Compact microstrip lowpass filter with very sharp roll-off and ultra-high figure-of-merit for wireless applications

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This study presents a novel lowpass filter (LPF) with very sharp roll-off and ultra-high figure-of-merit (FOM). The −3 dB cutoff frequency of the proposed LPF is set nearby 1.39 GHz to eliminate the higher frequencies for wireless applications in communication systems. The measurement results indicated that the transition band of the proposed LPF is 0.04 GHz from −3 dB to −20 dB and 0.06 GHz from −3 dB to −40 dB, which shows a very sharp roll-off. Also, an ultra-wide stopband from 1.39 GHz to 20.2 GHz and low insertion loss and high return loss in the passband region are achieved. The proposed LPF has an ultra-high FOM which is probably the highest FOM ever reached in related works until 2014, thus showing its strong sufficiency. The proposed LPF is fabricated and tested, and there is a good agreement between the simulation and measured results.