Design of Multilayer Balun Filter With Independently Controllable Dual Passbands

In this letter, a novel multilayer dual-band balun filter using two dual-mode cross-slotted patch resonators placed back-to-back is proposed by converting two-port bandpass filter to a three port device. The operation frequency and bandwidth in each passband can be individually controlled, which is a new design concept for a dual-band balun filter. The detailed design procedure based on the coupling matrix is theoretically presented. A transmission zero between the two passbands is realized to improve the band to band isolation; meanwhile a good unbalanced to balanced conversion in both bands is realized. To demonstrate the design methodology, a dual-band balun filter is designed and fabricated. The measured results show good agreement with the simulation data.