Design of dual-mode tunable filter with constant fractional bandwidth using varactors

This paper presents a 2nd-order Chebyshev bandpass filter with tunable constant fractional bandwidth. The filter is designed using a short-stub loaded dual-mode resonator, which readily achieves a constant bandwidth while tuning. Two sets of varactors are used for tuning the center frequency and the return loss level. Within the tunable range of the varactors, the frequency tuning range of the proposed filter is from 1.7GHz to 2.1GHz with a 3.5% constant fractional bandwidth. Additionally, 20 dB return loss and insertion loss below 1.64dB can be achieved over the entire tuning range.