Balanced Bandpass Filters Using Center-Loaded Half-Wavelength Resonators

This paper presents a balanced filter that is designed by using the center-loaded half-wavelength resonator. To demonstrate the design ideas, the differential- and common-mode equivalent half circuits are built and studied. The proposed technique is novel in the way that the common-mode suppression can be made independent of the differential-mode response in the balanced filter, making use of a center-loaded half-wavelength resonator. This is very important to simplify the design and tuning processes of the balanced filters. Making use of this feature, the resonator is not only utilized to design the balanced single-band bandpass filter but also the balanced dual-band bandpass one, possibly being the first to be presented. To demonstrate the design ideas, one single-band and two dual-band balanced filters are presented. The experimental results agree well with the theoretical predictions and simulations.