Compact Low Pass Filter Using Sharp Roll-off Ultra-Wide Stopband T-shaped Resonator

In this paper, an ultra-wide stopband microstrip low pass filter with sharp roll-off and compact size is designed and fabricated. To provide very small dimensions, T-shaped 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 L-shaped structure is utilized.  This filter has an ultra-wide 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.

Keywords:

Microstrip filter , Low pass filter , Sharp roll-off , Ultra wide stopband , T-shaped structure