Synthesis of Equi-Ripple Dual-Band Filters With Complex Reflection Zeros

In this letter, an iterative equi-ripple dual-band synthesis method is presented based on reduced Chebyshev functions. The complex reflection zeros (CRZs) are introduced to synthesize novel dual-band filtering functions. Through regulating the CRZs, additional synthesis freedom can be obtained to achieve fully independent control of the return loss levels, orders, and the locations of the transmission zeros (TZs) of each band. Moreover, the number of negative couplings can be effectively reduced aided by the CRZ. The relationship between the CRZ positions and the return loss levels is analyzed quantitatively. Meanwhile, there exists a tradeoff between out-of-band and interband attenuation when setting the CRZ positions. The detailed recommendations are given for the selection of the CRZs to synthesize a satisfactory dual-band filter response. Finally, several synthesis examples are provided to demonstrate the advantages of this approach compared to previous methods.