A Miniaturized Low Pass Filter with Extended Stopband and High Passband Selectivity

In this work, an ultra-wide stopband low pass filter (LPF) with high selectivity is proposed using coupled stepped impedance resonators (SIRs), open shunt stubs and circular slots in the ground plane. The proposed LPF has been modeled using a lumped equivalent circuit which is extracted from the EM model. The design has been validated through the simulation and experimental results. The fabricated prototype has a 3-dB cutoff frequency (fc) of 2.44 GHz and an ultra-wide stopband extended up to 20.5 GHz (8.4 fc) with an attenuation level > 20 dB. The transition bandwidth (from 3 dB to 20 dB) is 0.09 GHz and the roll-off rate is 225 dB / GHz (reference to 30 dB). The passband insertion loss is 0.35 dB at 1.22 GHz and the normalized circuit size of the filter is 0.045.