A Compact Step-Impedance Combline Filter With Symmetric Insertion-Loss Response and Wide Stopband Range

A microstrip combline filter using step-impedance resonators with input/output coupling by tapped-transformer is presented. This filter has impressing performance of more symmetric insertion loss response around the passband and wider stopband range, compared with the conventional combline filter. A demonstrated circuit was designed, simulated and measured. The measurement results show that the return loss is greater than 25 dB and the insertion loss less than 1.2-dB over the passband of 2400 to 2500 MHz. On both sides of the passband, there exists one transmission zero on each side, resulting in better skirt factor. The 30-dB stopband suppression level is obtained up to four times the center frequency of the passband