Design And Realization Of Filters Based On Mixed Coupling With Multiple Transmission Zeros

As a crucial passive component in RF and microwave circuits, whose performance will directly determine the quality of the entire system, microwave filters play a more and more important role in modern wireless communication systems. Therefore, the design on the filter which posses the advantage including good frequency selectivity, high out-of-band rejection, small size, low price has become a research hotspot in modern filter design. In this paper, a series of compact filters with high selectivity, good out-of-band rejection are proposed based on the mixed electric and magnetic coupling theory.First of all, the paper presents the basic theory in filter design and analyzes the filter transfer function and low-pass model in detail. Then the paper presents and defines the concept of mixed electric and magnetic coupling by K converter theory. Based on traditional coupling matrix, mixed coupling matrix is established according to the generalized chebyshev function model. Varieties of filter design with high selectivity, broad stopband and compact size are proposed based on the theory above.Secondly, a half-wavelength stepped impedance resonator(SIR) mixed-coupled dual mode structure with short-circuit stub this paper was analyzed in this paper and a new circular SIR mixed-coupled structure was presented. A series of planar filter with high performance and compact size was presented based on this SIR mixed-coupled structure. Transmission zeros close-to-passband are introduced due to the cancelling effect of mixed electric and magnetic coupling to improve the frequency selectivity. By introducing the source-load coupling and π-type circuit in the filter, the stopband rejection can be effectively enhanced. An extra transmission zero was introduced near the passband owing to the combined effect of cross coupling and mixed coupling in the circular third-order mixed-coupled structure to realize the excellent filter with compact size and high frequency selectivity. High out-of-band rejection and frequency selectivity is achieved due to the transmission zeros caused by mixed coupling through cascading a couple of two-order mixed-coupled structure. Besides, the paper also proposed a compact dual-band filter. Another passband was employed due to the new mixed-coupled path. The filter has the advantage of compact size, high frequency selectivity and high passbands isolation.Finally, the paper proposed a new dual-layer structure SIR and then put forward a two-order dual-layer mixed-coupled filter which has compact size and high frequency selectivity by applying the dual-layer SIR in the mixed-coupled structure. A third-order mixed-coupled filter is designed based on the two-order filter. Transmission zeros near the passband, which is generated by the cross coupling and mixed coupling in the filter, are introduced to improve the frequency selectivity and out-of-band characteristics effectively.