High- $Q$ Narrowband Tunable Combline Bandpass Filters Using MEMS Capacitor Banks and Piezomotors

This paper presents the design and implementation of a new class of evanescent tunable combline bandpass filters based on electronic tuning with the use of RF microelectromechanical systems (RF-MEMS) capacitor banks and also mechanical tuning using piezomotors. The use of microelectromechanical systems tuning circuit results in compact implementation of the proposed filters with high-Q and near to zero dc power consumption. The proposed filter structures consist of combline resonators with tuning disks that are either mechanically moveable using piezomotors or are loaded with RF-MEMS capacitor banks. Two- and six-pole tunable bandpass filters are designed and measured based on the proposed tuning concept. The two-pole tunable filter operates at 2.5 GHz with a bandwidth of 22 MHz and demonstrates a tuning range of 110 MHz, while the quality factor is better than 374 (1300-374 over the tuning range). The six-pole tunable filter with RF-MEMS capacitor banks operates from 2.634 to 2.59 GHz (44-MHz tuning range). The proposed tunable filter structures can also be implemented using alternative technologies such as barium-strontium-titanate varactors.