Research On High Performance And Compact Microstrip Filters For Wireless Communication Systems

There has been a phenomenal growth in the wireless industry. With the advancement ofwireless systems, the finite electromagnetic spectrum has to accommodate more and moresystems and thus it is becoming increasingly crowded. As a result, RF signals should bestrictly confined to the assigned spectrum ranges. As the key components to confine signals,bandpass filters are accordingly demanded with more stringent requirements. Hence, thisdissertation concentrates on bandpass filters for wireless communications. Dual-and tri-bandoperation，tunability, compact and high-performance of bandpass filters are highlighted asresearch focuses. Theory and design are included in this dissertation. One ultra-widebandbandpass filter, two dual-band bandpass filters with a tunable one and two tri-band bandpassfilters are presented in this thesis. The work can be divided into five parts as follows.(1)A design technique of a ultra-wideband filter is proposed using centrally-loadedresonators. This filter comprises of the short-and open-stub loaded resonators arranged inparallel. The short-stub loaded resonator is designed to resonate at the lower portion of theultra-wideband passband range and the open-stub loaded resonator operates at the upperportion. Hence, the signals at lower portion of the passband range mainly pass through theshort-stub loaded resonator while the signals at upper portion mainly go through theopen-stub loaded resonators. Interdigital coupling structure with the figure length of aroundquarter-wavelength at5.8GHz is employed to feed the signals to resonators and generated anotch band at5.8GHz. By adjusting the figure length, the frequency of notch band can betuned to desirable values. Since two paralled resonators which operate at different frequenciesare used, the coupling at lower and upper frequencies portion can be individually tuned bycontrolling the coupling dimensions between feeding lines and the corresponding resonators.The upper transmission zero is generated by the open stub of the resonator. The location of theupper transmission zero can be tuned by changing the open-stub length. The lowertransmission zero is generated by the two transmission path of signals. The location of thelower transmission zero is mainly controlled by the short-stub length of the resonator. Twotransmission zeros are created near the passband edges, exhibiting a quasi-elliptic functionresponse.(2)A design technique for dual-band bandpass filter design is proposed based onStepped Impedance Resonators with source-load coupling. The proposed filter consists of twosets of quarter-wavelength stepped-impedance resonators. Both the resonators are folded to Lshape to reduce the total size. The feed line is folded and source-load coupling can be introduced. Transmission zeros are generated close to passband edges, resulting inhigh-selectively.(3)A design technique for dual-band bandpass filter with independently-tunablepassbands is proposed. The proposed filter consists of a feeding scheme and two pairs ofvaractor-loaded resonators. Each set of resonators generates one passband, resulting indual-band responses. Using this configuration, it is convenient to tune the center frequency ofeach passband, while the responses of the other passband remain unaltered. At the upperpassband, two sets of bias voltages are applied to the varactors, enabling the independentpassband tuning. The two resonators are fed by short-ended transmission lines through thecoupling structure. Source-load coupling is realized by the interdigital capacitors to generatetransmission zeros. The interdigital coupling strength can be used to control the locations ofthe transmission zeros. At the lower passband, the voltage is applied to the varactor to changethe electric length, allowing the tuning of the lower passband. A tap-coupled structure isutilized to feed the two resonators and to enhance the coupling strength at the lower passband.(4)A design technique of a tri-band bandpass filter is proposed using a short-endstub-loaded resonator. The filter consists of one set of short-ended resonators. This resonatorutilized a short-ended main transmission line and a center loaded open stepped-impedancestub. The resonator is theoretically analyzed. The first three resonant frequencies can beindividually adjusted and this enables the convenient designs of tri-band bandpass filter. Theresonators are folded to reduce the circuit size. They are connected together using via hole asthe coupling structure. Inter-digital coupling is realized between the input and output, formingsource-load coupling. Several coupling fingers are utilized to provide sufficient degrees offreedom to control the locations of the transmission zeros generated by the source-loadcoupling.(5)Adesign technique of a tri-band bandpass filter is proposed based on a double-sidedparallel-strip line. The proposed filter is based on a double-sided parallel-strip line with aninserted conductor plane in the middle of the substrate. It is formed by the stub-loadedstepped-impendence resonators of top layer and the half-wavelength resonators of bottomlayer, which produces three passbands. The frequencies of the1stand3rdpassband arecontrolled by the stub-loaded stepped-impendence resonators of top layer, while the frequencyof the2ndpassband is controlled by the half-wavelength resonators of bottom layer.