Research And Design Of Broadband Directional Coupler

In recent years the Internet of everything has become the development trend of society,wireless communication technology is developing rapidly,the RF front-end circuit in the RF microwave devices continue to broadband,miniaturization,anti-jamming ability of the rapid development of the direction,as a class of important passive devices in the RF microwave devices,the indicators of the directional coupler also have higher requirements.Currently common is the microstrip line and ribbon line directional coupler.As the microstrip line transmits quasi-TEM waves,while the ribbon line transmits TEM waves,the ribbon line can be used as a coupling transmission line to design strongly coupled,highly directional coupled line directional couplers,which is beneficial to enhance the performance of directional couplers.The traditional coupled-line type weakly coupled directional coupler requires a high distance between the coupled lines and is prone to processing errors.This thesis proposes a new resistive type weakly coupled directional coupler,consisting of a transmission line and a resistor,which makes the coupling between the transmission lines negligible and reduces the processing difficulty.The main contents of this thesis are:1.For the structure of an n-section parallel coupled-line directional coupler,an ideal coupler model is established in a commercial electromagnetic tool,and the advantages and disadvantages of a symmetrical type directional coupler with n（n is an odd number）sections of coupled lines directly cascaded and a directional coupler using two symmetrical couplers cross-cascaded are investigated and compared.The simulation results show that the former is smaller in size and the latter larger in size for the same bandwidth and coupling degree;the even-mode impedance value of the latter intermediate coupled line is smaller than that of the former without regard to size,making it easier to process and fabricate and thus reduce errors.2.Design of a broadband 3 d B Tandem directional coupler with an operating frequency range of0.7-4.2 GHz.The structure of this directional coupler consists of a five-section symmetrical weakly coupled directional coupler and a single-section weakly coupled directional coupler cross-cascaded.The simulation results for this directional coupler are good,with return loss and isolation better than18 d B,in-band fluctuations of up to 0.4 d B at the pass-through end S₂₁ and the coupling end S₃₁,and a phase difference of（90±0.5）°between the pass-through and coupled outputs.3.Design a quadrature directional coupler that can maintain good performance at low temperatures（24 K）.The directional coupler uses suitable materials,a parallel bias-coupled ribbon line as the coupling transmission line,an overall structure symmetrical about the centre,a coupling degree of 3 d B,an operating frequency range of 0.7-4.2 GHz,and the output signal of the straight-through end and the coupling end in phase orthogonal.Through repeated optimisation and adjustment,a directional coupler with good operating performance is obtained.The simulation results show that the return loss and isolation are better than 20 d B,the in-band fluctuation between the straight-through end S₂₁ and the coupling end S₃₁ is 0.25 d B at maximum,and the phase difference is（90±1.4）°.4.Resistive weakly coupled transmission line directional couplers with coupling degrees of 20,30 and 40 d B were designed using transmission lines and resistors respectively,and their operational performance was summarised.The results show that the return loss is greater than 22 d B,the isolation is greater than 22 d B,the coupling is around 20 d B and the insertion loss is a maximum of 2.7 d B in the 1-3 GHz bandwidth.