Yttrium Iron Garnet (YIG) is a crystal that has very high Q characteristics. This high Q provides very low phase noise in oscillators and multi-octave frequency tuning for both oscillators and filters.
YIG crystals are “grown”, similar to silicon crystals. The pulled crystal is “sliced and diced”, resulting in small YIG cubes. (Unfortunately, the cube shape is non-uniform, and as a result has non-uniform coupling in a resonator circuit.) These small YIG cubes are then put into a “tumbler” that slowly shape the YIG cube into a YIG sphere (very similar to smoothing a stone for jewelry). The size of the YIG spheres range from 10-30 mils.
The YIG sphere is typically mounted on the end of a thermally conductive rod (normally beryllium). This is done for two reasons: 1) the rod acts as a “tuning stick” for orienting the YIG sphere in the resonant circuit, and 2) YIG has best performance when it’s temperature is kept constant; the rod is a thermal conductor to/from a proportional heater and the YIG sphere. (YIG oscillators and filters have been designed without the rod and heater for low cost. However, the impact on performance limits applications.)
YIG is a ferrite material that resonates at microwave frequencies when immersed in a DC magnetic field. This resonance is directly proportional to the strength of the applied magnetic field and has very linear “tuning” over multi-octave microwave frequencies.
The DC magnetic field is generated using an electromagnet, a permanent magnet, or a combination of both. The magnetic field of an electromagnet can be “tuned” using a variable current.
YIG crystal resonance is the alignment of external electron paths at the molecular level (precession), creating a “combined” magnetic dipole: a magnetic field resonating at microwave frequencies around the YIG sphere.
Current generates magnetic fields, and magnet fields can generate current when coupled to a conductive “loop”. Using small conductive “loops” allows coupling to & from the YIG spheres resonant magnetic field.
Want to learn more about the specifications and uses of YIG? Everything from magnet specifications to RF specifications and power consumption specifications can be found in our tech brief: Technology Description of YIG Tuned Oscillators.