Design of a Quad-Band Bandpass Filter Using Split-Ring Resonators

In this paper, design, simulation and fabrication of compact quad-band bandpass filters (BPF) using Split Ring Resonators (SRR) are presented. These types of filters are inherently narrow bandwidth. The bandwidths of the passbands of the filter are increased using an interdigital capacitor between two adjacent cells of the filter, and by increasing the substrate thickness. A quad-band BPF with center frequencies of 1. 5, 2. 24, 3. 07 and 4. 43 GHz was designed and simulation results show the bandwidths of these bands are 120, 219, 492 and 280 MHz respectively. In order to adjusting the center frequencies and their bandwidths for practical applications, parametric study and required simulations are presented. The proposed structure has sharp passband selectivity, acceptable insertion loss and return loss in each band, and controllable distance between the bands. Final filters (two structures) are implemented on Rogers 5870 substrate, and simulation and measured results are compared. Second fabricated filter has a compact size of that shows 18%, 69% and 43% size compacting in compare with three samples of quad-band filters of other references.