Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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 small loops around the sphere couple fields into and out of the sphere. 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.