Tunable narrowband bandpass filter using higher order microstrip ring resonator

A type of tunable narrow-bandpass filter is proposed in this paper. Fourth-order microstrip ring resonator with magnetic coupling is employed and loaded with varactors for achieving agility in the filter response. The simulation result showed that the center frequency of the proposed filter can be adjusted from 698 to 886 MHz and tuning range of 188 MHz, 1-dB constant absolute bandwidth of 9 MHz and insertion loss less than 10 dB are obtained. Additionally, the out-of-band rejection is improved by two transmission zeros which located on lower-side and higher-side of the pass-band. The measured performance of the fabricated filter shows good agreement with the proposed theory.