Overview of Ferrite-based RF Technology

Ferrites are high electrical resistance, magnetic, and ceramic materials constructed from a mix of metallic oxides, often including Iron (Fe), Zinc (Zn), Manganese (Mn), Nickel (Ni), Cobalt (Co), Barium (Ba), and Strontium (Sr). These metallic oxides exhibit magnetic behavior that allows for their use in RF circuits as inductors, transformers/baluns, isolators, circulators, phase shifters, chokes, filters, and even switches. Generally, the ferrite material is placed near conductive wires, these conductive wires are run through, or wrapped around the ferrite a prescribed number of “turns”. An exception to this is Yttrium Iron Garnet (YIG), which is actually a ferrite crystal grown much like a silicon crystal and is typically polished into a sphere and mounted on a thermally conductive rod suspended within a DC magnetic field.

A ferrite’s purpose is to concentrate the magnetic energy from a magnetic field, which enables the frequency dependent control of the electromagnetic energy. Electromagnetic waves travel through ferrite materials with little attenuation, but with some degree of phase shift dependent on the intensity of the DC magnetic field induced by the ferrite. Hence, with the proper design, ferrites can be used to control the phase and direction of flow of electromagnetic fields.

• Common RF Ferrite Devices
• Broadband Transformers
• Common Mode Chokes
• Converter and Inverter Transformers
• Differential Mode INductors
• Narrow Band Transformers
• Noise Filters
• Power Inductors
• Power Transformers
• Pulse Transformers
• EMC/EMI Chokes
• Circulators
• Isolators
• Filters
• Switches
• Phase Shifters
• YIG Oscillators
• YIG FIlters
• YIG Multipliers
• YIG Synthesizers

Important features of ferrites for RF applications are frequency dependent magnetic permeability, coercivity, and performance over temperature. As the various materials used to construct an RF ferrite have different performance features, the metal oxide mixture for a ferrite is specifically chosen to achieve a desired response. Ferrites are constructed by mixing the metallic oxide powders with binding agents until a slurry is made. This slurry is then injected into molds, heated, and pressurized to achieve the desired ceramic material features (sintering). If a magnetic field is applied while the ferrite material is still hot, a set permanent magnetism can be achieved, though this is not generally desirable for RF applications.

Depending on the mix and fabrication process, a ferrite’s permeability across a desired frequency range can be set. This is how some ferrites can be designed with extremely high or low loss for a given range of frequencies. Hence, a ferrite can be made for use as a high frequency transformer operating in the gigahertz, and another can be designed for high suppression of RF energy beyond a few megahertz for electromagnetic compatibility/electromagnetic interference (EMC/EMI) applications.

Ferrite-based RF electronic components are often still, at-least partially, hand assembled, tested, and tuned. This is a result of the high tolerances on material properties of ferrites and mechanical tolerances of the construction and assembly processes used to fabricate these devices for desired performance criteria. Virtually every physical and electrical property of the materials and structures used for RF ferrite fabrication impacts the performance of these components, making automated manufacturing difficult.