Performance Improvement of Microstrip LPF Based on Transfer Function Analysis

A high performance microstrip LPF with adjustable cut-off frequency and outstanding specifications is designed, analyzed, and implemented. The structure comprises a meandered transmission line and modified and improved T-shaped resonators. In order to describe the design theory, the transfer function based on an LC equivalent circuit is calculated to adjust the cut-off frequency and the location of the transmission zeros (TZs). The proposed LPF has a -3-dB cut-off frequency equal to 1.208 GHz and a narrow transition band of 0.188 GHz. The proposed LPF achieves a wide stopband with 14th harmonic suppression, insertion loss less than 0.4 dB, and low VSWR. Finally, the fabricated LPF reaches an extremely high figure-of-merit of 102182, which is considered as a significant achievement.