Hexagonal Ferrite Self-biased Microwave Circulator

With the development of mobile telecommunication, works on microwave devices have a general objective to reduce the size of components, to be planar and to be integrated. Microwave circulator was one of the most important devices in telecommunication, which was irreplaceable. However, traditional circulators need additional permanent magnets, which leaded to giant volume and weight, so this kind of circulators was incompatible with the MMIC (Monolithic Microwave Integrated Circuit). Hexagonal ferrites, especially M-type barium ferrite, had high uniaxal magnetocrystalline anisotropy. When it was used in microwave circulators, the high uniaxal anisotropy can produce suitable internal field so that it is easy to integrate with microwave circuits, thereby realizing self-biased and significantlly reducing the size and weight of circulators. Therefore, self-biased microwave devices based on hexagonal ferrites have vital significance on light, miniature, integrated circulators.Working principles of circulators were studied and a self-biased dual Y-junction circulator was designed using hexagonal bulk materials, whose operating frequency is 31.5 GHz. The simulation results showed that the circulator had a good performance between 30.5 GHz and 32.5 GHz with insertion loss less than 1 dB and isolation more than 20 dB. The circulator was prepared and measured. The results showed that the prepared circulator had a good nonreciprocal characteristic. When the operating frequency was 27.75 GHz, the insertion loss of the circulator was 9 dB and the isolation was 37 dB. When the operating frequency was 30 GHz, the isolation was 45 dB and the insertion loss was 15 dB.Integrated circulators were designed with embedding and full magnetic substrate type in order to integrate with microwave circuits. The emdedding self-biased microstrip Y-junction circulator had a good performance around 32 GHz with insertion loss less than 0.5 dB and isolation appropriate 30 dB. Samples were prepared according to the design. Measurements showed that a good performance could be got around 35.8 GHz with insertion loss about 21 dB and isolation about 50 dB. The self-biased Y-junction circulator with full magnetic substrate had a good performance around 32 GHz with insertion loss less than 1dB and isolation about 40 dB. Measurements showed that a good performance could be got around 34.8 GHz with insertion loss about 20 dB and isolation about 52 dB. It could be clearly seen that from the testing results the insertion loss of integrated circulators was too large compared the bulk circulators. This was due to that the thick film of barium ferrites was not dense and the surface was rough.The design method of self-biased circulator with coplanar waveguide structure was also studied. A self-biased circulator with coplanar waveguide was also designed using ferrite thin films of 10μm based on the former method. Simulation illustrated that the operating frequency of the circulator is 34.1 GHz with insertion loss less than 1 dB and isolation around 20 dB. Measurements showed that a good performance could be got around 35.8 GHz. The insertion loss reached the minimum around 18 dB when the frequency is 33 GHz. The isolation reached the maximum about 45 dB when the frequency was 34 GHz.