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.