Parallel-coupled stub-loaded resonators compact size tunable filter

A novel topology of tunable compact band-pass filters based on stub-loaded parallel coupled resonators is demonstrated. These resonators behave like short-circuited quarter wavelength resonators, which explains the compactness of such a structure of filter. By loading the stubs by a tunable capacitance, a narrow-band tunable filter is obtained, with relative pass-band widths of 3.5 % with only two resonators. For a proof-of concept, three two-pole filters with different loading capacitors have been realized. The tunability reaches 6 %, i.e. twice the relative bandwidth. Insertion loss between 2.2 dB and 2.8 dB is achieved with a return loss better than -12 dB. The bandwidth is quite constant for all the working frequencies.