| 摘要 |
The performance of broadband isolators and circulators can be characterized by the ratio fmax/fmin, where fmin and fmax are defined as the edges of the frequency band in which the devices have acceptable operating characteristics. For the most advanced isolators and circulators available today this ratio is approximately 3:1. This invention teaches how to improve broadband performance substantially. The present limitations are shown to be primarily due to two causes: 1.) lack of bias field homogeneity, and 2.) previously unrecognized low-field loss due to excitation of magnetostatic surface waves. These surface waves are excited at the dielectric/ferrite interfaces on the side faces of the ferrite platelets or discs in the devices. For stripline edge-mode isolators and stripline circulators, the undesired low-field loss can be reduced by using certain rf device structures in combination with suitable bias magnets. These rf structures have a high-magnetization ferrite in the center region and lower-magnetization ferrites in the peripheral regions of the device. The bias magnets generally include high-permeability pole pieces, either in close proximity to the rf structure, or separated from it by composite pole shoes containing the same magnetic microwave materials inserted into the rf structure. It is estimated that fmax/fmin ratios of about 6:1 are possible for properly designed devices using two microwave ferrites, whose saturation magnetizations are in the ratio of 2:1. Higher values of the fmax/fmin ratio are possible when more than two microwave ferrites are used.
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