Tunable Balanced Bandpass Filter With Wide Tuning Range of Center Frequency and Bandwidth Using Compact Coupled-Line Resonator

A tunable balanced bandpass filter using a short coupled-line resonator is proposed. The design aims at achieving a wide tuning range in the differential-mode for both of the center frequency and bandwidth with high common-mode suppression. The proposed filter uses a short section (about one-eighth of one wavelength) of parallel-coupled lines connecting the differential ports of the device and five pairs of varactors. A theoretical analysis using the even-odd mode theory is presented to verify the tunability of the design and calculate the initial values of its design parameters. For validation, a prototype is designed, fabricated and tested. The measured results indicate that the differential-mode tuning range for the center frequency and 3 dB- bandwidth extend from 0.58 to 1.22 GHz, and from 65 to 180 MHz, respectively, with more than 30 dB of common-mode suppression in the operating differential-mode passband. Moreover, the proposed device has a compact size (with respect to the guided wavelength) of only
$0.12\lambda_{G}\times 0.08\lambda_{G}$
.