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Novel microstrip coupled-line bandpass filters with extended rejection band are proposed using the transmission zeros inherently associated with the shortened coupled sections to suppress the odd spurious harmonics at (2n+1)fo (n=1,2,...), where fo is the passband center frequency. The proposed filters feature compact size and low insertion loss, and have two cross- coupled-induced transmission zeros near the passband edges for improving the filter selectivity. To demonstrate this novel technique of shortening coupled sections, the second-order filter with the stopband extended up to 4.6 fo is implemented and carefully examined. To further improve the filter response, the fourth-order filter with the rejection extended up to 7.4 fo and better than 30 dB is also implemented.
Microwave Theory and Techniques, IEEE Transactions on (Volume:54 , Issue: 2 )
Date of Publication: Feb. 2006