Quasi-absorptive substrate-integrated bandpass filters using capacitively-loaded coaxial resonators

This paper reports on the RF design of highly-selective substrate-integrated (SI) bandpass filters (BPFs) with two-port symmetrical quasi-absorptive characteristics. The proposed filter concept is based on a two-pole/one-transmission-zero (TZ) bandpass (BP) filtering section that is loaded at its input/output ports with resistively-terminated second-order bandstop (BS) sections. It features size compactness and low inband insertion loss (IL) that are obtained through: i) high-quality-factor SI capacitively-loaded coaxial resonators and ii) a BP section that is shaped by two resonators and a frequency-dependent coupling. Various synthesized examples and practical implementation aspects are discussed through coupled-resonator-based synthesis and linear simulations. For proof-of-concept demonstration purposes, an experimental prototype shaped by six SI resonators was designed, built, and measured. It exhibits a highly-selective symmetrical quasi-absorptive filtering transfer function with one TZ at the upper passband side and the following characteristics: center frequency: 3.68 GHz, 3-dB bandwidth: 590 MHz (16%), minimum in-band IL: 1 dB (including SMA connectors), in-band group delay: 1.79±0.5 ns, and return loss<10 dB for frequencies between 2.9 GHz and 4.9 GHz (i.e., 1.69:1 spectral ratio).