Compact Band-Pass Filter with Controllable Frequency Characteristics

A compact microstrip tunable bandpass filter (BPF) is proposed and investigated. The filter synthesis and structure are based on parallel microstrip lines that are printed over an RT/ duroid 6006/6010LM substrate, with a permittivity of 10.2 and thickness of 1.27mm. A parallel topology method can be adjusted for the filter tunablility in the center frequency, to allow fully tuned frequency response. In order to tune the targeted pass band frequency, a lumped element capacitor is used. This capacitor was optimally located over the middle strip resonator. The pass band frequency was smoothly shifted downwards from 2.4GHz to 1.8GHz, when the capacitance of the capacitor varies from 0.15pf to 0.9pf. The proposed filter demonstrates in-band insertion loss of around 0.25 dB and return loss better than 20 dB over the whole pass band frequency. This presents the filter as a favourable candidate for numerous smart future applications.