Based On The Parasitic Passband Microstrip Dual-band Bandpass Filter Design

Development of dual-frequency and multi-frequency communication systems becomes an important trend of wireless communication technology. As a pivotal component of the communication systems, dual-band or multi-band RF filters play a more and more important role. In this dissertation, the design of dual-frequency filters has been intensively investigated based on studies on the synthesis theory of cross-coupled filters and the parasitic passband characteristic of resonators.First, the synthesis theory of cross-coupled filters has been discussed, with which excellent stopband restrained performance can be realized by introducing appropriate transmission zeros. Then we conduct theoretical analysis on SIRs and demonstrate that SIRs have flexible parasitic passbands. Thus, a method of designing dual-passband filters is proposed by utilizing parasitic passband of SIRs to perform the second passband .Next, a dual-passband filter has been designed with the slow-wave SIR resonators. Simutation studies of the slow-wave structure is conducted to obtain the desired dual-band passband. Dual-frequency impedance-matching technology is considered to meet the need of dual-band. A 0°feed structure has been introduced to improve the stop-band characteristics. The filter has been fabricated and measured. It is shown that the filter operates at 2.4GHz and 5.1GHz, with the in-band losses of 1.1dB and 2.3dB, and the fractional bandwidth of 4.58% and 2.56%, respectively.Finally, another filter has been further developed with miniatured hairpin resonators and cross-coupled structure. Miniaturization of resonators is extensivelly investigated. Cross-coupled structure has been studied in detail to acquire better stopband attenuation of the filter. The design is mainly focused on the miniaturized design of resonator and the extraction of cross-coupled matrix. Simulated and measured results agree very well, which shows that dual-band filter resonates at 2.35GHz and 5.15GHz, with the fractional bandwidths of 3.83% and 2.46%, respectively.In conclusion, we have demonstrated that the performance of dual-band filters can be effectively improved based on the cross-coupled filter synthesis theory and the parasitic passband characteristic of resonators.