Integrated Tunable LC Higher-Order Microwave Filters for Interference Mitigation
Amin, Farooq Ul
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Modern and future communication and radar systems require highly reconfigurable RF front-ends to realize the vision of Software-Defined Radio (SDR), where a single digitally-enabled radio is able to cover multiple bands and multiple operating standards. However, in the increasingly hostile RF environment, filtering becomes a bottleneck for SDRs as the traditional off-chip filters are fixed frequency and bulky. Therefore, tunable filtering is a critical building block for the reconfigurable RF front-ends and on-chip implementations are needed to meet size and weight constraints. On-chip passive components are lossy, especially inductors, and to fulfill the tunability requirements a number of active circuit techniques, e.g. N-path, Q-enhanced, discrete-time filters etc., have been developed. Most of these active filtering techniques, however, are limited to RF frequency range of few GHz and below. Additionally, these techniques lack or have very limited bandwidth tunability. On the other hand, Q-enhanced tunable LC filtering has the potential to be implemented at Microwave frequencies from 4~20 GHz and beyond. In this dissertation, a number of Q-enhanced parallel synthesis techniques have been proposed and implemented to achieve high-order, frequency tunable, and wide bandwidth tunable filters. First, a tunable 4th-order BPF was proposed and implemented in Silicon Germanium (SiGe) BiCMOS technology. Along with center frequency tuning, the filter achieves first ever reported 3-dB bandwidth tuning from 2% to 25%, representing 120 MHz to 1.5 GHz of bandwidth at 6 GHz. A new set of design equations were developed for the 4th-order parallel synthesis of BPF. A practical switched varactor control scheme is proposed for large tuning ratio varactors to reduce the nonlinear contribution from the varactor substantially which improves the tunable LC BPF filter linearity. Second, parallel addition and subtraction techniques were proposed to realize tunable dual-band filters. The subtraction technique is implemented in SiGe BiCMOS technology at X and Ku bands with more than 50 dB of out-of-band attenuation. Finally, a true wideband band-reject filter technique was proposed for microwave frequencies using parallel synthesis of two band-pass filters and an all-pass path. The proposed band-reject scheme is tunable and wide 20 dB attenuation bandwidths on the order of 10s of MHz to 100s of MHz can be achieved using this scheme. The implementation of the proposed parallel synthesis techniques in silicon technology along with measured results demonstrate that Q-enhanced filtering is favorable at higher microwave frequencies. Therefore, such implementations are suitable for future wireless communication and radar systems particularly wide bandwidth systems on the order of 100s of MHz to GHz. Future research includes, high-order reconfigurable band-pass and band-reject filters, automatic tuning control, and exploring the parallel synthesis techniques in Gallium Nitride (GaN) technology for high RF power applications.
- Doctoral Dissertations 
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