Design and Simulation of Microstrip Directional Coupler with Tight Structure and High Directivity*

The design of Cyclotron CYCHU-10 has been carried out at Huazhong University of Science and Technology (HUST). Because of the low centre frequency (99MHz) of its RF system, we should choose suitable directional couplers for the RF system which is supposed to be highdirectivity and tight-structure. This paper analyzes and synthesizes kinds of directional couplers, especially microstrip structure and lumped element coupler, for their tinier volume at the low centre frequency compared with other structures. The achievement of the high-directivity with proposed configuration is carried out by adding an inductor to decrease the even and odd mode phase difference. Tight-structure is performed by the Lumped elements, which is easy to fabricate due to the multilayer microstrip line techniques used in MICs and MMICs. INTRODUCTION After the first directional coupler was reported, various types of directional couplers consisting of one quarter wavelength transmission lines (TLs) have been used in many RF systems [1]. And many researches on the theory, design, fabrication and simulation for the TEM-lines such as coupled-line directional couplers were made and numerous relevant papers were published [2]. Due to the higher level of integration of RF circuits on a single substrate, microstrip directional couplers have gotten much attention and been widely used for realizing such directional couplers in the design of RF systems [3]. However, because of the inhomogeneity of microstrip lines, the even and odd mode propagation velocities for coupled microstrip lines are not equal resulting in poor directivity, which becomes worse as the coupling is decreased[4], it is necessary to apply phase velocity compensation techniques to improve coupler directivity. Several previously techniques have been used to resolve the problem of poor directivity in parallel coupled microstrip lines. Generally speaking, the design theory for TEM TL couplers is based on an assumption of the same phase velocities of the even and odd propagation mode. This is not suitable for coupled microstrip lines, since they have unequal evenand odd-mode phase velocities. Previously compensation techniques can be classified into distributed and lumped compensation approaches. The former is to modify the parallel coupled-line structure such that the phase velocities of both modes are equalized. Such as typical wiggly-line coupler which increases the physical lengths of the adjacent edges of the microstrip lines. The latter involves adding external reactive components which are capacitive and inductive compensation techniques. Compensated microstrip directional coupler and the edge capacitors or inductors added affect the odd mode by equivalent extension of the TL electrical lengths, with almost no effect on even mode [5]. Furthermore, lumped elements (LEs) are very attractive in the sense of smaller size and lower cost compared with TL circuits, due to their smaller element dimensions and the multilevel fabrication process. With these characteristics, the LE design methods have been already applied to TL circuits such as a branchline coupler or a rat-race [6]. LUMPED ELEMENT DIRECTIONAL COUPLERS By taking the longitudinal symmetry, the TL directional coupler can be divided into the evenand odd-mode TLs [7]. And because the LE coupler circuit is fully based on equality with a TL coupler in the both operations, the proposed LE coupler model shown in Fig.1 has been derived by investigating the both modes TLs. Shown in following circuit figures, there’re such equalities as follows 2 / 2 / 2 1 2 e1 L L L L e (1) e4 e3 e2 e1 C C C C (2)