RF Tunable Resonators and Filters

Abstract

The recent remarkable growth in the telecommunications industry has caused a huge advance in filter technology. The new communication systems demand stringent filters with high specifications, low costs, compact features, and the ability to accommodate multiple wireless standards (2G, 2.5G, 3G, WiMAX, Mobile WiMAX, LTE and LTE-A). In order to realize a multi-standard RF front end and cover all frequency bands, a transceiver requires a customized RF device that includes tunable filters. The main building block of any tunable filter is a tunable single resonator. The thesis introduces two novel techniques to extract an equivalent circuit model for RF resonators. Both techniques are successfully tested and validated for different case studies. Using a systematic approach, the lumped element technique can be used to relate the EM model to an adjusted parallel RLC model that takes into consideration the input coupling. The node-to-node technique is based on using a transmission line equivalent to model combline resonators. Several examples are used to apply the technique to gain more insight into understanding and thus improving resonator performance. The thesis also presents the design and implementation of a high-Q bandpass filter using the proposed angular tuning technique that maintains a constant Q value over a relatively wide tuning range. The traditional technique for tuning combline filters is achieved by changing the gap between the post and the tuning disk. Such a technique is known to yield a Q value that degrades considerably at the lower edge of the tuning range. The proposed angular tuning technique shows a 25% improvement in the Q value at the lower edge of the tuning range, in comparison to what is typically achieved using the traditional tuning technique. Using the proposed angular tuning technique, a 1% bandwidth 2-pole filter is designed, fabricated, and tested with a 430MHz tuning range at a center frequency of 3.6 GHz. The filter is integrated with miniature piezoelectric motors, demonstrating an almost constant insertion loss over the tuning range.