Wide-stopband miniaturized bandpass filter based on slow-wave structure and slot resonator with quarter mode substrate integrated waveguide cavity

Abstract

In this paper, a miniaturized bandpass filter (BPF) based on a quarter-mode substrate integrated waveguide (QMSIW) cavity with slow-wave structure and slot resonator is reported. The filter mainly consists of an internal capacitive-coupled QMSIW cavity, two source-loaded coupled microstrips and defect-ground structures. Here, the slot resonator is etched on the top layer of the QMSIW cavity, while the slow-wave structure is etched on the bottom metal surface of the QMSIW cavity. The combination of the two structures can effectively enhance the product of the equivalent capacitance and inductance and eventually reduce the resonant frequency of the QMSIW cavity. Moreover, a lumped capacitor is placed in the middle of the QMSIW cavity after etching the slot resonator to enhance impedance matching in TE101 mode. To verify its effectiveness, a bandpass filter is designed and implemented. The simulation results show that its center frequency (fc) is 0.37 GHz and the insertion loss (IL) of passband is 0.34 dB. In addition, the passband has a fractional bandwidth (FBW) of 55.2% and an out-of-band rejection of 16 dB up to 4.7 GHz, corresponding to 12.7fc. Compared with the conventional QMSIW filter, its resonant frequency is reduced from 2.3 GHz to 0.37 GHz, and the size is reduced by more than 83%. Therefore, the proposed miniaturized bandpass filter is suitable for the broad-stopband applications in practical microwave systems.