Microwave Bandpass Filters Tuned by the Magnetization of Ferrite Substrates

C-band (around 5 GHz) microwave bandpass filters fabricated on yttrium-iron-garnet (YIG) and Al-doped-YIG ferrite substrates are tuned by an external magnetic field of 0 to 720 Oe. The filters on Al-doped YIG with lower saturation magnetization exhibit lower tuning ranges and sensitivities but reduced insertion losses and expanded bandwidths. The tunabilities of the filters are related to their partial or full magnetization states. Full magnetization states have three regimes: low field, resonance, and high field. The resonance regime cannot be used for bandpass filters. The detailed tuning mechanism is analyzed based on the dynamic magnetization states of magnetic materials. The results will provide a framework for the design of magnetically tunable bandpass filters.