Tunable Band Pass Filters for Communication Systems
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The ever-increasing demand for high communication data rate and high-quality multi-media services; over past few decades, has ignited new avenues in radio architectures. Frequency reconfigurable (or frequency agile) communication systems are among the key architectures for efficient and cost-effective utilization of the allotted frequency spectrum. The emerging concept of on-orbit flexible payload (or programmable payload) in satellite communication is another encouraging development on the horizon. In-addition, tunability in filters used for remote radio unit (RRU) is highly preferred by network operators owing to the high cost of installing RRU both in low density remotely accessed locations and in high density expensive urban locations. Such frequency reconfigurable radio architectures typically demand reconfigurability (tunability) of components within the physical layer as well. Hence, tunable filters play a vital role in realization of frequency reconfigurable communication systems. In general, any fixed frequency filter can be transformed into a tunable filter by introducing tuning elements dedicated to tuning the resonators and the coupling structures. Thus, a tunable filter of order N would require 2N+1 tuning elements to maintain a constant absolute bandwidth (BW) over the tuning range. This use of large number of tuning elements not only increases size and cost, but also adds to the complexity of the tuning control mechanism, particularly when configured in a closed loop system. Over the past decade, a significant research has been carried out to reduce the number of tuning elements by roughly 50% (i.e. with only N tuning elements). The coupling structures are suitably designed to maintain their performance over the tuning range, eliminating N+1, while only N tuning elements are used for tuning the N resonators. The goal here is to further reduce the number of tuning elements to a ‘single tuning element’. The thesis presents several novel configurations for a high-Q tunable band pass filter employing a single tuning element, while maintaining a constant BW, return loss performance and location of the transmission zeros over a wide tuning range. Advanced filter synthesis techniques for both tunable filter and fixed filters are also proposed. A tunable double-septa waveguide (WG) filter is presented employing a single tuning element. The theory of coupling behavior of single septum and double septa to achieve constant absolute BW is explored. The tuning mechanism of the proposed filter is explained with measurement results presented for a Ku-band tunable WG filter designed at 15 GHz with a 2% fractional BW to achieve 15% tuning range. BW variation is observed to be within ±5% while the center frequency is tuned from 14.65 to 17.15 GHz. The filter promises to be useful in emerging 5G millimeter-wave applications, where the filter size is very small to accommodate multiple mechanical tuning elements. Furthermore, the proposed design methodology is scalable, i.e., the tuning mechanism is independent of the filter order. A frequency reconfigurable dual-mode WG filter having an elliptic response is presented. The proposed filter maintains a constant absolute BW and a constant rejection BW (i.e. constant frequency spacing between transmission zeros) over the tuning range. Furthermore, the filter can be tuned using a single tuning mechanism. A 4th order prototype filter at 11.5 GHz with 50 MHz bandwidth and 2 symmetric transmission zeros (± 45 MHz) is fabricated and measured. A novel configuration of a BW reconfigurable WG filter that uses only two tuning elements irrespective of the filter order is proposed. The proposed filter configuration demonstrates that it can achieve a relatively wide BW variations without deviating the center frequency. A 4 pole prototype filter is designed, fabricated and tested at Ku-band. The measured BW tunability of the filter is nearly 35 % from 225 to 320 MHz at 13.375 GHz. To the author’s knowledge, this is the only BW reconfigurable filter that can be tuned with only two tuning elements regardless of the filter order. The thesis also demonstrates the feasibility of realizing a high-Q lambda/2 resonator based tunable coaxial filter, which is tuned by a single rotational tuning element irrespective of the filter order. The proposed filter has low variations in the absolute BW and insertion loss (IL) over a relatively wide tuning range. A prototype four-pole filter is developed at 2.5 GHz with a fractional BW of 4% to verify the concept. The measured tuning range of the filter is 20%, within which the BW variation is better than ±10% and IL variation is better than 0.05 dB. The proposed concept is easily expandable to filters with higher order. Furthermore, the concept is adopted to design a tunable diplexer using only a single tuning mechanism while maintaining the frequency performance of each channel and the frequency spacing between the two channels over the tuning range. The proposed high-Q tunable filter is promising for use in the frequency-agile communication architecture at the cellular base-station and aerospace applications. A novel configuration of a High-Q coaxial tunable filter which employs a single rotational mechanism to tune the filter, while using fixed lambda/4 resonators is also presented. The rotational tuning concept is different from that proposed for the tunable coaxial lambda/2 resonators. A prototype filter is designed for the proof of concept, which has a tuning range of 11.6% from 685 MHz to 770 MHz, over which bandwidth variation is within 10.5±0.7 MHz.. In-addition, the proposed design methodology can be scaled to realize higher order filters. The proposed filter promises to be useful in a wide range of telecommunication applications including flexible payload in aerospace applications.
Cite this version of the work
Gowrish Basavarajappa (2021). Tunable Band Pass Filters for Communication Systems. UWSpace. http://hdl.handle.net/10012/16961