Miniaturized Quint-Band Bandpass Filter Based on Multi-Mode Resonator and Resonators With Mixed Electric and Magnetic Coupling

Based on multi-mode resonator (MMR) and λ/4 resonators, a miniaturized microstrip quint-band bandpass filter (BPF) with mixed electric and magnetic coupling (MEMC) has been presented. The proposed MMR is employed to generate the 1st, 2nd, and 5th passbands, while the 3rd and 4th ones of close band proximity are achieved by using four coupled λ/4 uniform impedance resonators (UIRs). Due to MEMC cancelling effect, virtual grounds in feed lines, and source-load cross coupling, multiple transmission zeros (TZs) can be yielded to obtain sharp passband roll-off skirts. To verify the validity of the design approach, a quint-band BPF centering at 2.1, 3.0, 4.0, 4.7, and 7.2 GHz (denoted as f1, f2, f3, f4, and f5) with respective 3 dB fractional bandwidths of 13.7%, 5.6%, 10.5%, 5.1%, and 2.9% has been designed and fabricated, whose measured results show good consistence with the predicted frequency responses.