High-Selectivity and Linear-Phase Dual-Band Bandpass Filter With Large Frequency Ratio

In this paper, an aperture-coupling and cross-coupled mechanism are put forward to exploit two bandpass unit cells for a dual-band bandpass filter (BPF) with a large frequency ratio. It includes a cascaded step-impedance resonator (SIR) BPF and a low-pass filter (LPF) on the top layer and a sixth-order SIW BPF with cross-coupling on the bottom layer. As for the SIW BPF of the bottom layer working at the millimeter-wave (MMW) frequency band, a specific-length microstrip line is utilized as an inverter to connect two non-adjacent resonate cavities, which can introduce the inverse cross-coupling and improve the frequency selectivity of the filter. On the top layer, a BPF operating at the microwave band was designed using a cascaded form of SIR BPF and LPF. Based on the proposed design theory, a dual-band BPF with a large frequency ratio up to 6.83 was carefully designed and simulated. The desired results can be obtained and the concept of the proposed structure can also be verified. Thus, both two frequency bands exhibit the characteristics of flat group delay.