The Sound the Air Makes: High-Performance Tunable Filters Based on Air-Cavity Resonators

Tunable filters have a wide range of applications from software-defined radio to reconfigurable satellite payloads. They are a key building block for any flexible transceivers. A variety of tunable filter technologies can be found in the literature. Examples include: planar tunable filters employing solid-state or microelectromechanical systems (MEMS) varactors [1]-[6], and ferroelectric variable capacitor tuned coaxial filters [7]. The choice of technology is driven by the application. In this article, we focus on applications requiring high performance, including low loss, high-power handling capability, and high stability, mainly for communications satellites or wireless base stations. These requirements immediately rule out any low-quality factor (Q) technologies. For instance, besides low Q, planar-type tunable filters typically suffer from poor selectivity and transmission-response variation over the tuning range. Technologies based on substrate-integrated-waveguide (SIW) offer better Q than microstrip circuits and advantage in packaging [8]-[10]. However, in most cases, their Q is comparable to strip-line circuits with the same volume. Air-cavity resonators, on the other hand, offer high-Q in the range of thousands to tens of thousands and high-power handling and are therefore one of the obvious choices. The addition of each requirement, such as power, selectivity, vibration, and temperature stability, further limits available choices. We are not aware of an existing technology that satisfies all these requirements. The search for a viable solution for the targeted high-end applications is indeed a difficult journey, with years of experience accumulation from past good and bad designs.