt zero. The filter (102) has a predetermined passband and stopband, and an input (104) and an output (106), and a variable reactance element (108) for adjusting the stopband frequency of maximum attenuation, also defined as a shunt zero, coupled to at least one of the input (104) and the output (106) of the filter (102), whereby the shunt zero is adjustable over a range of frequencies.

Wide-stopband chirped fibre moire grating transmission filters

Abstract: Efficient, wide-stopband fibre transmission filters with single narrow transmission bands produced using multiply exposed chirped fibre grating resonators are reported. Tailored combinations of these structures with additional chirped gratings have achieved single passband transmission with an effective finesse of 48. This demonstrates the feasibility of fabricating all-fibre transmission filters with arbitrary passband/stopband combinations.

SAW impedance elements

Abstract: In the past few years a new class of SAW devices has been under development based on the use of one port SAW resonators as lumped elements with their impedance strongly changing in a frequency range of effective SAW excitation These 'SAW impedance elements' can be connected in ladder type, balanced bridge type, or others types of networks to provide desired filter characteristics In this paper the admittance characteristics of a single element are analyzed using coupling of mode (COM) model simulations It is found that resonant type characteristics of a long transducer can be described by very simple formulas The frequency gap between resonance and anti-resonance, important for filter design, is determined practically by the /spl Delta/V/V parameter of the piezoelectric substrate used, while the minimum magnitude of the impedance at the resonant frequency is determined by the SAW attenuation The internal reflections only shift the resonance to the left edge of the stopband not changing other characteristics A case of a synchronous resonator comprising a relatively short transducer and two reflectors placed on both sides of it is also analyzed This type of impedance element has smaller frequency shift between resonance and anti-resonance points >

Design of predictive IIR filters via feedback extension of FIR forward predictors

Abstract: FIR predictors for polynomial signals and sinusoids are easy to design because of the available closed-form design formulae. On the other hand, those FIR predictors have two major drawbacks: the passband gain peak is usually greater than +3 dB, and a long FIR structure is needed to attain high attenuation in the stopband. Both of these characteristics cause severe problems, particularly in embedded control applications when the predictor operates inside a closed control loop. In this paper, we present a novel feedback extension scheme for FIR forward predictors. This extension makes it possible to easily design IIR predictors with low passband ripple and high stopband attenuation. The new approach is illustrated with design examples.

Longitudinal leaky surface waves for high frequency SAW device applications

Abstract: Longitudinal leaky surface waves with a phase velocity of 6700 m/s and low propagation loss on lithium tetraborate (LBO) are reviewed. Moreover, the dispersion properties of longitudinal leaky surface waves propagating under the periodic Al strip grating on LBO are described theoretically and experimentally, for applications of the mode to the high frequency SAW devices. The theoretical method is based on Floquet's theorem using space harmonics as an orthogonal function set. It is found that the propagation loss at the lower edge of the first Bragg stopband for shorted gratings is relatively low. The parameters used in the electrical equivalent circuit model for designing the SAW filters, are obtained from the calculated dispersion curves. A 1.5 GHz SAW filter using the mode has been designed and fabricated for the filter of the global positioning system (GPS). A low loss SAW filter can be obtained easily without submicron fabrication techniques, by using the longitudinal leaky surface waves on LBO.

Design of FIR filters using quadratic programming approach

Abstract: The paper presents a simple computer-aided design approach for designing FIR digital filters. The design problem is formulated as a quadratic programming problem with quadratic constraint. Two design methods are considered. One method involves minimizing the mean-square error between a desired response and the filter response over a passband subject to a mean-square stopband constraint. The other method involves minimizing the mean-square value of the filter response in the stopband subject to a mean-square error constraint over the passband. Numerical techniques based on matrix factorization are proposed to reduce the computational complexity. Relationship of the approach with the eigenfilter method is established. Numerical results are presented to illustrate the performance achievable. >

Improved filter sharpening

Abstract: We propose a natural extension to Kaiser-Hamming (1977) filter sharpening methods to allow for a piecewise linear desired amplitude change function (ACF). The primary advantages of the proposed ACF over piecewise constant ACFs is that we obtain better control of selective improvement (or degradation) in either the passband or stopband or both, and we are not restricted to applying our methods to filters with piecewise constant pass and stopbands, since linear segments of slope 1 can be used to retain existing filter performance in either passband or stopband. The proposed ACF approximating polynomial (AP) is easy to compute, may be constrained to have simple (or integer) coefficients, and may be expressed as the AP of Kaiser and Hamming plus a correction polynomial. We also provide applications for motivation.

High frequency fundamental resonators and filters fabricated by batch process using chemical etching

Abstract: The small sized 1st intermediate frequency (IF) filters at center frequency range of 70 MHz to 150 MHz and passband widths of /spl plusmn/5 to /spl plusmn/100 kHz with sharp selectivity are required in mobile communication systems such as mobile and portable cellular phone. Our solution to employ fundamental mode monolithic crystal filter (MCF) assembled in surface mountable package. We describe here in detail, the design approach including batch process etching technology. Through photolithography, 56 patterns are chemically etched on one wafer (25 mm/spl times/20 mm). Then, a similar etching process automatically adjusts the wafer thickness in accordance with frequency. For the MCF, the frequency of split electrodes and the degree of coupling between them are automatically adjusted by an accurately positioned mask evaporation process controlled by a computer. Further, we describe suppression of the spurious response, technology for realizing wide bandwidth and high stopband attenuation characteristics. By our above developed technology, we achieved 90 MHz miniaturized IF filter with 1/15 volume reduction of conventional 3rd overtone mode MCF, still possessing the same characteristic of conventional one. Also, we achieved 130 MHz of middle band with suppression of spurious response in wide stopband frequency range, and 71 MHz linear phase wide /spl plusmn/88 kHz band with the group delay distortion 0.9 /spl mu/s over f/sub 0//spl plusmn/80 kHz.

Design of selective lowpass sampled‐data and digital filters exhibiting equiripple amplitude and phase error characteristics

Abstract: A method is presented for the design of reciprocal reactant sampled-data and digital filters exhibiting equiripple passband and stopband amplitude characteristics and approximating a linear phase characteristic such that the phase error response is equiripple too. the passband amplitude characteristic approximates unity transmission and the stopband amplitude characteristic approximates zero. Odd- and even-degree cases are considered separately. the stopband amplitude response is controlled by an arbitrary number of finite transmission zeros together with two (one) zeros at infinity for even (odd) transfer functions. All the available degrees of freedom are used for controlling the filter performances. Thus the resulting solutions are selective and have good amplitude and phase characteristics. A comparison between our method and a recently published method shows that this approach has higher stopband loss and smaller phase error ripple with wider bandwidth.

Theoretical modeling of resonant modes of composite ultrasonic transducers

Abstract: Although a great deal of effort has been devoted to the modeling of composite piezoelectric materials, most of the earlier works are based on the assumption that the structure of the composite relative to the wavelength is very fine. Such approximation cannot address the complete dynamic behavior of composites. In order to understand the overall characteristics of composite ultrasonic transducers, a dynamic model was developed, in which the acoustic waves propagating in 2-2 composites along the thickness direction were analyzed by solving the coupled elastic equations of the constituent phases. By neglecting the boundary conditions of the free surfaces and simply taking the resonator thickness as half a wavelength, the resonant modes of the composite transducers as functions of aspect ratio of the ceramic plate elements and volume fraction of ceramic phase can be calculated from this model. The theoretical dispersion curves for the thickness mode and the lateral periodical mode agree with the experimental results. The vibration distribution in the ceramic and polymer phases at the resonant frequency as a function of the composite thickness as well as the volume fraction of the ceramic phase are obtained, and through the discussion of the vibration field the variation rule of the resonant frequency is well explained. For the resonant frequency the results of the isostrain model, the stopband resonance model, and the T-matrix model are consistent with the predictions made by this model under the special condition of very fine structure. >

Effect of spatial filtering on the spontaneous emission spectrum of a sub-threshold VCSEL

Abstract: We have measured the optical spectrum of a vertical cavity surface-emitting laser (VCSEL) operating below threshold. The spectrum consists of a strongly asymmetric central peak with weak structures in the wings that denote the edges of the DBR stopband. We show that the striking asymmetry of the central resonant part is caused by rays emitted at an angle with respect to the surface normal. The asymmetry can be removed by spatial filtering in the far-field. Analysis of the so obtained Lorentzian spectrum yields the current-dependent cavity loss rate and resonance frequency and thus, provides a new tool to analyze VCSEL's. >

Wavelength division multiplexer with wide passband and stopband for 1.3 /spl mu/m/1.55 /spl mu/m using silica-based planar lightwave circuit

Abstract: A wideband 13/155 µm wavelength division multiplexer (WDM) is demonstrated It is composed of point-symmetrically connected Mach-Zehnder interferometers (MZIs) and fabricated using silica-based waveguides with relative index differences of 045% and 7 × 7 µm2 cores The passband and stopband are successfully widened by factors of ~15 and 2, respectively, compared with an ordinary MZI-type WDM

Non-uniform Q self amplitude equalized bandpass filter

Abstract: A bandpass filter having 4-degrees of freedom includes a plurality of resonant cavities having respective Qs where at least one of the Qs is different. A plurality of main couplings couple successive resonant cavities to establish a main signal path that provides a first degree of freedom for controlling the shape of the filter's frequency response over its passband. A plurality of bridge couplings couple pairs of the resonant cavities so that the cavities are connected in a canonical circuit topology. The bridge couplings provide second and third degrees of freedom for controlling the sharpness of the frequency response 's transition between its passband and stopband and controlling the linearity of its phase, respectively. The cavities' non-uniform Qs provide a fourth degree of freedom for controlling the amplitude of the filter's frequency response in the passband so that the amplitude is within a predetermined tolerance of a desired passband shape.

Resonant modes in coupled resonator filters and the unique equivalent circuit representation

Abstract: An equivalent circuit representation, which has been successfully used in SAW transversely coupled resonator filter (TCF), is extended to longitudinally coupled resonator filters (CRF). In TCF case usually two fundamental modes are accurate enough to represent the passband but more modes (most of them are 'quasimodes' located outside of the stopband of the structure) are needed for CRF. A mode identification method is introduced in terms of equivalent motional parameters, R, L, and C and then applied to two experimental examples, a narrow bandfilter on quartz and a 3% bandwidth filter on 36/spl deg/ YX-LiTaO/sub 3/. The results confirm that it could be a useful tool for system designer as it is compatible property with commercial electronic circuit CAD software. The profiles of the main resonant modes detected by a laser probe are presented in this paper.

Improvements in superconducting linear phase microwave delay line bandpass filters

Abstract: Microwave delay line filters can be designed to specified amplitude and phase characteristics using a synthesis procedure which takes into account multiple reflections and losses in the delay lines. The time domain synthesis procedure has now been extended so that frequency-dependent loss and delay line dispersion can also be allowed for. Meanwhile experimental results on linear phase microstrip and coplanar filters show significant improvements on stopband attenuation and phase linearity over previously published data on microstrip devices. >

Compact low loss IF balanced bridge filters

Abstract: Balanced bridge SAW impedance element filters (BBF's) developed recently have some important advantages such as wider passband, better suppression in the stopband, small size when compared with coupled resonator filters (CRF's). Compared to ladder type IEF's, a balanced bridge network gives lower insertion loss in passband, better suppression in the stopband and allows balanced input/output operation. The geometrical and COM parameters were found close to optimal to decrease size, technological sensitivity and suppress spurious signals. To avoid high level of feedthrough, care must be taken to provide the filter layout being also balanced with respect to parasitic capacitances.

Compact low‐pass electrical filters for cryogenic detectors

Abstract: We describe the design and performance of compact, low‐pass electrical filters, suitable for operation at cryogenic temperatures. The filters are two pole LC filters with a cutoff at ≊100 kHz, an attenuation of −24 dB/octave, and a stop band rejection of −80 dB between 5 and 20 MHz. Their function is to suppress rf voltage noise to sensitive cryogenic detectors. The low‐pass filters are realized using surface mounted devices on thin PC boards which can be stacked for maximum packing density. Compact modules containing 14 and 35 filters have been developed and installed in the Infrared Telescope in Space.

112/spl deg/-LiTaO/sub 3/ periodic waveguides

Abstract: The waveguide properties of periodic waveguide structures on 112-LiTaO/sub 3/ are studied experimentally and theoretically at frequencies close to the Bragg stopband. On admittance curves of long transducer type test structures it is seen that additional attenuation appears at both the high and low frequency sides of the stopband in that the resonances at these frequencies are suppressed. In addition, strong spurious resonances are seen in the lower half of the stopband region. Direct laser probe measurements have shown that the amplitude distribution in the transverse direction across the waveguide changes significantly versus frequency. Propagation ranges from (a) weak single mode guiding at low frequency to (b) strong multi-mode guiding near the lower stopband frequency then (c) a region without any guided modes and strong side radiation near the upper stopband frequency and (d) a return to weak single mode guiding at frequencies well above the stopband. The theoretical explanation of the observed effect is proposed based on Haus and Wang (1978) and Schwelb (178) models. This effect "side-radiation" from waveguide structures in devices at the frequencies corresponding to the upper edge of the stopband can give rise to spurious responses due to acoustic coupling between different parts of the device.

COM-theory analysis of STW resonator structures

Abstract: The dispersion characteristics of surface transverse waves (STWs) on 36/spl deg/ Y cut quartz substrate have been analysed numerically. A closed-form dispersion relation has been introduced in the equations of the coupling-of-modes (COM) theory to account for the variation of trapping with frequency. The transmission and reflection coefficients at the finger edges have been determined from the stopband characteristics. Introducing an electromechanical coupling which varies with the electrode mass loading, quantitative agreement with experimental results on 650 MHz two-port STW resonators has been achieved.

Novel slow-wave structure for narrow-band quasi-transversal filters

Abstract: Slow-wave transmission lines for producing large delays have employed multilayer patterning, increased width, or very fine features requiring accurate photolithography. A narrow-band slow-wave structure with none of these disadvantages is proposed for use with tapped delay line (transversal) narrow-band filters. This line has periodic discontinuities, resulting in a pass- and stopband structure in which the group delay in the passbands is several times that of an ordinary transmission line. The discontinuities are spaced typically by one or two wavelengths so that fine details are avoided. A linear-phase transversal stripline filter in which delay is increased by a factor of five is demonstrated. Using a previously published design procedure to calculate the filter tap weights, multiple reflections, dispersion and frequency-dependent losses were allowed for and both amplitude and phase of the filter were synthesised. A wide range of both phase and amplitude characteristics should eventually be possible and superconducting delay lines should ultimately permit losses of only 1 or 2 dB

A novel SAW filter for IF-filtering in DECT systems

Abstract: We have developed an improved SAW filter for IF-filtering in DECT systems. The filter is a two track reflector filter using EWC-SPUDTs as in- and output-IDTs and reflectors in the center of each track The propagation path of the SAW is folded in order to make better use of the total chip length. A similar configuration with identical reflectors in the tracks is known in literature, however, we used different reflectors in the tracks and this way we could improve the filter performance and especially the stop band attenuation considerably. The SPUDTs are withdrawal weighted, for the reflector design we used an electrode width weighting technique. The filter fits into a very small QCC10 package (9.1/spl times/7.1/spl times/1.8 mm/sup 3/) and has high performance so that it can be used in a DECT base-station, too. The substrate is LiTaO/sub 3/, X112.2Y, the center frequency is 110.59 MHz, the minimum IL is 8 dB typically, the 3-dB-bandwidth is 1.1 MHz and the stopband attenuation is better than 40 dB.

Wide Stop Band Optical Filters from Photonic Band Gap Air Bridges

Abstract: Photonic band gap materials can produce optical filters with large stop bands and sharp transmission resonances The transmission spectrum and radiation characteristics of one such filter are described using a modified finite difference time domain program

A filter and a radio communications device comprising the same.

Abstract: To provide a low pass filter (10) with good harmonic rejection and minimal insertion loss, a relatively low impedance (L6) is selectively switched in parallel with a shunt impedance (L3) located in a network of the filter (10), whereby the series resonant frequency provided by the shunt impedance in combination with a relatively high impedance point is altered and the filter characteristic is changed; whereby the passband is selectively widened and narrowed whilst stopband rejection is kept below a specific level thereby filtering out harmonics in the stop band. In a radio communications transceiver, Fig 4, not shown, a microprocessor (48) is used to control the switching voltage Ux in response to a transmission frequency.