Design of compact bandpass filter with enhanced stopband characteristics

In this paper, a compact third-order bandpass filter with enhanced stopband characteristics is proposed. The filter consists of three stepped impedance resonators, where one is embedded into the other two resonators for compact size. Multiple transmission zeros (TZs) can be realized near the passband by introducing frequency-dependent coupling between the nonadjacent resonators for enhanced frequency selectivity. Besides, defected ground structure is employed to suppress the harmonics over a broad bandwidth. A new filter centered at 1.51GHz has been analyzed, simulated, and fabricated to verify the validity of the proposed method. The measured results show that there are three finite TZs near the passband, which are located at 1.1GHz with 62.1dB rejection, 1.76GHz with 34.2dB rejection, and 2.48GHz with 37.2dB rejection, respectively. And, the spurious frequencies of the upper stopband are suppressed below 20dB from 1.7 to 25GHz.