Electronically Tunable Filters

Abstract

Microwave filters are essential building blocks in communications systems. As the communications industry evolves, smaller and more flexible filters with a high quality factor (high-Q) are in great demand. The deployment of tunability and reconfigurability in the designing of microwave filters provides greater system flexibility as well as economic benefits.

This thesis investigates a concept for improving the roll-off performance of filters that employ low-Q resonators. The approach is to combine two filters (sub-filters) with non-ideal characteristics to realize a band-pass filter with better roll-off characteristics, i.e. an effective high-Q value. Lossy filter and pre-optimization techniques are proposed to design the sub-filters.

To demonstrate the concept, two tunable band-pass filters are designed in microstrip, with slots etched on the ground plane as sub-filters. The sub-filters are simulated in the EM software, Sonnet. The simulated structures are fabricated, integrated with varactors, assembled, and then tested. The measured results are in good agreement with simulations, confirming the validity of the proposed concept. The thesis also investigates the use of the proposed concept in the design of tunable diplexers.