The Research On Microwave Dual-Band Bandpass Filters For Wireless Communication

In recent years, dual-band portable telephones and WLAN (wireless local area network) become quite popular due to the need of wireless mobile communications. Dual-band filters become key components in the front of these communications systems. This dissertation gives a detail study on the design technique and theory of dual-band filters for wireless communication systems. A synthesis theory of dual-band bandpass filter (BPF) is firstly proposed by using successive frequency transformation and circuit conversion techniques.The author's main contribution includes four parts as follows:Firstly, a two-pole 900MHz/1800MHz dual-band bandpass filter is proposed by using microstrip open-loop resonators. Dual-band characteristics are obtained by using the fundamental and first spurious resonance of the resonators. Both the internal and external couplings of the filter are appropriately designed to simultaneously satisfy the dual-band requirements. Then a novel compact microstrip three-pole dual-band bandpass filter is developed with remarkably improved stopband characteristics. Two square open-loop resonators are embedded in a U-shaped resonator to achieve a significant size-reduction of the filter. Meanwhile, microstip open stubs as well as a cross-coupling between nonadjacent resonators are exploited to produce multiple transmission zeros in the stopbands of the filter. As a consequence, wide stopbands with sharp and large attenuations are obtained. Theoretical and measured frequency responses of the designed filter show a good agreement.Secondly, a dual-band bandpass filter using microstrip slow-wave resonators is developed for WLAN applications. The fundamental and higher order resonant harmonics of a microstrip open-loop resonator are analyzed against the length of the loading open stub. Then a dual-band BPF with controllable second passband is designed in the form of slow-wave open-loop resonators. A zero-degree feed structure is employed to produce transmission zeros near the passband to increase the rate of attenuations, and a quarter-wavelength feed line for impedance matching is added to get desired external quality factor at the input and output. The theoretical analysis and design procedures are successfully verified by experimental results.Thirdly, a novel method is proposed to synthesize dual-band bandpass filters from a prototype lowpass filter. By implementing successive frequency transformations and circuit conversions, a new filter topology is obtained which consists of only admittance inverters and series or shunt resonators, and is thereby easy to be realized by using conventional distributed elements. A microstrip dual-band BPF with central frequencies of 1.8GHz and 2.4GHz is designed and fabricated using microstrip lines and stubs. The simulated and measured results show a good agreement and validate thereby the proposed theory.Finally, a design theory for dual-band bandpass filter with controllable bandwidths is proposed by using new dual-band resonators. The resonator consists of a one-section and a two-section microstrip open-stub. Two passbands are separated by three transmission zeros. The first transmission zero and the third transmission zero are caused by the quarter-wavelength and three-quarter-wavelength resonant of the two-section open stub. The transmission zero in-between the two passbands is caused by the quarter-wavelength resonant of the one-section open stub. Formulas for designing fully controllable bandwidth dual-band BPF are given. A dual-band BPF operating in 2.4GHz-band and 5.2GHz-band is designed and measured. All the theoretical analysis and design procedures have been successfully verified by experiment results.