First spurious suppression in microstrip parallel-coupled line filters using compensated capacitors and stubs

In this paper a suitable method is presented which allows it to effectively remove the first spurious pass-bands. The proposed filter configuration is constructed by adding three appropriate capacitors between each coupled line and three attached open-circuit stubs at each line of the conventional coupled-line filter. The capacitors and stubs are placed at the middle and each end of the coupled-lines. Placing the capacitors lead to equal modal phase velocities and shorten coupled-line lengths. In order to demonstrate the characteristics of the proposed filter, a third order band-pass filter centered at the operating frequency 1 GHz with 20% fractional bandwidth is designed and simulated. The simulated results show for such a filter prototype, the first spurious response is effectively suppressed.