Spurious passband suppression in microstrip coupled line band pass filters by means of split ring resonators

In this letter, spurious passband suppression in microstrip coupled line band pass filters by means of split ring resonators (SRRs) is demonstrated for the first time. By etching SRRs in the upper substrate side, in close proximity to conductor strip, strong magnetic coupling between line and rings arises at the resonant frequency of SRRs. This inhibits signal propagation in the vicinity of that frequency, allowing the rejection of undesired passbands by properly tuning SRRs. To test this novel technique, we have designed and fabricated two different SSRs-based filters. In one case, the rings have been designed to suppress only the first spurious band, and SRRs have been etched at both sides of the 50-Ω access lines. For the other prototype, SRRs have been etched on the active device region (i.e., surrounding the parallel coupled lines) and have been tuned to eliminate the first and second undesired bands. The measured frequency responses for these devices confirm the efficiency of this technique to suppress frequency parasitics, with rejection levels near 40 dBs, leaving the passband unaltered. Since SRRs are small particles (with sub-wavelength dimensions at the resonant frequency), this approach does not add extra area to the final layouts. Moreover, the conventional design methodology of the filters holds.