Full Study of the Parallel-Coupled Stub-Loaded Resonator: Synthesis Method in a Narrow Band With an Extended Optimal Rejection Bandwidth

This paper gives a complete theoretical study in a narrow band for the parallel-coupled filters based on short-circuited stub-loaded resonators. It describes a synthesis method for this filter topology, and a complete resonant mode analysis to fully control the spurious frequencies positions. To enlarge the out-of-band rejection, an original technique, based on simple design steps, is used to address tuning of extra transmission zeros. Harmonic was suppressed to better than 35 dB with a wide stopband of more than six time the fundamental frequency. Design equations and design rules are given. Finally, several three-pole bandpass filters are designed and characterized in stripline and miscrostrip technologies to demonstrate the efficiency of the proposed concepts.