Novel Microstrip Coupled-Line Bandpass Filters With Shortened Coupled Sections for Stopband Extension

摘要

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) f_(o) (n velence 1, 2,...), where f_(o) is the passband center frequency. The proposed filters feature compact size and low insertion loss, and have two crosscoupled-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 4.6f_(o) is implemented and carefully examined. To further improve the filter response, the fourth-order filter with the rejection extended up to 7.4f_(o) and better than 30 dB is also implemented.