Compact Low Pass Filter Using Sharp Rolloff UltraWide Stopband Tshaped Resonator

In this paper, an ultrawide stopband microstrip low pass filter with sharp rolloff and compact size is designed and fabricated. To provide very small dimensions, Tshaped and stepped impedance have been used. The cut off frequency at 3 dB is 3.1 GHz. In the proposed structure, the insertion loss is lower than 0.12 dB and the return loss is greater than 15 dB. In order to achieve a 20dB attenuation level in the stopband, a modified Lshaped structure is utilized.  This filter has an ultrawide stopband that is expanded from 3.37 GHz to 37.5 GHz. Also the suppression level is greater than 20dB. The fabricated LPF has a size about 16.6 × 13.5 mm2 which is equal to 0.28ƛg × 0.22ƛg, where ƛg is the guided wavelength at 3.1 GHz, which is very small. The proposed filter is designed, simulated, optimized, fabricated and measured and we can see a good agreement between the simulations and measured results.