Compact high-rejection wideband bandpass filter using asymmetrical resonators

In this paper, a compact wideband bandpass filter using asymmetrical resonators is proposed. A lossless transmission-line model calculation is used to determine the resonant responses of the asymmetrical resonator, which consists of a uniform resonator tapped by an open stub. By adjusting the stub length and the tapping position, one can control the resonant frequencies for wideband bandpass filter design purpose. With the two inherent transmission zeros at both sides of passband, a sharp rejection response is achieved to improve the stopband performance. For the proposed wideband bandpass filter, from 2 GHz to 4.8 GHz (90%), the measured return loss is greater than 11 dB, and the measured insertion loss is less than 2 dB. The maximum group delay variation is 0.7 ns. Furthermore, there are two transmission zeros at 1.8 and 5.15 GHz for a sharp cutoff response. The effective area measures only 0.34λ_g ×0.08λ_g at center frequency.