GaAs-Based IPD-Fabricated Center-Frequency-Controllable Bandpass Filter With Asymmetrical Differential Inductor and Air-Bridge Enhanced Capacitor

In this paper, a design of bandpass filter (BPF) comprised of an asymmetrical differential inductor and an air-bridge enhanced capacitor is developed using the GaAs-based integrated passive device (IPD) fabrication techniques. The <inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-factor can be improved by the asymmetrical differential inductor and differential structure, providing relative less loss for the BPF design. In addition, the capacitor is specially built with different turns inside the blank area of the proposed inductor, and several air bridges are applied to enhance its capacitance with its value varying from 0.295 to 0.318 pF without extending any extra space. Hence, this makes it more feasible to control the center frequency of the proposed BPF. Moreover, the area as compact as <inline-formula> <tex-math notation="LaTeX">$800\,\,\mu \text{m}\,\,\times 988\,\,\mu \text{m}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$0.015\lambda _{0} \times 0.018\lambda _{0}$ </tex-math></inline-formula>) has been successfully achieved, including the contact pads by the BPF chip fabrication. After the BPF chip is assembled on a sub-board printed circuit board (PCB) and attached on a die sink of the iron cube, the measurement has been effectively conducted. The measurement results of the proposed BPF give the insertion loss of 0.38 dB and the return loss of 17.35 dB at the center frequency of 1.56 GHz. Furthermore, the 3-dB fractional bandwidth (FBW) is calculated as 51.3% and the isolation is higher than 10 dB from 2.62 to 5.45 GHz. By changing the structure in terms of different numbers of turns of the capacitor and with or without the air bridge, variation can be achieved from 1.64 to 2.28 GHz for the center frequency and from 4.31 to 5.81 GHz for the frequency of transmission zero (TZ). Such results validate that our proposed BPF design is capable of offering both sound performance and controllable properties.