YIG sphere

[1][2] A sphere made from a single crystal of synthetic yttrium iron garnet acts as a resonator.

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.

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 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.

Simplified schematics of YIG-resonator coupling to microstrip network
YIG filter partially disassembled. The electromagnet is on the left. The module with YIG sphere and input and output coils is on the right.