Chebyshev-type diplexers designed using coupling matrix method

This paper presents a method for the design of cross-coupled planar microwave filters, which have a pair of attenuation poles near cut-off frequencies and are applicable to diplexers. The attenuation poles will improve the selectivity of the filter and can be obtained by modifying the traditional Chebyshev frequency response. A practical design technique using coupling matrices for the Chebyshev filters is introduced, including theoretical and simulated results. By synthesizing the coupling matrix, the coupling coefficients between the resonators and the input/output singly loaded quality factor Q are determined. The result obtained from analyzing the coupling matrices shows that the return loss and frequencies of attenuation poles can be adjusted by tuning the coupling coefficients. The design example of a new type cross-coupled filter with fractional bandwidth of 1.375% at 2.4 GHz is described. The coupling coefficients of different dimensions are derived from an ideal model and the proper values to realize the coupling matrix are chosen. This method is general and useful for the design of high performance microwave filter in a wide variety. By integrating two filters with different resonant frequencies, the application to a diplexer design is demonstrated.