Controlled Agility: Frequency-Agile Planar Filters with Advanced Features

High-performance filters with electronically controlled transfer functions are needed for future microwave systems. In commercial applications, frequency agility is necessary to address ever-evolving standards and an increasingly congested spectrum, while military requirements include cosite interference mitigation for broadband systems and adjacent-channel simultaneous transmit and receive. These filters will enable new system architectures, which will, in turn, allow for improved performance and new capabilities. For example, as analog-to-digital converter (ADC) technology continues to advance, it is expected that, eventually, intermediate-frequency (IF) stages will not be needed; instead, direct detect schemes could be used, with the entire radio-frequency (RF) chain consisting basically of an ADC preceded by a filter. This places very high performance requirements on the filter, not only in terms of loss and selectivity but also frequency agility. Optimally, these filters would be tunable in the sense that center frequency and/or bandwidth can be changed, as well as reconfigurable in the sense that transfer function order and/or number and location of transmission zeroes can be changed. A filter that is both electronically tunable and reconfigurable allows the filter to be optimized for a given application or spectral situation [1].