Advanced microwave miniature and lossy tunable filters for wireless communication applications
Microwave filters play very important roles in many RF/microwave applications, which are employed to separate or combine different frequencies. Emerging applications, such as wireless communications, challenge the design of microwave filters with more functionalities and higher performance, such as reconfigurable or tunable, compact size, light weight and lower cost. In order to meet the increasing challenge requirements, the objective of this dissertation is to develop new multilayer miniaturized filters, compact lossy microstrip filters, and reconfigurable lossy filters. To achieve this, this dissertation is divided into three main parts. The first part focuses on the design of novel miniature bandpass filter with improved performance. In this aspect, a novel microstrip bandpass filter using slow-wave open-loop resonators is presented, which concentrates on the stopband rejection performance to suppress the harmonic standing wave rather than the passband performance by using multilayer LCP technology. The multilayer open-loop slow-wave resonator has not only very compact size, but also exhibits an excellent wider upper stopband resulting from the dispersion property. Based on this type of resonator, a five-pole bandpass filter has been proposed, which has good stopband rejection and high selectivity as well as compact size and light weight. The second part is devoted to the design of compact lossy filters with improved performance characteristics. To achieve this, lossy synthesis and extracted-pole technology are combined together to design microstrip filters with flat passband and high selectivity. Two six-pole filters has been analysed from the theoretical circuit model to EM simulations, fabricated to demonstrate the response performance in narrowband and wideband respectively. The third part concentrates on the designs of novel varactor-tuned microstrip lossy bandpass filters. Firstly, state-of-the-art literature review is given to have a general view of reconfigurable bandpass filter with different tuning centre frequency and bandwidth characteristics. Then, three types of tunable microstrip bandpass filters with resistor loading under symmetric tuning method are presented to introduce additional loss into the passband to make it flat over the entire tuning range. The first filter is designed to control the bandwidth and selectivity. The second one is designed to control the bandwidth at fixed centre frequency, while the third filter is extended from the first one to combine resistor loading and cross coupling. Finally, microstrip tunable bandpass lossy filters with extracted-pole technology are proposed. Three six-pole filters of this type have been analysed and fabricated. Due to the asymmetric tuning method, the number of tuning components and dc bias schemes are increased, which is a kind of tradeoff with performance. For all the presented filters, theoretical analyses, implementations and measurements have been given. All of the results achieved in this thesis make the proposed filters attractive for their applications in modern wireless communication systems.