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

Abstract

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 $(f_\mathrm{even})$ 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 $(f_\mathrm{odd})$ 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 $f_\mathrm{odd}$ and its odd-order harmonics, which cannot be excited accordingly. Thus, the wideband tuning range of the passband $(f_\mathrm{even})$ 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.