Microstrip Filters With Enhanced Stopband Based on Lumped Bisected Pi-Sections With Parasitics

A procedure for improving the stopband response of planar bandpass filters is presented in this letter. The technique is based on the introduction of transmission zeros by using lumped-element bisected-pi sections at the filter input/output. As an example, a 3-pole 10% FBW bandpass filter with Chebyshev response centered at 1 GHz, based on strongly loaded combline microstrip resonators has been designed, manufactured and measured. The proposed solution can be used on any planar topology in order to improve the stopband performance with a negligible additional footprint.