Integration Junction Circulator Design Method Based On Perturbation Theory

Microwave ferrite circulator has important applications in modern communication and radar systems. With the rapid development of microwave integrated circuit, it's necessary to study ferrite integrated junction circulator. For this purpose, the design theory and method of microwave integrated planar junction circulator have been studied in this work.Accoding to the cylindrical cavity consisting of ferrite layer and dielectric substrate, the boundary conditions of the cavity are discussed firstly. Based on the Maxwell equations and perturbation theory, the electromagnetic field distribution in the ferrite / dielectric substrate cavity have been obtained. The formulas of the junction radius and junction input impedance have been deduced. The applicable conditions of this design method have also been discussed.With the design method, an integrated self-biased microstrip junction circulator using non-incremental quarter wavelength impedance transformation had been designed and optimized. The simulation results show that the insertion loss of the integrated circulator is less than 3 dB, and the isolation loss is better than 10 dB in the frequency range of 29.57 GHz to 29.82 GHz. The maximum isolation loss is about 23 dB at 29.8 GHz while the minimum insertion loss is about 0.2 dB. There is only about 250 MHz frequency bandwidth more than 10 dB isolation.To improve the bandwidth of the circulator, considering that the real part and imaginary part of the junction impedance change with frequency, a broadband impedance matching network of the junction circulator had been designed using Smith Chart impedance matching tool in ADS simulation software. The simulation results show that the insertion loss of the circulator is less than 3 dB, isolation loss is greater than 10 dB in the frequency range of 27 GHz to 31 GHz, and the bandwidth is about 4 GHz. The designed circulator was prepared and measured. The experiment results show that the circulator has a good broadband performance between 29 GHz and 31 GHz with isolation more than 10 dB. Finally, we have studied the coplanar waveguide integrated junction circulator. The field distributions of the coplanar circulator had been modified by adding a round bottom electrodes. The influences of geometric parameters on device performance had been discussed. Then we analyzed this kind of coplanar waveguide junction circulator. The simulation results show that the insertion loss is less than 3 dB, isolation loss is better than 10 dB in the frequency range of 30.4 GHz to 31.5 GHz, the maximum isolation loss is about 26 dB at 30.9 GHz while the minimum insertion loss is about 0.1 dB.