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Yttrium iron garnet spheres (YIG spheres) serve as magnetically tunable filters and resonators for microwave frequencies. These filters are used for their high Q factors.  A sphere made from a single crystal of synthetic yttrium iron garnet acts as a resonator. These spheres are on the order of 0.5 mm in diameter and are manufactured from slightly larger cubes of diced material by tumbling, as is done in the manufacture of jewelry. The garnet is mounted on a ceramic rod, and a pair of small loops around the sphere couple fields into and out of the sphere; the loops are half-turns, positioned at right-angles to each other to prevent direct electromagnetic coupling between them and each is grounded at one end.
The field from an electromagnet changes the resonance frequency of the sphere and hence the frequency it will allow to pass. The advantage of this type of filter is that the garnet can be tuned over a very wide frequency range by varying the strength of the magnetic field. Some filters can be tuned from 3 GHz up to 50 GHz. YIG filters usually consist of several coupled stages, each stage consisting of a sphere and a pair of loops.
The input and output coils are oriented at right angles to one another around the YIG crystal. They are cross-coupled when energized by the ferrimagnetic resonance frequency, which depends on the external magnetic field supplied by an electromagnet.
Another YIG application is YIG oscillators, where the sphere acts as a tunable frequency-determining element. It is coupled to an amplifier which provides the required feedback for oscillation.
- J. Helszajn, "YIG Resonators and Filters," John Wiley & Sons (New York: 1985)
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