Miniaturized ultra-wideband bandpass filter using bridged-T coil

In this letter, a miniaturized ultra-wideband bandpass filter based on thin-film integrated passive device process is proposed. The multi-mode resonator is adopted to achieve a wide bandwidth from 3.1 to 10.6 GHz. By using the bridged-T coil to replace the transmission line of multi-mode resonator, the circuit size is largely reduced while the multi-mode property can be retained. The filter size without pads is 2.21 mm$\,\times \,$1.04 mm, which is only about $0.050 \lambda-{{{0}}} \times 0.024 \lambda-{{{0}}} $ at the center frequency of 6.85 GHz. To our knowledge, this is the smallest multi-mode resonator filter ever published.