A Tunable Filtering Attenuator Based on Lossy-Tuned Multimode Resonators

A tunable filtering attenuator based on lossy-tuned multimode resonators is proposed in this letter, featuring wide tunable attenuation range with low minimum insertion loss, improved in-band flatness, and high frequency selectivity. The design consists of one lossy-tuned triple-mode resonator coupling with two lossy-tuned dual-mode resonators. The improved in-band flatness and in-band matching at high attenuation levels are implemented through fine-tuned lossy resonator modes using p-i-n diodes. The mechanism behind high-attenuation flat passband and switch-off function is clarified. The p-i-n diode equivalent circuits and shunt capacitors are used to achieve circuit optimization and miniaturization. A prototype was fabricated and measured, demonstrating a tunable attenuation range of 1.14–41.83 dB, a 77.7% fractional bandwidth at 2 GHz, and a switch-off capability with over 34.8-dB in-band suppression.