Background and Objectives: In this paper, an ultra-wide stopband microstrip low pass filter (LPF) with sharp roll-off and compact size is designed and fabricated. Also, proposed filter is designed, simulated, optimized, fabricated and measured and we can see a good agreement between the simulations and measurement results.
Methods: T-shaped and stepped impedance have been used, because these structures have a very small dimensions. This method provides a suitable cut off frequency.
Results: The cut off frequency at -3dB is 3.1GHz. In the proposed structure, the insertion loss is lower than 0.12dB and the return loss is greater than 15dB. 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 3.37GHz to 37.5GHz. 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 3.1GHz, which is very small.
Conclusion: A microstrip LPF with 3.1GHz cut-off frequency consisting of T-shaped and stepped impedance resonators has been proposed, simulated and fabricated. The transition band (-3 to -40dB) is 0.38GHz, which is a very sharp response. The stopband of the proposed filter is from 3.37 to 37.5 GHz which is equal to 12.1fc. There was a significant agreement between the simulation and the measured results. With all these features, these filters are widely used in wireless communication systems.
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