A lowpass filter implementation with an ultra sharp transition band and extended stopband bandwidth based on even-mode and odd-mode analysis of lumped circuits

Abstract

In this paper, a microstrip lowpass filter employing a T-shaped resonator and high-impedance curved bend transmission lines loaded by rectangular patches operating at 2.4 GHz is presented. To improve the transition band slope, the above-mentioned cells have been coupled. Next, four attenuators composed of L-shaped and radial patches are placed symmetrically at both sides of the coupled resonators to broaden the stopband with an acceptable suppression level. By exploiting the even-mode and odd-mode analysis of the equivalent lumped circuit of each resonator and their combination, the relations of the transmission coefficient, reflection coefficient and the transmission zeros of them have been derived, independently. Studying these equations can clarify the roll of each lumped element and consequently its corresponding microstrip structure in the scattering parameters of the utilized cells. Employing the main resonators and the introduced suppressors results in a very steep roll-off rate (860.46 dB/GHz) and an acceptable relative stopband bandwidth (1.73) with the suppression factor of 2.53. From DC to 2.389 GHz, the insertion loss and return loss are better than 0.178 dB and 16.95 dB, respectively. Furthermore, the occupation area of this lowpass filter is 10.8 mm × 14.65 mm. Eventually, the designed filter has a high figure of merit equal to 110308.34.