Research On Microstrip Bandstop Filters And Balanced Bandpass Filters

Nowadays,the advance of wireless communications technology has brought convenience to people’s learning, work and life. However, this technology is facing the bottleneck of limited frequency resources. As a frequency selective device, the property of the bandstop filter affect the performance of the entire communication system directly. On the other hand, a communication system faces the interference noise, which limits the operable minimum signal level. Compared with the traditional non-equilibrium filter with single-ended signals handle ability,the most important advantage of the balanced filters is high immunity to the environmental noise. As the wireless communication equipments develop toward the direction of miniaturization, low loss, anti-noise and integration, the study of the balanced filter is becoming a hot topic of research. Based on planar microstrip structure, this paper mainly studies the design method of bandstop filters and balanced filters. The main innovation of this article are as follows.(I) The research of bandstop filters with high selectivity. Bandstop filters are the basis of the balanced filter design.Multi-path by embedding short-circuited stub in the conventional branch-line bandstop filter are presented in the broadband or dual bandstop filter design. This method shows high selectivity and reduced size.(II) Source-load coupling is presented in this paper to design balanced filter with four controllable transmission zero, which improve the selectivity of passband and common-mode rejection.(III) Modified coupled feed-lines are proposed and implemented in the design of balanced bandpass filters with highly selectivity and high common-mode rejection. Modified coupled feed-lines are implemented by loading short- and open-circuited stubs at the end of the traditional parallel-coupled feed-lines to perform bandpass and bandstop response, respectively, which realized the common-mode rejection in the feed port. To achieve higher selectivity, coupled-line stub and its stub–load resonator also been applied.(IV) The wideband balanced filter with multilayer dielectric and back-to-back structure is presented. SIR slot-line resonator can achieve strong coupling, and back-to-back microstrip SIR resonator separates differential-mode and common-mode resonant frequencies, and also introduce two transmission zeros to achieve high selectivity in differential passband and high common-mode rejection, simultaneously.(V) The implement of slot-line structure in balanced filters are proposed. Ultra-wideband balanced filters with notch-bands are designed based on the slot structure and embedded spur-lines. By controlling the three resonant modes of the slot resonator and the coupling of the microstrip-slot structure, a UWB differential-mode can be achieved. Also, the using of embedded spur-lines can achieve notched-bands, which controlled by its length and offset. In addition, the slot-line coupling feed structure and its application in multi-band balanced filters and balanced diplexers is presented.