Design-Oriented Modelling of Microstrip Ferrite Circulators

A modeling approach for the design of microstrip ferrite circulators is presented and validated on several examples using the same commercially-available ferrite disk. A baseline narrowband 4.25-GHz microstrip circulator is first demonstrated with a commercial 4.97-mm radius ferrite disk operated in saturation and below the ferromagnetic resonance. The non-uniform DC magnetic field distributions of a cylindrical permanent magnet is taken into account by spatial discretization of the ferrite properties in full-wave simulations. Several design parameters are shown to affect the frequency response; the ferrite thickness relative the microstrip substrate thickness shifts the operating frequency, while external matching networks can increase the fractional bandwidth from 10% up 40%. Another degree of freedom is the applied DC magnetic field, which can be reduced to set the ferrite operation below the ferromagnetic resonance with significant miniaturization of the overall device, as demonstrated with a 1.6-GHz circulator designed with the same 4.97-mm radius ferrite disk, resulting in an almost factor of 3 reduction in linear electrical size.