Microwave Filters Utilizing the Cutoff Effect

Two band-rejection microwave filters employing the waveguide cutoff effect are discussed. One type utilizes the cutoff property in the series arm of an E plane tee to improve the filter's characteristics, while the other utilizes this property in the E and H arms of a magic tee. Experimental data for both single and multistage filters are presented. Methods of obtaining low standing wave ratios over a broad pass-band are also presented.     

Non-Minimum-Phase Microwave Filters

Several restriction are normally imposed on the type of transfer functions which can be realized as practical microwave filters. Conventional waveguide filter structures cannot realize general nonminimum-phase functions with transmission zeros in the right half-plane. These networks are, however, of great importance as much better combinations of phase and amplitude characteristics can be achieved through the use of these filters. In this paper a new type of filter is analyzed, in which reactive elements are combined in a special way with a magic-T or a 90-degree microwave hybrid. A synthesis procedure is given for the realization of transfer functions with zeros in any or both halves of the complex frequency plane. This realization leads to canonic networks with ladder type structures. It is also shown how antireciprocal phase shifters can be used to realize modulators, or special types of filter functions.     

High-Power Microwave Filters

In order to obtain filters capable of handling very high power, the use of radial lines and uniform line discontinuities was investigated as the most promising approach. In this connection, it was necessary to consider the equivalent circuit and interaction for H-type radial line mated at each end to uniform TE/sub 10/ waveguide for taper angles of 45°. It was found that the equivalent circuit was valid for taper angles of 45°, and that for engineering design purposes the interaction could be neglected. The author utilized the 45° tapers and the uniform lines to design a high-power microwave filter capable of handling 700 kw at 10 pounds pressure in 0.900 by 0.400 ID waveguide. The design procedure for a multielement filter is described utilizing a partly graphical approach.     

Nonuniform Waveguide High-Pass Filters with Extremely Steep Cutoff

The design of a tapered waveguide high-pass filter with very steep cutoff characteristics near the cutoff frequency and very low reflections for frequencies beyond the cutoff is studied on the basis of nonuniform or inhomogeneous transmission line theory. The complex input reflection coefficient due to the presence of a section of nonuniform waveguide is obtained through a new approach by formulating the problem in terms of a pair of coupled differential equations of forward wave and reflected wave with varying propagation constants and nonuniform coupling coefficients. The solution of the reflection coefficient appears in the form of an infinite series of integrals and can be reduced, for the case of very gentle tapering to the simple form of fourier integral previously obtained by others. The general solution thus obtained is valid even if 1) the tapering along the waveguide is not gradual, and 2) the tapered section is terminated in an arbitrary impedance. It is shown that among many illustrated simple trial functions of impedance variation along the taper, the exponential function raised to cosine square yields reflection characteristics with the steepest rise near the cutoff and the lowest reflections for all frequencies beyond the cutoff. The steep rise near cutoff frequency is phenomenal, since, for example, at the nominal cutoff of 55 kMc the reflection reduces to about -50 db within 0.18 kMc, i.e., the transition region from the stop band to pass band at -50 db reflection is only about 0.33 per cent of cutoff. The same design procedure for the high-pass filter can be used for waveguide transitions of extremely wide band and very low reflections.     

Electronically Tunable Microwave Bandpass Filters

Combline filters with novel input and output coupling networks which enable broad-band tuning to be achieved with minimum degradation in passband performance are discussed. Explicit design formulas for these filters are presented. Computer analysis of varactor tuned combine bandpass filters including the small signal varactor equivalent circuit is presented enabling filter performance to be easily evaluated. The design and experimental performance of a varactor tuned combline filter, realized in suspended substrate stripline is described. This filter toned from 3.2 GHz to 4.9 GHz exhibited low passband insertion loss and its performance was is close agreement with theoretical expectations.     

基于MEMS技术的微波滤波器研究进展

基于MEMS技术的滤波器是现行RF结构中一个关键的MEMS器件。与传统的采用金属矩形或圆柱波导以及半导体元件制作的滤波器相比,MEMS滤波器具有低损耗、高隔离度、线性好、体积小、易于集成等优点。对利用MEMS技术制作的滤波器做了分类总结,综述了近几年MEMS滤波器的研究进展,包括硅体微加工滤波器、LIGA传输线型滤波器和基于MEMS开关/电容实现的可调滤波器。指出可调滤波器的开发适应微波、毫米波波段的多频段、宽带无线通信系统的迫切需要,具有重要的现实意义。     

Electronically Tunable Microwave Bandstop Filters

The design procedures for varactor tuned bandstop filters are presented. A novel circuit technique for realizing bandstop filters with a symmetrical frequency response is described and explicit design formulas are presented. The physical design and experimental performance of a three cavity varactor tuned bandstop filter tunable around 4 GHz is presented. Experimental results are substantiated with computer analysis which includes the effects of varactor resistance.     

微波滤波器预失真技术

介绍了一种交叉耦合微波滤波器的自适应顸失真理论。该理论在传统预失真理论的基础上,实现了传输极点向如轴的异步移动。每一个极点移动的距离,可以通过最小二乘法优化得到,避免了传统预失真中极点同步移动的盲目性。虽然传统预失真技术可以使滤波器通带内损耗变化及群时延变得较为平坦,但这是以增大插入损耗、回波损耗为代价的。采用自适应预失真技术,滤波器的电参数将得到较好的折衷。同时,通过选择合适的反射零点,可以综合出物理结构对称或者同步调谐滤波器对应的耦合矩阵。     

Circularly Polarized Microwave Cavity Filters

A new group of circularly polarized microwave cavity filters is described. With a single circularly polarized cavity, a reflectionless filter is achieved that couples nearly 100 per cent of the energy from the main waveguide at the cavity resonant frequency. Two degenerate cavity modes may be excited, to produce a circularly polarized field, by coupling to the transverse and longitudinal waveguide magnetic fields or to the transverse electric and magnetic waveguide fields. A theoretical analysis is presented as well as experimental results. The loss between the band-pass terminals of the reflectionless circularly polarized filter is identical with the loss in a conventional reflectiontype band-pass filter with the same bandwidth and cavity-wall losses. The null at resonance between the band-elimination terminals of the reflectionless circularly polarized filter is limited only by the asymmetries of the cavity and not by the cavity-wall losses. Design equations and curves are given for eight of the lower order, circularly cylindrical, degenerate cavity modes that are coupled to a rectangular waveguide at the point of circularly polarized waveguide magnetic fields.     

Compact, Wideband Bandstop Filters With Sharp Rejection Characteristic

A compact unit of parallel coupled transmission line is presented for design of a compact, sharp-rejection, wideband bandstop filter (BSF). The rejection depth and bandwidth can be easily controlled by the coupled-line parameters. A lossless transmission line model is used for filter analysis. Design equations and graphs are presented in order to facilitate the design procedure. Theoretical prediction is verified in microstrip line by fabricating a prototype single section coupled-line BSF.     

Input Manifolds for Microwave Channelizing Filters

The following sections describe two techniques for interconnecting the input ports of microwave filters suitable for sorting signals into large numbers of contiguous frequency bands. Both techniques are capable of extension to very large numbers of channels, and both are believed to be new and useful. Both have been tested at microwave frequencies and give results in reasonable agreement with those predicted. One technique uses the impedance transforming property of a uniform transmission Iine to achieve the equivalent of the directional filter. It has proved especially valuable in the design of practical filters providing large numbers of channels in several microwave frequency bands. The other is based upon a width-tapered waveguide, which serves as a space-distributed high-pass filter.     

Microwave Filters to Combat RF Interference

Microwave filters are useful to combat radio-frequency interference. A large variety of filter structures and design techniques are now available to the designer. The purpose of this paper is to review these filter types, both for high-power and low-power applications.     

Exact Design of Band-Stop Microwave Filters

An exact method for the design of band-stop filters which adapts synthesis techniques due to Ozaki and Ishii is discussed. This method places no theoretical limit on the width of the stop-band, although, for practical reasons, different (but equivalent) circuit configurations are used for stop-bands of different widths.These configurations include a form having open-circuited shunt stubs separated by lengths of line; a second form using resonators which are separate from the main line but parallel to it, so that coupling takes place by way of fringing fields; and a third form in which the resonators are attached directly to the main line, but are folded parallel to it so that coupling is both by direct connection and by fringing fields. Easy to use formulas are given for the exact design of band-stop filters from low-pass prototype filters and equations are given for converting from one form of filter structure to any of the other equivalent forms. The experimental results of trial designs are presented.     

Microwave Filters, An Advancing Art

From the beginnings of microwave engineering until today, filter design has been a persistent and fruitful field for investigation. As this issue of the Transactions shows, interest in microwave filters remains acute, and probably is greater now than ever before.     

Modeling Long-Pass Filters Based on Fundamental-Mode Cutoff in Photonic Crystal Fibers

Index-guiding fibers presenting a Ge-doped photonic-crystal cladding and a pure-silica core are numerically studied for the development of efficient optical long-pass filters. They present a fundamental-mode cutoff (FMC) toward short wavelengths that leads to a high extinction ratio, sharp band edge, and bend-tunable transmission profile. The simple design poses no experimental challenge for fabrication and the relatively large obtained modal area makes these waveguides fully compatible with standard telecommunication fibers. The proposed long-pass filter compares favorably with standard, bulk, technology, and with a previously demonstrated photonic crystal fiber presenting a FMC.     

Synthesis of a Class of Microwave Filters

This paper deals with the development of a new model for a class of microwave filters. With this model one can reproduce and systematize from a general viewpoint results now in the literature. Its most prominent feature, however, is that the use of the model permits the development of a synthesis procedure for the wide-band filter. From this single model a wide variety of structural realizations are readily obtained. Designs employing this model, and the appropriate synthesis, show significant improvement of desired characteristics over conventional designs. The general multiple quarter wave matching transformer problem is also discussed.     

Design of Microwave Filters by Sine-Plane Approach

Using a new sine-plane approach, an easy-to-use design procedure for microwave filters is developed. The design formulas are very simple and are valid for filters of wide bandwidths. Furthermore, the new design offers many advantages over other presently available designs.     

Ferrite-Loaded, Circularly Polarized Microwave Cavity Filters

CircuIarly polarized cavities have made possible a group of compact, high-Q, microwave waveguide filters having useful directional properties. When these cavity filters are ferrite loaded, frequency sensitive circulators result and magnetic tuning becomes possible. This paper presents several new three- and four-port ferrite-loaded filters, some with 3-db waveguide couplers, which can be used as tunable band-pass filters, tunable band-rejection filters, or as passive, selective duplexers. As duplexers, they can be operated at a fixed frequency or can be magnetically tuned over a one to five per cent frequency range at X band depending upon the allowable loss. Experimental loss, bandwidth, isolation, and tuning data are presented. Temperature stability and power handling capacity are also discussed.