In this paper, dual-band and tri-band bandpass filters (BPFs) with fully independent and controllable passbands based on multipath-embedded resonators are presented. The dual-band BPF consists of two double open-ended stub-loaded terminal-shorted resonators (DOESL-TSRs) with a common via-hole connected along the symmetric plane of the filter. Based on DOESL-TSRs, a triple open-ended stub-loaded terminal-shorted resonator (TOESL-TSR) is proposed in the design of tri-band BPFs. The resonant characteristics of DOESL-TSR/TOESL-TSR are analyzed by the numerical calculation method. The measured results of the dual-band BPF show that the center frequencies (CFs) are located at 2.595 and 5.75 GHz, respectively, with 3 dB fraction bandwidth (FBWs) of 15 and 12.8%. The measured CFs of the tri-band BPF are located at 2.545, 3.775, and 5.95 GHz, respectively, with 3 dB FBWs of 9.8, 9.3, and 5.5%. Both of the filters exhibit the merits of fully independent and controllable passbands, high selectivity, and compact size.

In this letter, a passive high-selectivity dual-band filter with two controllable transmission zero (TZ) pairs is proposed, while synthesis method and control mechanism of the two TZ pairs are investigated. Specifically, by employing the magnetic/electric mixed coupling (MEMC), source–load coupling (S–L coupling) and stepped-impedance resonators, a dual-band bandpass filter with two pairs of controllable TZs is constructed. Two controllable TZ pairs can be independently adjusted by re-modifying the associated coupling structures. To validate the synthesizability and controllability of the TZ pairs, mathematical synthesis, and EM simulations are carried out. Two demonstrative filters with identical passband performance and different central TZ distributions for GSM (0.9/1.8 GHz) are designed and measured. The analysis and experimental results show that the synthesis-controllable TZ pair (fz2, fz3) introduced by MEMC can be synthesized and controlled using inline mixed coupling synthesis, and the optimization-controllable TZ pair (fz1, fz4) because of S–L coupling is controlled by S–L coupling strength optimization procedure.