High‐selectivity tri‐band bandpass filter with ultra‐wide stopband

A tri‐band bandpass filter (BPF) with an ultra‐wide stopband is proposed. Based on tri‐mode unequal‐length shunted‐line stepped impedance resonators (SIRs), some extra resonator sections are added in unequal‐length shunted‐line SIRs to increase the degrees of freedom in extracting the coupling coefficients of the tri‐band BPF. Unequal‐length shunted‐line SIRs can provide an efficient way to obtain a tri‐band BPF with a small frequency ratio between the first two centre frequencies. Moreover, two transmission zeros can be achieved near every passband by employing cross‐coupling, resulting in high passband selectivity, excellent band‐to‐band isolation and an ultra‐wide stopband. As an example, a tri‐band BPF is demonstrated. Simulated results of the filter agree well with the measured ones.