Design Of Miniaturized And High Performance Microstrip Filter

The current tendency of microwave technology is towards miniaturization, highintegration, and low cost. Microwave filter is one of the most important components ofcommunication system. And its performance will play a key role in the wholecommunication system. As the rapid progress and application of wireless communicationtechniques, the finite electromagnetic spectrum has been occupied by more and moresystems. And the useful frequency bands are becoming very crowded. Therefore,miniaturization, high frequency selectivity and good out-of-band suppression of filtershave attracted much attention from researchers. Planar microstrip filters not only have theadvantages of smaller sizes, lower weight, lower fabrication cost but also integrate withother planar circuits more easily when compared to conventional metallic waveguidefilters. Based on the background mentioned, my work involved in this thesis focused onhow to realize miniaturized and high performance microstrip filters.The main work and innovation can be summarized as:1. Design and implement a fourth-order cross coupled microstrip bandpass filter. Thenovelty includes the following three points:First of all, locate a T-shaped stub-loaded dual-mode resonator between two coupledhalf-wavelength resonators, and realize a novel fourth-order cross coupled filter withoutadding any extra circuit sizes. Its structure is compact and a pair of transmission zeros aregenerated around the passband to improve the frequency selectivity by the proposedfourth-order cross coupling topology.Secondly, adopt the asymmetrical input/output tapped lines. Introduce one more pair oftransmission zeros around the passband to make the transition from passband to stopband more sharper without adding any extra structures.Finally, load the lowpass structure composed of the anti-coupled lines with aopen-circuited resonator at the input/output tapped lines. The cutoff frequency of thelowpass structure is set far away from the center frequency of the bandpass filter.Thus theharmonic is successfully suppressed without degrading the in-band performance and thestopband bandwidth is extended to approximately five times center frequency.2. Design and implement a compact dual-band filter composed of two pairs ofhalf-wavelength resonators. The main work in this section has two parts:First, propose and verify a novel coupling scheme which can be used to achieve adual-band filter. The dual-band filter based on this coupling scheme owns the advantagesof flexibly adjusting the distance between two passbands, the bandwidths of each bandsand so on.Second, further miniaturize the lowpass structure used before. And make use of itslowpass character to suppress the harmonic and extend the stopband bandwidth.3. Utilize the double-sided parallel-strip line to design a balun filter exhitbing baluncharacters and frequency selection performance at the same time. The novelty is listed asfollows:First, no extra structures are needed to achieve180degree phase difference between twooutput ports due to the inherent out-of-phase character of DSPSL. Thus, the circuit size isreduced and it is suitable for miniaturization.Second, propose a novel second-order coupling topology with source-load coupling andapply it into the design of dual-band balun filter.