The frequency selective properties of non-linear spin wave interactions in ferrite materials enable a family of passive devices with a unique capability for selectively filtering signals above a threshold power level. Frequency Selective Limiters (FSLs) and Signal-To-Noise Enhancers (SNEs) operate autonomously, without external power or control signals. The non-linear process involves the interaction of the signal with spin waves at half the spin wave frequency, which results in absorption of the signal above a threshold input power level. Frequency selectivity is a direct result of the half-frequency spin wave interaction, which has a bandwidth determined by 2.8x?Hk (MHz), where ?Hk˜0.5oe is the spin wave linewidth. Two types of FSL have been demonstrated that exploit the non-linear process in different ways, the transmission line FSL and the Magnetostatic Surface Wave (MSW) FSL. Transmission line FSLs rely on the absorption of excess, above threshold, power by the spin waves but MSW FSLs exploit the limiting of the magnetization or spin precession angle at the signal frequency which results in the reflection of excess power in a MSW transducer. Epitaxial yttrium iron garnet (YIG) films have enabled development of both types of devices in broadband, planar chip scale configurations, with threshold power in the -40 to +10 dBm range.
A signal-to-noise enhancer is formed by placing a suitably biased YIG film on a narrow microstrip transmission line. At low power levels MSW are launched by the signal currents in the microstrip thus attenuating the signal but for signal powers above threshold the power carried away by the MSW is fixed at the threshold value, thus attenuation in the microstrip decreases with excess power above threshold. Bandwidths up to 1GHz have been achieved with YIG film/microstrip SNEs and wider bandwidths and higher operating frequencies demonstrated with YIG film/slot-line and lithium ferrite/microstrip devices. This presentation will provide a summary of the spin wave theory behind the limiting process and frequency selectivity, and describe the principles of operation, device structures, performance and potential applications of FSLs and SNEs.