High-Q Multi-band Filters

Abstract

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.