Varactor-tuned microstrip bandpass filters with different passband characteristics

Two novel types of tunable bandpass filters are introduced in this study. The first type is composed of varactor-loaded parallel-coupled lines and short-circuited stubs, which allows for the lower passband edge and two transmission zeros around the lower passband skirt to be reconfigured separately while the corresponding upper ones maintain fixed. The second type of tunable bandpass filter is built by cascading the first type of tunable bandpass and another tunable lowpass filter, which results in a great flexibility to tune both centre frequency and passband bandwidth. The designed prototype tunable filters are implemented using liquid crystal polymer (LCP) bonded multilayer printed circuit board (PCB) technology. The measured results demonstrate very promising performance in terms of wide tunability and high selectivity.