Tunability and performance enhancement for planar microwave filters
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Radio-frequency (RF) spectrum is exploited as a valuable resource for wireless applications such as mobile and satellite communications. As a result, communication systems including satellite communication and emerging 5G are trending to have frequency-agility to adapt to highly complex RF environments. However, due to the nature of materials and components, electrically-tunable planar filters, which play essential roles in frequency agile RF systems, have their disadvantages of low-order, high-loss, and poor selectivity. This has limited the overall performances of the frequency agile RF systems. In the light of this scenario, the objective of this thesis is to develop efficient performance-enhancement techniques, including the lossy technique, and the active technique, into the high-selective tunable planar filters to boost the performances of tunable RF systems. First of all, an electrically reconfigurable microstrip dual-mode filter is demonstrated with nonuniform-quality-factor lossy technique. The 4-pole bandpass filter exhibits a continuously bandwidth tuning and centre frequency tuning capability. By making use of the doubly tuned resonant property of the dual-mode microstrip open-loop resonator, passband flatness can be improved by simply loading resistors on the even-odd mode symmetrical plane of resonators. Moreover, two intrinsic transmission zeros are in upper and lower stopbands enhancing the filter selectivity. The coupling matrix synthesis is introduced to describe the nonuniform-quality-factor distribution in a filter network. The experiment of this type of four-pole tunable lossy filter has presented a good agreement with the simulation. Then, the thesis reports a novel 5-pole lossy bandpass filter with the bandwidth tunability. In order to improve the filter selectivity, we choose a hybrid filter structure consist of hairpin resonators and dual behaviour resonators to produce two adjustable transmission zeros for high selective responses. A novel lossy technique named centre-loaded resistive cross-coupling is developed to efficiently reduce the insertion-loss variation of the tuned passband. The fabricated filter demonstrates an insertion loss variation of less than 1 dB for all bandwidth states. To compensate the loss within the varactor-tuned narrowband filter, a tunable 2-pole active filter is presented with a constant absolute bandwidth. The negative resistance generated from active circuits successfully cancels the loss within the varactor-loaded resonators resulting in high quality-factor resonator filter responses. With the transistor small-signal model, the value of the negative resistance of the active circuit can be predicted by network analysis. Experiments were carried out to validate the design.