Miniaturized mixed-cross coupling bandpass filter with source–load coupling

Abstract

A miniaturized trisection bandpass filter (BPF) with improved stopband characteristics is presented. The proposed BPF consists of three stepped impedance resonators (SIRs), where the nonadjacent SIRs share a common shorted stub, and the other SIR is embedded inside the nonadjacent SIRs. To improve frequency selectivity, mixed-cross (M-C) coupling and source–load (S–L) coupling were introduced in the nonadjacent resonators and source/load ports, respectively. Owing to the M-C and S–L couplings, multiple transmission zeros (TZs) can be employed for high skirt selectivity and upper wide stopband. A new BPF centered at 2.45 GHz with 17.6% fractional bandwidth has been designed and fabricated to verify the validity of the proposed method. The measurement result shows four finite TZs in the stopband, located at 2.09, 2.98, 4.64, and 12.47 GHz, respectively. The circuit only occupies 6.70 × 8.60 mm2