High-Q Multi-band Filters
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Recent development of multifunctional communication systems capable of processing large amount of data has triggered the demand for novel payload configurations with advanced filtering functions. To increase the payload flexibility, a large number of multiplexer and filter networks with different frequency plans are usually employed for the transmitting downlink. Multi-band filters are the required function in many cases for minimizing integration complexity and reducing size and mass of space systems. The multi-band filters combine the frequency spectrums of non-contiguous channels before transmitting through antenna beams, and provide sufficient rejection to the frequency spectrums of the adjacent channels, thus maintaining a high signal-to-interference ratio especially in multi-beam frequency-reuse communication systems. Traditional approaches to realize multi-band filters do not offer advantages in terms of size and mass reduction. Multi-mode resonators have the advantage of size reduction; however they are not often used in multi-band applications due to the challenges of operating the multiple modes in prescribed passbands simultaneously. The main research objective of this thesis is to investigate the feasibility of designing multi-band filters based on high-Q multi-mode resonators. Various multi-mode waveguide and dielectric resonators are explored to realize multi-band filters. The proposed multi-band filters do not require junctions and can achieve an equivalent performance with fewer cavities, thus significantly reducing the footprint when compared to traditional approaches. Furthermore, tunable multi-band filters with a constant absolute bandwidth and minimum degradation during the tuning process is investigated and developed. A systematic design approach of designing multi-band filters based on multi-mode resonators is established in this work starting from the coupling-matrix synthesis of the multi-band network. Following that, dual-band filters based on elliptical and rectangular dual-mode resonators are proposed. The two passbands of the dual-band filter are carried by two independent cavity modes and realized by an inline direct-coupled configuration. The inline dual-band filter design can convert to a diplexer structure by modifying the output ports at the end-resonators. To improve near-band frequency selection of both channels, multiple configurations to realize quasi-elliptic dual-band filter functions are proposed. The first quasi-elliptic design is based on a combination of dual-mode and single-mode rectangular resonators resulting in multiple transmission zeros and improved spurious response. The second structure is a side-coupled design based on dual-quadruplet configuration featuring a pair of transmission zeros on each of the passband and a very compact layout. Limitations of the quasi-elliptic design are investigated and modified structures have been proposed with improved RF performances. Triple-band filters are realized by three types of high-Q cavity resonator structures. Each cavity resonator employs triple-modes with resonant frequencies associated with the three passbands. The first design was an elliptical waveguide triple-band filter with an in-line configuration. Each passband of the filter was controlled by a dedicated polarization and represented by an inline direct-coupled set of resonators. The second design was a rectangular-cavity triple-band filter with a folded configuration. The folded configuration overcomes a number of drawbacks from the elliptical in-line design including an improved tunability and ease of assembly. The last design was a triple-band filter design based on dielectric loaded cavity resonators. The unique dielectric resonator structure results in triple-band filters having a very compact size, high Q, and stable thermal response. Further adding tuning capability to the multi-band filter can provide an additional degree of flexibility for the communication payload. A tunable multi-band filter with a constant absolute bandwidth is developed based on combline resonator and requiring only a single tuning element. The performance is demonstrated with an in-house-developed tuning station. It achieves a constant selectivity over a tuning range of 170 MHz and an unloaded Q better than 3000. The novel filter configurations proposed in this thesis promise to be useful not only for satellite payload applications but also for a wide range of wireless base station applications.
Cite this version of the work
Li Zhu (2019). High-Q Multi-band Filters. UWSpace. http://hdl.handle.net/10012/15023
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