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Yttrium iron garnet filter (YIG filter) is a magnetically tunable filter for microwave frequencies. YIG spheres are used for their high Q factors.
YIG filters use a synthetic single crystal yttrium iron garnet sphere acting as a resonator. The garnet sits on ceramic rod, and small loops around the sphere couple fields into and out of the sphere. An electromagnet changes the resonance frequency of the sphere and hence the frequency it will allow to pass. The advantage of this method is that the garnet can be tuned over a very wide frequency 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 energised by the ferrimagnetic resonance frequency, which depends on the external magnetic field, supplied by an electromagnet.
YIG filters are often used as preselectors. YIG filters tuned by a sweep current are used in spectrum analyzers.
An other application of YIG is a tunable oscillator, the YIG oscillator, where the sphere acts as the frequency-determining resonator and is coupled to a transistor that provides the required feedback for the oscillation.