Resistorless Implementation of Lossy Filters Using Coaxial SIW Resonators With Non-uniform Q

A technique for designing bandpass filters with very flat amplitude response is presented in this paper. The approach is based on the use of non-uniform-Q resonators implemented in coaxial substrate integrated waveguide (SIW) technology. An important unloaded Q-factor ratio can be obtained by adequately controlling several layout parameters of the basic resonator cell, thus enabling to optimize the in-band response of the filter. The approach is experimentally demonstrated at X-band frequencies by designing two resonators in Rogers RO4003C (ϵ
r
= 3.55, tan δ = 0.0027) with unloaded Q-factors going from 53 to 270. Then, a 4th-order filter example with 2 transmission zeros is designed, fabricated and measured based on the former results. The filter is centered at 10 GHz with a 280 MHz bandwidth showing a in-band flatness better than 1 dB-pp, which would require an unloaded Q-factor about 600 for a uniform-Q implementation, therefore showing the advantages of the proposed approach for implementing bandpass filters for high-end RF and microwave applications.