Study And Design Of Several Compact And Highly Selective Microstrip Filters For Wireless Communication Systems

In RF/microwave and millimeter wave communication system, filters play very important roles. With the rapid development of wireless communication system, it put forward higher requirements for the filters with small size, high performance, multi-function and low cost. Therefore, filters with these characteristics have become a hot research for several years. The dissertation makes deeply insight into the important issues concern the miniaturized and highly selective filtering components, such as multi-band bandpass filters, multiplexers, wideband bandpass filters, ultra-wideband bandpass filters and balanced filters.The major works are concluded as following:1. Multi-band bandpass filters based on multi-mode resonators have been studied. The resonance characteristics of the three-section stepped impedance resonators(SIR), combined stub-loaded resonators(CSLR) and stepped impedance stub-loaded resonators(SISLR) are analyzed in detail. The design graph about resonance characteristic and corresponding dimensions is plotted. And the design Procedure of multi-band bandpass filters is also introduced. Based on the proposed theory and analysis method, several bandpass filters, such as tri-band bandpass filter with high selectivity, high-order dual-band bandpass filter and compact tri-band bandpass, are designed and fabricated.2. Multiplexers using slotline stepped impedance stub-loaded resonators have been studied. A novel slotline stepped impedance stub-loaded resonator(Slotline SISLR) is proposed. Then, its resonance characteristics and transmission zeros positions are deeply analyzed. First, by using parallel coupled feedline, two compact bandpass filter with high selectivity can be achieved. Second, based on the analysis of T-junction, a diplexer with high selectivity is designed. To further reduce the circuit size, multiplexer are proposed based on common reosator. Finally, a novel multiplexer structure is introduced by applying distributed coupling technique. Since there are no extra matching networks, the proposed quadruplexer is very small.3. Wideband bandpass filters with wide upper stopband have been studied. First, the resonance characteristics of coupled stub-loaded resonator(CSLR) have been analyzed. Then, a wideband bandpass filter with controlled bandwidth is proposed. To suppress the harmonic response, the defect microstrip structures(DMSs) are adopted. Compared with former one, the proposed wideband bandpass filter has a wide upper stopband with the better suppression level, but the bandwithd almost keeps unchanged. Second, based on the analysis of the traditional branch-line structure, a modified branch-line structure(MBLS) is developed. A wideband bandpass filter based on this novel srtucture has an improved out-of band characteristic. Then, based on the transversal signal-interaction sructure and open/short-circuit stubs, the frequency selectivity and the harmonic suppression level have been greatly improved.4. UWB bandpass filters with high frequency selectivity have been studied. First, a design theory for UWB banpsass filter with high frequency selectivity is proposed by using novel multi-stub loaded ring rsaonators. Then, based on the analysis of frequency characteristic of the multi-mode resonator, a compact UWB bandpass filter with high performance can be obtained. Finally, the design procedure of a lowpass filter based on crossed DMS is introduced. By introducing a quasi-highpass structure into the lowpass filter, a UWB bandpass filter with high frequency selectivity and wide harmonic suppression is proposed.5. Balanced filtering compenonts have been studied. First, exploring the resonance characteristic of the open- and short-circuit crossed resonators(CR). By using this structure, a novel balanced wideband bandpass filter can be achieved. Second, a wideband balanced bandpass filter based on the novel modified branch-line structure is deeply analysized. Compared with the initial branch-line structure balanced filter, both the differential- and common-mode characteristics are obviously improved. Finally, novel and compact balanced filter using half wavelength Double side short-circuit resonators and quarter wavelength resonators with wide upper stopband and high common noise Suppression is proposed. Then, based on this structure, a novel balanced to balanced diplexer is designed to enhance the balanced circuits’ application in communication system.