Coplanar Stripline-Fed Dual-Mode Ring Resonators With a Pair of Transmission Zeros

A differential-fed strategy is proposed for the first time by using coplanar stripline (CPS) to provoke two types of the ring-based resonators with a pair of transmission zeros (TZs). Firstly, a two-pole response can be exhibited with two TZs at either the real or the imaginary frequencies. Unlike the microstrip line-based schemes loaded with capacitive elements, the real (imaginary-axis) TZs of the CPS- and the double-sided parallel-strip line (DSPSL)-based counterparts are found when an inductive discontinuity is dominant, as a perturbation of two degenerate modes of a ring resonator. Moreover, the short-ended CPS-fed configuration is constructed to provide the main magnetic coupling between the external transmission line and the CPS-/DSPSL-based ring resonator. As a result, two 4th-order bandpass filters and a 3^rd-order filtering crossover are designed and simulated, by combining the dual-mode ring and the quarter-wavelength resonators in together. Simulated results show that the bandpass behavior with finite TZs is legitimately achieved.