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Transmission characteristic differences of a narrow bandpass filter between p- and s-polarized light, especially the central wavelength separation, will corrupt the performance of the filter when the incidence is oblique. In this paper, by adding high-index materials asymmetrically to both sides of a low-index spacer, which tunes the equivalent index of the spacer, the central wavelengths of the two polarizations coincide perfectly when in 20[degrees] incidence; with different reflected layers on the two sides of the spacers and replacing some reflected layers with equivalent layers, the 0.5 dB normalized passband width is kept at 0.2 nm, which meets the requirement of the 50 G dense wavelength division multiplexer filter. OCIS codes: 310.0310, 310.6860, 060.4510, 230.7408.

[Al.sub.2][O.sub.3] and Ti[O.sub.2] thin films have been deposited on Si wafers, quartz, BK7 glass, and polycarbonate substrates by atomic layer deposition (ALD). The refractive indices and growth rates of the materials have been determined by spectroscopic ellipsometry and transmission electron microscopy. The influence of substrate temperature and precursor on the refractive indices has been investigated. The refractive index of Ti[O.sub.2] significantly increases with temperature, whereas the [Al.sub.2][O.sub.3] films are temperature insensitive. The films deposited using [H.sub.2][O.sub.2] as oxygen source show a slightly higher refractive index than the films prepared with [H.sub.2]O. Multilayer narrow-bandpass filters and broadband antireflective coatings have been designed and produced by ALD. OCIS codes: 310.0310, 310.1210, 310.1620, 310.1860, 310.4165.

We propose a novel approach to designing an optical filter with a flat-top and ultranarrow transmission band by using a sampled fiber Bragg grating (SFBG) having multiple equivalent phase shifts (EPSs). In the proposed approach, multiple EPSs are fabricated by adjusting several sampling periods of the SFBG. The obtained multiple EPSs are then arranged properly to get a flat-top transmission band in the narrow--1st reflection band of the SFBG, leading to the generation of a flat and ultranarrow transmission band. Filters with two or five EPSs are designed by simulation and experiment, with low ripples and ultranarrow l dB bandwidths observed in transmission bands. OCIS codes: 050.2770, 120.2440, 060.2340, 050.5080.

This paper discusses the physics behind the operation of dual-mode bandpass filters from a field theoretical point of view. It is argued that the two degenerate modes of the empty dual-mode cavity, commonly taken as the vertical and horizontal polarizations, become nonphysical when coupling and tuning elements are inserted. Instead, the original degenerate modes are rotated, or modified in a complex way, to generate two new modes whose characteristics depend on the coupling and tuning elements. It is shown that the tuning elements, as placed in existing dual-mode filter designs, act as both tuning and coupling elements. A working dual-mode filter can be designed with only 'tuning' elements present. A physical representation of dual-mode filters in terms of the eigen-resonances of the dual-mode cavities, with the tuning and coupling elements present, is introduced. Two fourth-order dual-mode rectangular cavity filters with the same response in the passband and its vicinity are also presented to demonstrate the similar role played by 'tuning' and coupling elements in dual-mode cavities. The first filter uses only 'tuning' elements, while the second is based only on 'coupling' elements. Index Terms--Boundary conditions, coupling matrix, dual-mode filters, Maxwell's equations, resonator filters, synthesis.

A new filter composed of a cascade of strip gratings with a 3rd order Chebishev frequency response that operates within a prescribed frequency band, is presented in this paper. Theoretical development was undertaken to establish equivalence between electromagnetic characteristics of a frequency selective surface (FSS) configuration and equivalent model based on circuit parameters that are used in the realization of Chebishev frequency response. The FSSs replace the impedance inverter elements in circuit implementation of the filter and the spacing between FSSs, which are adjusted to obtain the desired electrical performance, correspond to resonant elements in the circuit implementation. Dielectric slabs similar to the ones that were employed in FSS stages were used as the spacers between the constituent FSS layers to increase the robustness and rigidity of the structure. The fabrication process is described in detail and methods are introduced to account for tolerances in the permittivity and etching process during the design stage. Close agreement between measurement results demonstrates the validity of the design procedure. Index Terms--Millimeter-wave filter, quasi-optical measurement, spatial filters, strip grating.

This paper presents a novel multipole miniaturized-element frequency-selective surface (FSS) having a very low thickness and a desired multipole frequency response. For this design, new miniaturized elements for the FSS are developed to achieve low thickness solution and improved functionality. The proposed FSS enables implementation of higher order spatial filters over low-profile conformal antenna arrays. First, design of a very thin-layer modified miniaturized-element FSS producing a single-pole bandpass response in addition to a transmission zero is presented. The modified design is just a single-sided circuit board with a particular unit cell consisting of a loop centered within a wire grid. Next, using a similar metallic pattern on the other side of a very thin substrate, a dual-bandpass frequency response is produced. This response is achieved by choosing proper dimensions for the loops and wire of each layer and by appropriately positioning the layers with respect to each other. To establish a benchmark, dual-pole FSSs using cascaded layers of a previously designed miniaturized-element FSS are considered. In comparison, the modified dual-bandpass design has only two metal layers, instead of four, and a single substrate, instead of three. The proposed multipole FSS has a thickness of [lambda]/300 which is six times thinner than the benchmark structures. Moreover, the frequency response of the new FSS shows higher out-of-band rejection values. Performance of the multipole screens is tested by fabricating FSSs with maximally flat and dual-bandpass responses and measuring their frequency responses using a standard measurement setup in a freespace environment. Index Terms--Dual-band, miniaturized-element frequency-selective surface (FSS), multipole.

Novel balanced-to-unbalanced (balun) bandpass filters are presented and carefully examined. By properly converting a symmetric four-port balanced-to-balanced bandpass filter to a three-port device, with two ports forming a balanced output and one port remaining unbalanced, a novel three-port balun bandpass filter with both functions of balun and bandpass filter may be realized. Specifically, a balun bandpass filter is implemented with its center frequency at 0.99 GHz and a 3-dB fractional bandwidth ([B.sub.w]) of 10.1%. The implemented balun bandpass filter presents an excellent in-band balance performance with common-mode rejection ratio ([R.sub.cm]) better than 41 dB (the corresponding amplitude and phase imbalances are within 0.003[degrees] and 1[degrees]) over the passband. In this study, a wideband balun bandpass filter is also implemented so as to improve the wideband performance of the quasiYagi antenna. Being integrated with the proposed wideband balun bandpass filter, the implemented quasi-Yagi antenna achieves a bandwidth of 48% for [absolute value of [S.sub.11]] < -10 dB, with front-to-back ratio better than 13 dB, cross-polarization smaller than -20 dB, and peak gain of 3-5 dBi within the wide operating bandwidth. Index Terms--Antenna feeding structure, balun, bandpass filter, low cross-polarization quasi-Yagi antenna.

A novel three-pole electrically tunable narrow bandpass filter has been fabricated from bi-layer YBCO/[Ba.sub.0.05][Sr.sub.0.95]Ti[O.sub.3] thin films. At 77 K, and without bias, the filter has a center frequency of 3.50 GHz and 5.8% bandwidth. Independent application of dc bias to each of the resonators of 120, 182, and 100 V shifted the center frequency to 3.68 GHz. The insertion loss was less than 1.4 dB and return loss was better than 20 dB. A similar filter was made using bi-layer Ag/[Ba.sub.0.5] [Sr.sub.0.5] Ti[O.sub.3] thin films and measured at room temperature. The insertion loss of the filter was, as expected, much higher, although it showed improved tunability as compared with the YBCO version. Index Terms--Ferroelectric films, high-temperature superconductor, tunable bandpass filter.

A new cost-function is introduced for the optimization algorithm in order to achieve a multiple passband filtering function. A high-temperature superconductor ten-pole filter with triple-band performance is designed, where each of the passbands has a fractional bandwidth of approximately 0.26%.

Novel ultra-wideband (UWB) bandpass filters are proposed based on quasi-lumped-element prototypes and implemented with multilayer liquid-crystal-polymer (LCP) technology. In this study, the broadside-coupled microstrip radial stubs and high-impedance microstrip lines are adopted as quasi-lumped elements for realizing compact UWB bandpass filters. By introducing a short-circuited high-impedance microstrip line as a shunt inductor and suitably designing quasi-lumped-element capacitors, a compact six-pole bandpass filter is implemented with the Federal Communications Commission (FCC) defined UWB specifications. To further improve the selectivity and wideband performance, an eight-pole filter of this type is developed by adding two shunt short-circuited microstrip stubs, which introduce a transmission zero at the upper passband edge. The proposed filters are fabricated using multilayer LCP technology. Good agreement between simulated and measured results of these filters are observed. The measured results show that the fabricated six-pole filter has good specifications for the FCC-defined UWB system. The fabricated eight-pole filter has an ultra-wide fractional bandwidth (139%) and a good stopband rejection level, which is higher than 38.1 dB from 10.57 to 18.0 GHz. The proposed filters are attractive for UWB communications and radar systems. Index Terms--Bandpass filter, liquid crystal polymer (LCP), microstrip filter, ultra-wideband (UWB).

This paper investigates the design and implementation of millimeter-wave narrow-bandpass filters in a standard 0.18-[micro]m CMOS technology. Filters with a measured 10% 3-dB bandwidth at 60 and 77 GHz are realized in a thin-film microstrip structure by using the lowest metallization layer as a ground plane. The impact of dissipation losses of the filters is also examined. It is found that the metallization losses in the coupled-line filter as well as the ground plane are the main reasons for the insertion loss. Index Terms--Bandpass filters, CMOS technology, millimeterwave, thin-film microstrip (TFMS) structure.

In this paper, design techniques for an integrated RF bandpass filter are discussed. A novel wide-tuning high-Q active bandpass filter utilizing the active inductors is presented. Issues of the active inductor related to Q-enhancement, noise, linearity, and stability are considered. The circuit has been fabricated in an 0.18-[micro]m CMOS process, and the filter occupies the active area of 150 x 200 [micro][m.sup.2]. Measurement results show that the filter centered at 3.82 GHz with about 36-MHz bandwidth (3-dB) is tunable in frequency from about 1.92 to 3.82 GHz, and it exhibits--15-to 1-dB compression point at 2.44 GHz with approximately 60-MHz bandwidth while the dc power consumes 10.8 mW. Index Terms--Active bandpass filters, active inductors, multiband RF front-ends, Q-enhancement, stability.

A novel electronically reconfigurable microwave bandpass filter based on a switched delay-line approach is presented. In contrast to conventional tunable filters, the approach enables the lossy and nonlinear switching elements to be used as part of the coupling elements rather than within the resonators. Therefore, the filter distinguishes itself through its ability to provide a wide tuning bandwidth with low passband loss and high linearity. Theoretical analysis of the approach is presented in this paper. The feasibility of the approach has been experimentally verified with microstrip circuit prototypes. The filter produced a maximum passband loss of 1.7 dB and a third-order intercept point of > 32 dBm with 35 % tuning bandwidth. Index Terms--Bandpass filter, electronically tunable filters, low-distortion tunable filter, switched delay lines.

This paper presents a new class of dual-mode dualband filters in which each polarization is dedicated to a selected band. The equivalent circuit is a parallel combination of two inline networks that represent each polarization. A transmission zero is generated between the two bands by properly adjusting the relative orientations of the input and output coupling apertures. For filters where each branch contains an odd number of resonators, the input and output apertures have the same orientations. For filters where the order of each branch is even, the two apertures are orthogonal to each other. Filters using three and four dual-mode cavities are designed and presented. Different arrangements of the dual-mode cavities are also presented. Index Terms--Bandpass filters, dual band, resonator filters.

Two microelectromechanicai systems switchable microwave bandpass filters are presented. Both filters are compact fourth-order symmetrically coupled filters--the first is designed with bridge capacitive switches, while the second uses cantilever capacitive switches. The bridge-switched filter tunes from 21.6 to 16.85 GHz with insertion losses of 0.9 and 1.0 dB, respectively. The cantilever-switched filter tunes from 21.85 to 15.15 GHz and has insertion losses of 0.6 and 1.0 dB, respectively. Both the bridge F and cantilever capacitive switches are electrostatically actuated using dc voltages of 70 and 55 V, applied through high-resistivity chrome-silicide lines. The reported filters show state-of-the-art performance in insertion loss and frequency tuning characteristics. Index Terms--Bandpass filters, coupling circuits, microelectromechanical devices, micromachining, microstrip resonators.

This paper proposes a novel compact balanced-to-un-balanced bandpass filter. Firstly, a pre-design circuit is presented, which is composed of an inductive coupled-line bandpass filter and an out-of-phase capacitive coupled-line bandpass filter. A novel compact circuit with three coupled lines configuration, derived from the pre-design circuit, is then proposed for miniaturizing the balanced-to-unbalanced bandpass filter. A 2.4-GHz multilayer ceramic chip type balanced-to-unbalanced bandpass filter with a size of 2.0 mm X 1.2 mm x 0.7 mm is developed to validate the feasibility of the proposed structure. The filter is designed by using circuit simulation, as well as full-wave electromagnetic simulation softwares, and fabricated by the use of low-temperature co-fired ceramic technology. The measured results agree quite well with the simulated. According to the measurement results, the maximum insertion loss is 1.65 dB, the maximum in-band phase imbalance is within 3[degrres], and the maximum in-band magnitude imbalance is less than 0.32 dB. Index Terms--Balanced-to-unbalanced bandpass filter, balun, bandpass filter, low-temperature co-fired ceramic (LTCC).