A synthesis technique for mono- and mixed-mode symmetrical filters

The paper introduces a synthesis technique for mono- and mixed-mode symmetrical filters. Following the approach, all symmetrical filter structures can be solved with closed form solutions by using their symmetrical properties. The outstanding features of this synthesis technique are its capability to consider Q-factor of resonators or losses of lumped elements, to deliver low group delay, and to include active elements. The synthesis theory is presented and applied for coupled resonator filters. A bandpass filter based on two coupled resonators with a third resonator for coupling at 900 MHz with a notch at 2700 MHz was realized in SMD-technology. Additionally, a novel bandpass filter optimized for differential systems with a very high common mode suppression is reported and tested. Experimental results have verified the validation of the proposed synthesis technique.