Open-loop tunable resonators and filters with constant bandwidth

This study presents a compact novel design for achieving constant bandwidth tunable filters with wide tuning range. A method for linearising the Q-factor against frequency is proposed. A modified coupling structure is introduced to compensate for variations in filtering characteristic across the theoretical 50% tuning range of the filter. Filter design equations are provided for designing Butterworth or Chebyshev filters. An experimental second-order filter is demonstrated with 22.5% pass-band shift with only 3% bandwidth variation and insertion loss better than 3%dB. Furthermore, a third-order tunable bandpass filter with tuning range of 45% and less than 5% bandwidth variation is illustrated through simulation and modelling to show feasibility with higher filter orders. There is very good agreement between the theoretical and experimental results.