II
Abstract
In modern communication and microwave systems, directional couplers have a wide
range of applications, and become an important part of many microwave circuits. With
the rapid development of communication technology, the targets of directional couplers
have become more and more high, and we hope directional couplers have wider
bandwidth, smaller size, higher directivity and better reliability. Because of the
advantages of small size and broadband, the coupled strip line directional couplers
become a research hotspot. I use the coupled strip line structure to design directional
coupler in this paper.
In this paper I complete the design of strip line directional coupler, and its operating
frequency is 30MHz-512MHz, coupling is 50dB, directivity is more than 13dB, coupled
flatness is ±0.7dB, VSWR is less than 1.25, IL is less than 0.25dB, the required size is
less than 40mm×30mm×10mm. In order to meet the target, the final design uses the three
coupled strip line structure, and adds attenuation network to achieve the coupled flatness
requirement.
First, working principle and technical indicators of directional couplers are
introduced. Since the coupled strip line structure is built on the basis of the strip line, so
the structure and fundamental characteristics of the strip line are analyzed. Then I
describe the method of analyzing coupled strip line in detail, which is odd and even mode
analysis method. The relationship between the coupling coefficient and the odd and even
mode characteristic impedances is given, and the odd and even mode characteristic
impedances or admittances of several common coupled strip line structures are derived.
Finally, I focus on the design of the coupled strip line directional coupler. According
to the technical requirements, the solution design is determined, and the coupled strip
line structure sizes are calculated. Then the circuit simulation (ADS) and modeling
simulation (HFSS) are finished, and the parameters of structure sizes are verified.
Production samples are produced and tested by vector network analyzer. The results of
