Multi-Band Differential Bandpass Filters with Quasi-Elliptic-Type Passbands and Multi-Transmission Zero Common-Mode Suppression

In this paper, the RF design of differential quasi-elliptic-type dual-and tri-band bandpass filters (BPFs) is reported. They are based on series-cascaded multi-resonant cells that result in: i) quasi-elliptic-type passbands in the differential-mode of operation that are shaped by 3N poles and 3N-1 transmission zeros (TZs) — N is the number of bands — and ii) high common mode (CM) suppression through multi-TZ generation — 5N-1 in total — and resistively-loaded poles. Coupling-routing diagrams that facilitate the RF design of these type of filters with standard coupled-resonator synthesis are presented. For proof-of-concept demonstration purposes, a dual-band microstrip prototype was manufactured and measured with the following RF performance characteristics: passbands centered at 2.53 and 3.49 GHz, fractional bandwidths of 13 and 12%, and minimum in-band insertion loss levels of 1.27 and 1.43 dB, respectively. Moreover, the CM is suppressed by >52 and >38 dB at the center of the bands.