Compact Hybrid Bandpass Filter Using SIW and CSRRs with Wide Stopband Rejection

A novel four-stage planar bandpass filter is proposed using substrate integrated waveguide (SIW) cavities with complementary spilt ring resonators (CSRR) etched on its top metal surface. Since the frequency of the CSRR's spurious higher order mode is different from that of the SIW cavities, and by applying an appropriate feeding position, wide and high stopband rejection is obtained. Transmission zeros (TZs) are generated through cross coupling. To validate the proposed method, a four-pole hybrid filter with two SIW cavities and two CSRR resonances is implemented on a 20-mil RT/Duriod 5880 substrate. The measured and simulated S-parameters of the filter are presented showing good agreement. Benefitted from the high-Q SIW cavities and compact CSRR structures, good filtering performance including low-loss, miniaturized size, and wide stopband rejection (more than 37 dB) is demonstrated.