A highly tunable radio frequency filter using bulk ferroelectric materials

The desirable attribute of software defined radios (SDR) implies that RF Front-ends must be multi-band and frequency agile. Wideband SDRs need a lower size, weight, and power (SWAP) tunable filter technology to meet the military's current and future communications needs. This is essential for the SDR operating in a battery powered environment such as man-portable Joint Tactical Radios (JTRS). The current method for building a tunable filter is to use discrete varactors at each section of the filter. These devices are nonlinear and their low third order intercept points make them highly susceptible to intermodulation which severely limits their dynamic range. Dielectric ceramics are inherently lossy and prone to breakdown due to the presence of materials defects. In this paper we will present a new approach to construction of tunable RF filters using low loss bulk ceramic high dielectric constant Barium Strontium Titanate (BST) ceramics. These ceramic compositions exhibit large change in dielectric constant with applied electric field. A prototype tunable filter has been built using these materials exhibiting 3:1 permittivity tunability. We will present a lumped circuit filter design for 30-450 MHz using the tunability of these paraelectric capacitors. The circuit design and experimental results along with the achievement of 1.7:1 frequency tunability of these filters will be shown.