A compact tri-band microstrip filter with independently tunable passbands and high selectivity

Abstract

This paper presents a compact tunable tri-band filter with independently controlled passbands and high selectivity. The filter is comprised of three pairs of varactor-loaded λ/4 short-circuited resonators. Each passband of the filter can be independently tuned by varying the voltage applied to the corresponding resonators. In addition, a source-load coupling structure is employed to produce transmission zeros (TZs) achieving high selectivity. A prototype tunable tri-band microstrip filter is implemented. The experiment results show that the three passbands have tuning ranges of 0.81–0.99 GHz, 1.34–1.54 GHz, and 1.8–2.03 GHz, respectively.