X-band RF MEMS tuned combline filter - Electronics Letters

Introduction: Microwave filters are critical components for electronic warfare, radar, and communications systems, but typical tunable filters are bulky, expensive, and consume large amounts of power. Radio frequency microelectromechanical systems (RF MEMS) technology provides the possibility of low-loss, high linearity filtering in volumes comparable to those of integrated circuits [1]. The maturation of this technology will allow further miniaturisation of radar and communication sub-systems. To date, there have been several examples of monolithic RF MEMS tuned filters [2–7]. The tuning range of these filters is limited either by the tuning range of the MEMS components [4, 5], or by the frequency range over which the filter elements maintain their reactances and coupling relationships. While traditional combline filters with tapped or coupled line transformer inputs fed from the grounded end of the resonators offer a broad tuning range [8], the tuning range can be increased by feeding the input and output coupled line transformers from opposite the grounded ends of the resonators, as described by Hunter [9]. In this design, the coupling in the transformer and coupling between the resonators change in offsetting directions, allowing extremely broad tuning ranges. In addition, this filter has fewer RF MEMS components than other designs [3–5]. Thus, this combline filter design is ideally suited for implementation using RF MEMS. To vary the loading capacitance, we use RF MEMS capacitive switches on the end of each resonator to select fixed metal-insulatormetal (MIM) capacitors, rather than using individual continuously tunable capacitors. In addition to allowing a broader tuning range, the digitally switched bank offers better reproducibility and stability across a broad range of environmental conditions than analogue tuned devices. This type of implementation will be essential for the acceptance of RF MEMS based components into fielded systems.