Modeling of a compact broad-stopbandlowpass filter using microstrip stepped impedance resonator for microwave application

Abstract

A compact elliptical-function lowpass filter using modified stepped impedance resonator is designed in this thesis. This is done by introducing two triangular coupled structures inside the filter proposed by Kai Chang in 2003. The model is evaluated with MoM simulation with good agreement. The proposed filter vastly improves the performance of the filter. The additional triangular structure suppresses the harmonic by adding poles at harmonic frequency. The filter is modeled using first order transmission line model as an equivalent L-C network. The equivalent circuit proposed here also can give the cutoff frequency, rejection rate, and stopband rejection within ±5% error range. The filter is fabricated on a 0.254mm thick RT/Duroid 5880 substrate with a relative dielectric constant εr=2.2. For single proposed filter, the return loss is better than 17 dB from DC to 1.488 GHz where the insertion loss is less than 0.13 dB. The rejection is greater than -14 dB from 2.0 GHz to 13 GHz. The proposed filter is about 30% smaller than the filter proposed by Kai Chang in 2003 for same cut-off frequency. In this thesis the proposed filter is modeled up to 5GHz lowpass response. By cascading these lowpass filters with different length and width, having different 3dB cut-offs, more rejection in stopband can be obtained to design efficient filter for practical wireless communication systems.