A study of tunable filters technology in RF/microwave engineering

Abstract

Due to increasing demand for wireless communication systems, and because of the stringent requirements of the congested RF-frequency spectrum, reconfigurable/tunable filters have advanced significantly in recent years. Tunable filters can be tuned to different frequency bands constituted a qualitative shift in the field of civil and military communications because of their great potential to minimise the size, complexity, power consumption and cost of traditional filter banks. At the same time, high performance is becoming increasingly important to meet the modern communication systems’ specifications. Against this background, this dissertation provides a study of tunable filters technology in RF/microwave Engineering. In order to accomplish this study, several tunable filters using different tuning approaches have been presented in this dissertation. A mechanically tuned lowpass filter is presented achieving a good tuning range over the filter’s passband. The suspended substrate stripline (SSS) topology has been utilized to obtain a high-quality response while the generalized Chebyshev responses has been applied to obtain flexible transmission zeros in terms of controlling their locations. Tuning was achieved by using a fabricated mechanical structure to tune a set of five SSS resonators synchronously. A systematic numerical design of this type of filters has been offered with full equations’ derivations and consequentially manufactured samples are validated experimentally and presented in this work. A novel design of a narrow tunable bandwidth bandpass filter with two transmission zeros has been developed. In this project, two different structures were combined containing the microstrip structure to simplify the integration with other system parts and the SSS structure to obtain high quality response. Moreover, it introduces two transmission zeros at both sides of the filter’s passband without the need of using the conventional cross coupling method. Furthermore, two different tuning approaches have been used, one for tuning the bandwidth and the two transmission zeros while the second one was for fine tuning. An extensive design methodology and a numerical design example for both, the fixed and tunable filters have been presented and consequentially the proposed design has been proven experimentally. A new cascaded bandpass filter is presented offering an outstanding response with relatively small number of cascaded elements. This filter utilizes the characteristics of the Step Impedance resonators (SIR) in terms of their flexibility of controlling the spurious response and the insertion loss by changing the ratio of the filter’s high to low impedance. In addition, it offers high quality responses by using SSS structures. The filter’s design methodology is presented, extensively illustrated with a numerical example and proven experimentally. A tuning feasibility of the cascaded SSS filter has been introduced where the electrical tuning has been used to tune the lower side of the passband while the upper side is mechanically tuned. The proposed tuning approach has been simulated by using an EM full-wave simulation software and presented whereas the manufacturing was unfortunately postponed due to the end of the research time.