COMPACT MICROSTRIP LOWPASS FILTER WITH SHARP ROLL-OFF AND WIDE STOPBAND USING SEMICIRCLE ENDED STUB RESONATOR

In this paper, a semicircle ended stub resonator, cascaded to a modifled radial patch to design a compact lowpass fllter with sharp roll-ofi and wide stopband is proposed. This fllter has 3dB cutofi frequency at 1.54GHz. The transition band is only 0.26GHz from 1.54GHz to 1.8GHz with corresponding attenuation levels of i3dB and i20dB respectively. Maximum insertion loss is 0.1dB in the passband, and the stopband bandwidth with the attenuation level better than i20dB is extended from 1.8GHz up to 13.93GHz. So, a wide stopband is achieved. The proposed fllter is designed, fabricated and measured, where there is a good agreement between the simulation and measurement results. The results show that a roll-ofi rate of 65.4dB/GHz together with a relative stopband bandwidth of 154% with the suppression level of i20dB is obtained while achieving a high flgure of merit (FOM) of 23509.