Compact Mixed-Cross Coupled Bandpass Filter with Enhanced Frequency Selectivity

In this paper, a compact three-order mixed-cross coupled bandpass filter (BPF) with enhanced frequency selectivity is proposed. Multiple transmission zeros (TZs) can be obtained near the passband for high frequency selectivity by introducing mixed-cross coupling between the nonadjacent resonators. The frequency-dependent mixed-cross coupling matrix of the proposed filter is presented to explain the occurrence of the TZs caused by mixed-cross coupling. A new BPF centered at 2.7 GHz with 11.5% 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 1.74 GHz with 52.16 dB rejection, 2.53 GHz with 24.67 dB rejection, 3.83 GHz with 47.52 dB rejection, and 7.75 GHz with 54.83 dB rejection, respectively. The circuit only occupies 6.2×7.6 mm.