An Integrated 800-MHz Coupled-Resonator Tunable Bandpass Filter in Silver With a Constant Bandwidth

This paper presents a fully integrated tunable lumped filter on silicon using a low-temperature silver micromachining process. A prototype 836-MHz bandpass filter with a 3-dB bandwidth of 5.9% and insertion loss of 4.8 dB is demonstrated in a second-order coupled-resonator configuration. Continuous tuning of 50 MHz is achieved by electrostatically actuating the lateral air-gap capacitors of the filter. To control the bandwidth while tuning the center frequency, reconfigurable termination impedance is proposed. As a proof of concept, a low-noise amplifier with tunable input impedance is designed to interface with the bandpass filter. The tunable impedance is realized at the input of the low-noise amplifier using a shunt positive metal-oxide-semiconductor transistor. The fabrication, design, and measurement results of the filter are detailed, and future research directions to improve the performance of such filters are discussed.