Novel Compact Lowpass Filter Using Dual-Layer Coupled Resonators

This article presents the synthesis and design of a novel compact lowpass filter (LPF) with low insertion loss and significant near-skirt selectivity. The LPF is designed based on a new structure called dual-layer coupled resonators (DLCRs) implemented with striplines embedded in air cavity, thus characterized with high quality factor and low insertion loss. To enhance band-edge selectivity and stopband attenuation, finite-position transmission zeros (TZs) are introduced with DLCRs to form pseudo-elliptical fitter functions. The equivalent circuit of DLCR is provided first, followed by detailed synthesis preconditions and procedures based on element extraction. With the elucidated synthesis process, TZs can be customized and arbitrarily placed at specified frequencies. The proposed synthesis theories are general and applicable to LPFs with arbitrary order and assigned TZs, thus providing effective guidelines for physical implementation. Besides, with smaller needed capacitances and avoided long-length high-impedance lines in TZ-producing resonant pairs, a more compact footprint is achieved with DLCRs. A 13th-order LPF with three TZs is synthesized and fabricated for navigation satellite applications, of which the good measurement results have validated the proposed theories.