Abstract :
Over the past decade, many time-domain signal processing functions have been demonstrated at microwave frequencies in bulk single crystals of yttrium iron garnet (YIG). These include pulse compression, variable delay, storage, pulse recall, and pulse frequency changing. However, none has reached product engineering status due to lens effects, low dynamic range, and the very limited range of available parameters which are all manifestations of the nonellipsoidal YIG geometry. In contrast, within the past four years Rayleigh type acoustic waves propagating in piezoelectric crystals at VHF/UHF frequencies have already led to practical devices, notably, color television filters and pulse compression loops. Nevertheless, delay lines exceeding 100 microseconds and programmable rime-domain filters of dynamic range greater than 50 dB have proved difficult to realize. This paper proposes a new approach to harness practically the known versatility of magnetic materials. Rayleigh type acoustic waves are utilized propagating at VHF/UHF frequencies in epitaxial films of YIG possessing ellipsoidal geometry grown on nonmagnetic single crystal substrates. Here acoustic waveguides can be fabricated readily in low loss media by selective etching techniques and programmability obtained by adjustment of the bias magnetic field. In common with acoustic waves in piezoelectrics, the philosophy is adopted that the wave should be predominantly acoustic with an energy admixture of less than 5 percent (in this case magnetic) for transduction and tapping purposes. Examples of the realization of devices for both frequency time-domain processing are delineated, and areas are highlighted where further research is necessary.
