Embedded band pass filter designed using embedded capacitor material

A new compact microstrip dual-mode bandpass filter with the center frequency of 2.4GHz is proposed based on embedded capacitance material (BaTiO₃-epoxy composite film). The filter consists of a defect ground structure (DGS) and an open-loop step-impedance resonator with a short-circuit line in the middle of the resonator. The bandwidth of the filter can be optimized by adjusting the width of the short-circuit line. A compact prototype was fabricated by embedding the bandpass filter into a multi-layered FR-4 packaging substrate through the PCB manufacturing process. The measured results show that the insertion loss is 2.23 dB, the return loss is greater than 30dB at the center frequency 2.231 GHz. The size of the proposed single one is 1.8 mm×1.85 mm. To improve the performance of the filter, a cascaded two-stage dual-mode filter with open-loop DGS was designed, fabricated and measured based on the single dual-mode filter. The size of the prototype is 3.55 mm×1.75 mm. The measured insertion loss is less than 4 dB and the return loss is higher than 15dB at 2.58 GHz, both agreeing well with the simulated results below 5GHz. It indicates that introducing the DGS can improve the pass band characteristic, miniaturization and selectivity.