Co-simulations of DC magnetic bias fields and RF performance for microwave ferrite circulators

This paper presents finite element co-simulations for DC magnetic field and microwave non-reciprocal performance of ferrite circulators. Permanent magnets provide a DC bias field, but need to be large for a uniform magnetic field distribution inside the ferrite material. Additionally, the ferrite effective permeability is a nonlinear function of magnetic bias. Smaller magnets give a non-uniform bias field distribution, which can be taken into account in the full-wave simulations. To demonstrate the effect of this nonuniform bias, a commercial ferrite material from Skyworks (TT105) is chosen for a circulator design around 3.9 GHz assuming first a uniform DC magnetic field that saturates the ferrite. A small commercial rare-earth permanent magnet is then simulated to obtain the nonuniform static magnetic field in the ferrite, and resulting HFSS simulations show that the effects of nonuniform bias field on circulator performance can be predicted using a combination of analysis methods.