Design of Microstrip Lowpass Filter with Sharp Roll-off using Elliptical and Radial Resonators

A new microstrip lowpass filter with sharp roll-off and high suppression level in stop-band is presented. To achieve the lowpass filter with high performance, coupled T-shaped, elliptical and radial resonators are adopted. New formulas for calculating equivalent circuit of elliptical and radial resonators are derived based on T-shaped resonators. Transfer functions of the resonators are calculated using the equivalent circuits. The designed filter with 3 dB cut-off frequency at 1.67 GHz has been fabricated and measured. Results demonstrate that a relative stop-band bandwidth (RSB) of 157 % (referred to a suppression degree of 30 dB) is obtained while achieving a sharp roll-off about 255.2 dB/GHz. The features of lowpass filter exhibit that the figure of merit is about 45372. Also good agreements are achieved between measured and simulated S-parameters of the fabricated and designed filter.