Out of band suppression in MSW filters

Out-of-band rejection > 20 dB in magnetostatic forward-volume wave (MFW) bandpass filters requires the selective attenuation of undesired width-mode responses. Previously, suppression of the m = 3 mode by > 30 dB was demonstrated with an array of resistive strips. Here an alternative technique is described which is based on the coupling between the low-loss epitaxial YIG filter element and an adjacent lossy YIG film. The lossy films studied were Ho:YIG with ΔH ≃ 50 Oe and YIG damaged by polishing with ΔH≃ 2 Oe. The 20 μm thick low loss film was doped with scandium so as to have an internal field approximately 70 Oe lower than the lossy YIG film. Below the low-frequency cut-off of the lossy YIG film, MFWs can propagate in the Sc:YIG film with a small increase in attenuation. Above the lossy YIG cut=off frequency the MFWs, which now propagate in both films, are highly attenuated. This approach has produced m = 3 mode suppression of > 30 dB while attenuating the desired m = 1 mode by < 4 dB which is comparable to results obtained with resistive strips.