Because of its incredible stability under extreme environmental conditions, ferrite material has been a trusted core material for many components. But in particular, they’re used in RF and microwave circulators and isolators for their unique ferromagnetic interactions with electromagnetic energy. The opposing magnetic fields of two ferrite disks coupled with a stripline, or y-junction, can be biased in such a way as to force signal transmission in one direction or halt it. This material property has led to the development of circulators, isolators, and isolator/switches, which are uniquely capable amongst RF/microwave components, as there are no effective semiconductor solutions that improve or replace this legacy technology. However, application and market demands in the military, defense, aerospace, and critical communications arenas require innovation with these vital components to meet modern expectations and increasing cost pressures.
RF and microwave circulators and isolators
Circulators and isolators – which are simply circulators with a matched load on one of the ports – have offered unique solutions in RF circuitry since their inception, from legacy military components to state-of-the-art satellite communications. Circulators are a cornerstone component in the RF signal chain, for protective isolation, bidirectional communications, and high isolation switching. These devices effectively leverage the interaction between ferrite materials to direct the flow of EM signals in one direction with little reflection and low leakage into the other ports, which is why this category of components are commonly dubbed ferrites. Most circulators are three-port passive components with a low insertion loss (S21, S32, S13) and high return losses (S11, S22, S33) where the return losses are essentially the same as the isolation between all unused ports, a benefit of the non-reciprocal nature of this device.
Conventional circulators tend to be bulky due to the ferrite disks and length of transmission line used for impedance matching. These components are also hand assembled and tuned with internal matching networks or variable capacitors for minimum insertion and maximum isolation. This process is viable when volumes for microwave modules are low and packaging is geared for maximum performance as opposed to automation. Now, with volumes creeping up and RF/microwave circuitry getting smaller, the pressure to keep costs down are increasing leading to the necessity for a complete suite of lower cost, faster to install surface mount components.