DESIGN OF COMPACT LOWPASS FILTER WITH ULTRA-WIDE STOPBAND USING THIN SLOTS

Abstract

This paper presents a compact microstrip stepped- impedance lowpass fllter with ultra-wide stopband by using back-to- back C-shaped and triple C-shaped thin slots. The properties of several thin slots in the ground plane have been investigated in this paper. With the full-wave simulation results and a simplifled equivalent model, the total equivalent inductance of the thin slots can be extracted at cutofi frequency for lowpass fllter design purpose. On the other hand, the thin slots work as bandstop fllter at the stopband of the lowpass fllter for a better stopband rejection. The proposed lowpass fllter with a cutofi frequency of 2GHz shows a wide stopband with an over 25-dB attenuation up to 17.5GHz. From dc to 2GHz, the insertion loss is less than 0.3dB and the return loss is greater than 20dB. In comparison to the conventional stepped-impedance lowpass fllter with the same passband performance, the proposed lowpass fllter shows not only a 24.3% size reduction but also a better stopband rejection.