Novel Frequency-Agile Bandpass Filter With Wide Tuning Range and Spurious Suppression

A new frequency-agile scheme for the loaded stepped-impedance resonator (SIR) is introduced, which can be employed for designing a wideband tunable bandpass filter (BPF). Two identical capacitors are added to the two open ends, and a capacitor is added to the center of the SIR. By properly controlling their respective values, the even-mode resonant frequency (feven) can be bidirectionally tuned, resulting in a wideband tuning range (approximately double those of the traditional tunable resonators). However, in such frequency range, the odd-mode resonant frequency (fodd) functions as a spurious resonance would appear. To overcome this, the feed line is attached to the center of the SIR where it is always short-circuited for fodd and its odd-order harmonics, which cannot be excited accordingly. Thus, the wideband tuning range of the passband (feven) in the proposed BPF is validated to be meaningful, and the selectivity of the passband is improved due to the suppression of odd-mode spurious near the passband. For demonstration, a wideband tunable BPF is designed, and the simulated and measured results are presented, showing good agreement. The results showcase that the tuning range of passband reaches 84.4%, i.e., from 0.77 to 1.42 GHz with insertion loss from 3.1 to 1 dB.