PCB embedded compact balanced filter with coupled LC resonators

In this paper, PCB embedded compact unbalanced-to-balanced filter was newly proposed for low cost Bluetooth and wireless LAN applications. In order to obtain excellent bandpass transmission characteristic, the proposed filter was configured by using two pair of modified magnetically coupled LC resonator. This proposed structure provides transmission zero pole at upper frequency regime. Out-of-band suppression under the passband was enhanced by applying parallel connected LC resonator at single-ended port. In order to reduce the size of PCB embedded balanced filter, BaSrTiO3 (BST) composite high Dk RCC film was applied to improve the capacitance density. This high Dk film provided the capacitance density of 12.2 pF/mm
²
. The fabricated filter had insertion loss and minimum return loss of 2.2dB and 11dB in the frequency ranged from 2.4~2.5GHz, respectively. The maximum magnitude and phase imbalance were 0.9dB and 1.5 degree in its passband, respectively. The out-of-band suppression was observed approximately 30dB and 22dB below 1.9GHz and over 3.5GHz, respectively. It has a compact volume of 1.7 mm times 2 mm times 0.7 mm (height).