Multi-Band Bandpass Filters and Filtering Power Dividers Based on Dual-Mode Resonators

Multi-band microwave components are expected to be used in modern high-integration wireless communication systems, which can support multi-services simultaneously without adding additional receive or transmit links. For this reason, multi-band microwave components are attracting great attention and becoming research hotspot in microwave passive components domain. Particularly, in the development of multi-band bandpass filters (BPFs) and filtering power dividers (FPDs), how to realize high selectivity, small circuit size, low loss, wide stopband, and flexible control of the channel frequencies and bandwidths are challenging problems. Moreover, balanced-to-unbalanced (BTU) filtering power divider is also one of the key passive components in RF/microwave front-ends for connecting the balanced ports and single-ended port to achieve conversion between balanced and unbalanced signals with common-mode noise suppression. This dissertation is intended to propose new types of multi-mode resonators (MMRs) and develop dual-band and tri-band BPFs, dual-band FPDs, and BTU filtering power dividers with small circuit size, independent control of channel frequencies and bandwidths, and other excellent frequency characteristics. Firstly, a novel type of stub-loaded stepped-impedance resonator (SL-SIR) is proposed. With different schematics of the loaded stubs, the SL-SIR has flexibly controllable dual-modes or tri-modes to construct dual-band or tri-band BPFs. Detailed mode analysis of the resonator is conducted, and parametric variations of the modes are investigated. Next, dual-band and tri-band BPFs are proposed and designed using the dual-mode or tri-mode SL-SIRs. Multiple geometrical parameters in both the external feeding structure and the internal couplings between neighboring resonators are employed to make the individual control of the two or three passbands possible. Furthermore, separately changeable multiple coupling paths between the resonators and the coupling paths between the source and load are devised to create multiple transmission zeros, which not only enhance significantly the selectivity of the passbands, but also widen greatly the stopband of the BPFs. A small H-shaped composite resonator is also proposed, and its even-mode and odd-mode are used to configure a miniaturized dual-band BPF with a flexible center frequency ratio. Three transmission zeros are produced by introducing a mixed electric and magnetic coupling between the resonators, which improves significantly the selectivity and out-band rejection performance. All the designed dual-band and tri-band BPFs are fabricated, and their measured frequency responses agree well with the theoretically predicted ones. Secondly, a novel compact dual-band filtering power divider (DB-FPD) is developed which can reduce significantly the circuit size of a RF/microwave front-end. The proposed DB-FPD consists of a small U-shaped Wilkinson power divider, two pairs of dual-resonance resonators (DRRs), and a pair of spur-lines. With the simultaneous use and appropriate design of the coupled feedlines, mixed electric and magnetic couplings between the DRRs, and spur-lines with different lengths, multiple TZs are produced which result in two passbands with desired power division, high frequency selectivity, good isolation, and an ultrawide stopband. A prototype DB-FPD is designed, fabricated and measured. The measured responses agree well with the design simulations, exhibiting a stopband up to 13.8 GHz (6f0) with 20-dB rejection level. This is the widest stopband of DB-FPDs reported thus far. Finally, two novel balanced to unbalanced filtering power dividers with variable bandwidth are proposed based on stub-loaded dual-mode resonators (SL-DMRs). Two SL-DMRs are used to replace the 90° horizontal transmission lines to realize filtering responses. With the even- and odd-mode analysis method and traditional transmission line theory, closed-form analytical equations and detail design procedures are derived. Moreover, a pair of parallel coupled-lines are used to feed the SL-DMRs, which not only realize variable bandwidth but also improve the stopband performance. Three transmission zeros (TZs) locate on both sides of the passband, which improve the selectivity of the passband. To verify the analytical theory and design method prediction, two prototypes are designed, fabricated, and measured. The measured responses agree well with the design simulations, exhibiting a good frequency selectivity, isolation, and common-mode suppression.