Microwave Dual-band Directional Couplers Based On Coulpled Microstrip-slotlines

The advent of the Internet of things and the era of intelligence have led to the development of modern wireless communication systems in the direction of dual/multi-band and miniaturization.As an important component in wireless communication systems,90-degree directional couplers have been widely used in circuits such as antenna feeding networks,balanced power amplifiers,quadrature mixers,fundamental frequency multipliers,and power monitors.Most of the existing dual-band directional couplers are designed based on branch-line couplers,resulting in problems such as narrow bandwidth and oversized circuit area.Dual-band couplers based on edge-coupled microstrip lines can effectively reduce the circuit size,but the microstrip line spacing is limited by the PCB processing process,making it difficult to achieve both high frequency ratio and tight coupling(e.g.-3d B).In addition,dual-band couplers are required to have unequal power output in some applications.To address these issues,this dissertation investigates coupled microstrip-slotline with high coupling level and designs three new dual-band directional couplers based on coupled microstrip-slotlines:Dual-band directional coupler based on coupled microstrip-slotlines,compact dual-band coupler based on coupled microstrip-slotlines,and unequal power dual-band coupler based on coupled microstrip-slotlines.The content of this dissertation is summarized as follows:1.Firstly,the basic theory of coupled microstrip-slotline is briefly analyzed.A novel 3-d B dual-band directional coupler based on coupled microstrip-slotlines is proposed.Dual-band operation is achieved by cascading two pairs of coupled microstrip-slotline and adding two symmetrical open-circuit branches.The designed dual-band coupler operates at 1/2 GHz,and the flared open end of slot line results in a large circuit area(0.3449?_L×0.9698?_L).In order to achieve the same coupling level on the same substrate,the microstrip line spacing required for edge-coupled microstrip lines is 0.01mm,which is difficult to fabricate in the PCB process.However,the width of the slot line in the coupled microstrip-slotline is 0.15mm,which is easy to fabricate.The proposed dual-band directional coupler has the advantage of being able to achieve a high coupling level.2.A compact dual-band coupler based on coupled microstrip-slotlines has been designed.By cascading three pairs of coupled microstrip-slotlines and reasonably designing the even and odd mode impedance of each pair of coupled lines,dual-band operation can be achieved.It is very difficult for the edge-coupled microstrip line dual-band directional coupler to achieve a frequency ratio of 2 under the-3d B coupling level condition;an additional rectangular slot on the metal ground is required to achieve such tight coupling.In the case of high frequency ratio(more than 3),such couplers can only achieve weak coupling.This problem is solved by using coupled microstrip-slotline in this dissertation.For demonstration,a 3-d B dual-band coupler operating at 1/3 GHz with the size of 0.387?_L×0.047?_L and a 3-d B dual-band coupler operating at 1/2 GHz with the size of 0.589?_L×0.024?_L are designed,fabricated,and measured.The simulated and measured results are in good agreement.3.An unequal power dual-band coupler based on coupled microstrip-slotlines is designed,fabricated,and measured.By using four pairs of coupled microstrip-slotlines to replace the transmission lines of traditional branch-line coupler,the unequal power output can be achieved in both frequency bands through a reasonable configuration of the even and odd mode impedance.Moreover,the problem that the different even-and odd-mode phase velocities of the coupled microstrip-slotlines in the middle section of the coupler proposed in Chapter 4 cannot be compensated,is solved.The coupler operates at 2.4/4.8 GHz and has a size of 0.1607?_L×0.1546?_L.Finally,the design has been fabricated and measured,and the simulated and measured results are in good agreement.The new design has the advantages of compact size and unequal power output.Since the length of the slot line in each pair of coupled microstrip-slotlines can be adjusted independently,the different even-and odd-mode phase velocities of coupled microstrip-slotlines can be compensated to improve the isolation,which is not available for edge-coupled microstrip line couplers.