This article proposes a generalized strongly coupled resonator quartet (GSCRQ) filter along with its synthesis approach. By introducing out-of-band reflection zeros (RZs), the proposed GSCRQ can generate a transmission zero on each side of the passband without negative couplings. The coupling coefficients in this coupling structure change with the positions of the out-of-band RZs. Thus, the GSCRQ configuration admits flexible design solutions. For GSCRQ coaxial combline filters, all couplings can be implemented as inductive couplings, simplifying the design and manufacturing process. In this article, a 6-2 filter in the GSCRQ configuration is synthesized and designed. The simulated results of the designed filter agree very well with the theoretical characteristics.

A new optimization method is proposed to realize the synthesis of duplexers. The traditional optimization method takes all the variables of the duplexer into account, resulting in too many variables to be optimized when the order of the duplexer is too high, so it is not easy to fall into the local solution. In order to solve this problem, a new optimization strategy is proposed in this paper, that is, two-channel filters are optimized separately, which can reduce the number of optimization variables and greatly reduce the probability of results falling into local solutions. The optimization method combines the self-adaptive differential evolution algorithm (SADE) with the Levenberg-Marquardt (LM) algorithm to get a global solution more easily and accelerate the optimization speed. To verify its practical value, we design a 5G duplexer based on the proposed method. The duplexer has a large external coupling, and how to achieve a feed structure with a large coupling bandwidth at the source is also discussed. The experimental results show that the proposed optimization method can realize the synthesis of higher- order duplexers compared with the traditional methods.