An Equivalent Circuit Model for Tunable Bandpass Filters Based on Ferromagnetic Resonance
An equivalent circuit model for transmission line loaded thin film ferrimagnetic material is proposed, which can be utilized to synthesize the bandpass filters. Theoretical derivation based on Landau-Lifshitz-Gilbert (LLG) equation and Kittel’s equation is applied to model thin film ferrite as a parallel RLC resonator, whose resonant frequency is determined by the ferromagnetic resonance (FMR) frequency. The transmission line loaded thin film ferrite structure is simulated and the resultant impedances match with that obtained from equivalent circuit model by tuning the effective thickness of the ferrite. The ferrite resonators are utilized to construct the 3rd order bandpass filters at FMR frequency. The filter center frequency tunability is realized by changing the magnetic bias field strength while the relative bandwidth tuning is realized by changing the coupling coefficients. The BaFe12O19 (BaM) ferrite is considered, whose internal anisotropy magnetic field is able to increase the FMR frequency to millimeter wave band. The effects of ferrite thickness, conductor width and transmission line structure are included.