Ultra-Compact WiMAX Bandpass Filter Embedded Into a Printed Circuit Board With a ${\rm SrTiO}_{3}$ Composite Layer

An ultra-compact bandpass filter has been developed for 2.3-2.7 GHz worldwide interoperability used for microwave access applications. The proposed filter was fully embedded into a multilayered printed circuit board (PCB) with a high dielectric strontium titanate (SrTiO₃) composite film layer. In order to reduce the size of the filter and to avoid unwanted 3-D electromagnetic coupling between the embedded passive filter circuit elements, the proposed filter was designed using J-inverter transformation technology. Two grounding via inductors and a single grounding capacitor were also utilized to generate three independent finite transmission zeros to enhance the rejection characteristics at the proper frequency bands. Since the J-inverter transformed filter circuit elements could be designed with relatively small inductance and large capacitance values, the high dielectric composite film was highly effective in reducing the size and improving the performance of the filter. The high dielectric film exhibited a dielectric constant of 17, a tangent loss of 0.01, and a capacitance density of 12.2 pF/mm² at 1 GHz. The measured maximum insertion loss in the passband ranged from 2.3 to 2.7 GHz was better than 1.8 dB, with a minimum value of 1.58 dB at 2.6 GHz. The return loss was higher than 15 dB in the passband. The transmission zeros occurred at 1.71 and 5.1 GHz, and provided suppressions of 45 dB at 1.71 GHz, and 54 dB at 5.1 GHz, respectively. The measured group delay was less than 0.8 ns in the passband. The embedded bandpass filter had a volume of 2.6 × 2.6 × 0.55 (H) mm³, which is the smallest one found, compared to previously reported devices, the filter, embedded into the eight-layered packaging substrate with the high dielectric composite layer, dramatically reduced the size up to 56%, and its performance was also much improved in comparison to the one embedded into a conventional six- layered PCB.