Investigation On Miniaturized Lossy And Frequency-dependent Coupled Filters

High-performance miniaturized filters with easy integration and low cost are desired in various modern microwave communication systems. However, resonators with small geometries usually have low unloaded Q-factors(Qu). Low-Qu resonator will degrade filter performance, especially for narrowband filters, i.e. increased insertion loss, rounded band-edges, and partially or completely obliterated in-band ripples, etc. In order to overcome these problems, several advanced synthesis techniques for lossy filters are proposed, which take the limited Qu of resonators into account during the design of filters and optimize frequency selectivity and passband flatness simultaneously.On the other hand, one of the most common ways to improve filter selectivity is to introduce transmission zeros. With the sophisticated approaches, transmission zeros are achieved through cross coupling(including source-load coupling), multimode resonator, nonresonating nodes and so on. Another promising approach is by using frequency-dependent coupling, which can yield a higher number of transmission zeros without cross-coupled topologies.In this paper, the investigations are focused on the miniaturized filters based on lossy synthesis and frequency-dependent couplings. The main contribution can be summarized as follows:(1) A lossy triple-mode microstrip filter is designed with flat passband by using the nonuniform-Qu method. From theoretical analysis and numerical simulations, it is known that the predistortion technique can improve the in-band flatness at the expense of significantly degraded return loss and that the matrix synthesis method of lossy coupling may lead to very complicated scheme to be realized. Compared with the former two methods, the nonuniform-Qu method is easy to implement with minor drawbacks. The triple-mode microstrip resonator has three eigenmodes which determine the specific resonant frequencies of the filter. By tuning the loaded resistor to adjust the Qu of odd-mode resonance independently, the power dissipation is distributed properly. Then, the passband flatness is improved significantly with the nonuniform-Qu method.(2) A filter and a four-way filtering power divider are designed with frequency-dependent couplings. The N+2 coupling matrix is first optimized with frequency-dependent couplings by fitting the eigenvalues of a transversal coupling matrix and its submatrix. Then, the lumped and distributed circuit models of linear frequency-dependent K/J inverters are presented and analyzed. A third-order inline microstrip filter with frequency-dependent couplings is further designed. Finally, it is extended to a four-way power divider with bandpass filtering response by the method of even- and odd-modes. The collaboratively designed component has both high frequency selectivity and high in-band port isolations.It is concluded that the passband flatness of multimode filter, due to the transversal topology, is improved by properly distributing dissipation of resonators while the multiway filtering power divider is constructed by introducing frequency-dependent couplings into an in-line coupled resonator filter/power divider. By these methods, the miniaturization of passive components can be achieved.