Design Of Filters With Source-load Coupling

With the rapid development of wireless communication and microwave integrated circuit technology, the miniaturization of electron equipment and filter becomes a very important problem. So, developing miniaturized microwave filter with high performance is a popular topic nowadays. The use of filter with source-load coupling can reduce filter order and insertion loss and improve skirt selectivity. This paper was given emphasis to the design of filter with source-load coupling, and studied related theories and technologies.Firstly, recent research works about miniaturization of microwave filter are introduced and summarized. The concepts about filter and resonator are introduced as basic theories.Secondly, we analyze the even-mode and odd-mode resonance frequency of dual-mode T-shaped resonator and its application in filter bandwidth design. Coupled lines and the equivalent circuit are introduced, and the equivalent capacitance is calculated.Then, a T-shaped resonator filter with source-load coupling by using lumped coupled elements between ports is proposed. The impact of source-load coupling strength on filter characteristic is discussed, it shows that the frequency response of filter has four extra controllable transmission zeros in the stopband at most because of the multiple signal transmission paths and controllable source-load coupling strength. The positions of transmission zeros in the stopband are controllable by adjusting the coupling strength between ports. At the same time, the center frequency and bandwidth are both not changed. The bandwidth of filter can be just adjusted by the different length of short-circuited stub of T-shaped resonator. Based on the filter proposed, other three filters with source-load coupling are proposed and discussed.1. A T-shaped resonator filter with source-load coupling by using open-circuited coupled lines between ports. The length of coupled lines can control transmission zeros. Fourth-order and six-order filters are designed by cascading two or three second-order filter. 2. A T-shaped resonator filter with source-load coupling by using open-circuited coupled lines between ports. The length of coupled lines also can control transmission zeros. In addition, there is no parasitic passband and the second harmonic is suppressed under 20dB.3. A dual-band filter composed by T-shaped resonator andλ/2 resonators with source-load coupling. T-shaped resonator control operating frequency and bandwidth of lower passband,λ/2 resonators is utilized to not only control operating frequency and bandwidth of the upper passband but also feed the lower passband resonator with source-load coupling, which makes filter generated five transmission zeros and have good skirt selectivity. The bandwidth of lower and upper passband is controllable.