Dispersive Box Section and Its Applications to Quasi-TEM Mode Monoblock Dielectric Filters

In this article, the comprehensive theory for synthesis of a dispersive box coupling section for full control of two transmission zeros (TZs) is presented and applied to the design of high- $Q$ monoblock dielectric resonator (MDR) filters. Compared to the traditional cascaded quadruplet (CQ) MDR section, the dispersive box section provides high flexibility in controlling TZs, a wide spurious free high rejection band, simplicity in physical realization, and higher unloaded $Q$ s. The analytic transformation for converting a traditional CQ section with diagonal cross coupling to a dispersive box is given to facilitate the direct synthesis and justifies the uniqueness of the realizable solution to all possible TZ arrangements. Circuit realizations of all possible TZ arrangements are discussed with synthesized coupling matrices. A practical eight-pole MDR filter with an asymmetric response is synthesized and prototyped. Excellent agreement between the synthesized and measured narrowband responses as well as the EM simulated and measured wideband responses affirms the great flexibility in controlling TZs and demonstrates a wide spurious-free high rejection band with the proposed dispersive box structure. The low in-band insertion loss demonstrates that the MDR filter using the dispersive box can be potentially applied in 5G and future wireless communication systems.