Inline Cavity Stepped Window Bandpass Filter With Two Transmission Zeros

This article proposed the design of bandpass filter (BPF) based on stepped window resonators (SWRs), which allows inline resonators to produce transmission zero (TZ). Compared with traditional cavity resonator using iris-coupled structure, the proposed SWR consists of two metal cavities, the larger cavity is regarded as a bandstop resonator to provide TZ near the passband. Therefore, each SWR pair can generate a transmission pole (TP) and a TZ. Two cavities are directly cascaded without any coupling iris, which increases the interface window between these two metal cavities and reduces the fabrication tolerance. By properly dimensioning the bandstop cavity, TZs can be generated in the lower and upper stopbands to improve the frequency-selective performance. An equivalent circuit model of SWR is proposed to explain the generation of TZ and synthesize the BPF. Finally, a fourth-order inline BPF with a fractional bandwidth of 1% at 10 GHz is designed using this SWR approach. The synthesis response and simulation results match well. The proposed BPF is fabricated and measured, and the good agreement between the measured and simulated results verifies the proposed design methodology.