The Integrated Design Of Broadband Coupled-line Directional Coupler

Typically, the directional coupler has been widely used in active circuits and passive circuits, such as balanced mixers, tuners, phase shifters, modulators, attenuators and filters, etc. It’s an important component of the microwave circuit.S.B.Cohn and other scholars have done a comprehensive analysis of the coupled stripline, which is convenient for the integrated design of broadband coupled-line directional coupler. This article will introduce the design methods of asymmetric multi-section coupled-line directional coupler, symmetric multi-section coupled-line directional couplers and asymmetric tapered-line directional coupler in detail. And writing the corresponding integrated design process to achieve rapid and convenient design.There are two main problems in the design of asymmetric multi-section coupled-line directional coupler. Firstly, the size of the low frequency directional coupler is too large. Secondly, the directivity and VSWR of couplers deteriorate with increasing frequency. we must take into account that making the structure smaller. In practice, a directional coupler working in the 70-960 MHz is designed. Getting the initial value of size by the integrated program. Optimizing the circuit model in the software of AWR. Then creating the three-dimensional model in HFSS and optimizing it. We made a detailed analysis for deterioration of directivity and VSWR. The main reason is the discontinuity of transition section between the coupler-lines. The problem can be solved by introducing compensating shunt capacitances in a number of cross-sections of coupled and signal lines. And this way achieves the requirements of engineering design.In the design of symmetric multi-section coupled-line directional coupler, the main problem is difficult to calculate any order odd polynomial in the comprehensive design theory. In this paper, using the Newton iterative method to approximate the polynomial. Getting the initial value of size by the integrated program. There is a good response curve in the circuit model that confirms the feasibility of this method. In practice, a directional coupler working in the 698-2700 MHz is designed. The discontinuity of transition section is also solved by introducing compensating shunt capacitances in a number of cross-sections of coupled and signal lines. The test curve of the coupler meets the engineering requirements.In the design of asymmetric tapered-line directional coupler, the main problem is the deterioration of the directivity and VSWR with increasing frequency. Currently, solving this problem by introducing periodic sawtooth in one end of tight coupling. However, the current theory doesn’t have quantitative analysis to determine the location, number, height and width of sawtooth. It only can be determined through optimization, which brings great inconvenience in the design. In HFSS model, comparing the change in the curve between introducing periodic sawtooth before and after. Directivity has a significant improvement in the model which introduces periodic sawtooth. It confirms the feasibility of this method.