High‐performance two‐pole filter using self‐coupled resonators with frequency‐variant internal and external couplings

Abstract In this article, a two‐pole filter with very high selectivity and second harmonic suppression is presented. The design is based on a novel configuration using self‐coupled half‐wave resonators with external frequency‐variant coupling (EFVC) structures. The self‐coupling allows placing two transmission zeros, produced by frequency‐variant interresonator coupling and the tapped feeds, close to the passband edges. The EFVC structure enables arbitrary positioning of these zeros without affecting in‐band performance. Thus, very high‐selectivity passbands can be realized. Additionally, the external coupling structures provide more zeros which are used to suppress spurious harmonics. The fabricated prototype second‐order filter at 2.35 GHz center frequency with 5.6% bandwidth shows a selectivity factor (ratio of 3‐ to 20‐dB bandwidths) of 0.535 and stopband up to 2.8 times the center frequency with minimum 20‐dB rejection.