Third-order tunable filter with CFBW using two quarter-wavelength resonators

A tunable third-order bandpass filter (BPF) with constant fractional bandwidth (CFBW) is presented. Only two quarter-wavelength resonators have been used in comb-line fashion to design the third-order BPF. The physical dimensions of the resonators have been decided to satisfy the filter specifications by synthesizing the coupling matrix. Using modal voltage waveform analysis, locations of the varactors to be connected are identified to tune the first three modes of the resonators to form the BPF. The varactors connected symmetrically between the two quarter-wavelength resonators can only control the odd modes, while the varactors loaded at the resonator ends can control the odd and even modes simultaneously. Moreover, the capacitance ranges of the varactors have been chosen to achieve the desired tuning of the filter. The center frequency of the filter is tunable from 640 to 840 MHz with CFBW of approximately 32%.