Design And Implementation Of X-band High Isolation Circulator

In the single antenna continuous wave radar system, the transmit signal leakage is an important factor limiting its development, so it is necessary to solve the isolation problem between the transceivers. With TR components used in continuous wave radar more and more widely, the circulator with duplexing has become one of the indispensable elements. In terms of its development prospect in continuous wave radar applications, in addition that the device will be keeping small and in a planar form, it also requires high isolation, wide working frequency band and other characteristics. Therefore it has an important significance of studying the high isolation characteristics of circulator.Based on the practical demand, firstly this paper conducted an investigation and analysis on the high isolation technical index of circulator. Considering the current design situations of circulator, this paper uses amplitude and phase cancellation techniques to achieve high isolation property. On this basis, depending on whether to use the ferrite materials to design circulator or not, this paper uses substrate integrated waveguide(SIW) and microstrip transmission line these two technical solutions and then analyzes the relevant devices. In detail, this paper describes the related design theories of ferrite circulator and coupler, the fundamental property of its necessary magnetic ferrite material in high isolation ferrite circulator, and the design theory of power divider in hybrid circulator. At last, this paper gives electromagnetic simulation and these two solutions are fabricated and tested.In the design of high isolation ferrite circulator program, firstly a dual Y junction circulator with microstrip transmission line is designed. Compared with the single Y junction circulator it has higher isolation, smaller size. Then a ferrite circulator using SIW transmission line is designed which adopts the properties of an inductive metal window and the embedded feed-line transition structure of SIW to microstrip transmission line. Thus it not only makes all metal via be through-hole reducing the difficulty of processing, but also that compared to the conventional linear transition, this method has the advantage of reducing the area by 41%, on the premise of having the same working bandwidth and isolation characteristic. In addition, a two sections microstrip branch line coupler is simulated with good amplitude and phase characteristics. Finally, a high isolation circulator using the simulated SIW circulator and coupler is fabricated and measured. Compared to the simulated result, the measured result has such problems as a larger insertion loss and a frequency offset.Given that the high isolation ferrite circulator program has a large dimension and its differences between the measurement and simulation, this paper designs a hybrid circulator, with its working principle based on the phase characteristics of 180°hybrid junction and fine isolation symmetry between the two power dividers to achieve high isolation characteristic. In order to improve the isolation characteristics in the design of two-port splitter, this paper uses slotted matching method to compensate for the capacitive effects generated by the mismatch of a 50 ohm resistor. Simulation results show that the hybrid circulator has a higher isolation, a smaller size and wider frequency band characteristics. Due to the high isolation characteristic of the hybrid circulator, which requires high processing precision, operating frequency band of the measured results produces a certain offset because of the deviation of machining errors and precision. However within the required operating frequency band of the system, it can fulfill the requirements of 40 dB high isolation.