Microwave systems basically consist of a microwave generator (an oscillator with an amplifier in the form of an electron vacuum tube or a solid-state power amplifier), which is operated via a distribution network into a microwave applicator (consumer). The applicator can be an antenna, a microwave oven, a plasma reactor or a resonant cavity of particle accelerators. The operating conditions of the applicators range from matched condition (the generator power is almost entirely absorbed by the applicator) to total reflection, in which the microwave returns to the source and can cause significant damage there. Furthermore, the forward and reflected waves overlap to form a standing wave, which, in the event of total reflection, leads to a doubling of the voltages and currents in the system. The higher the performance, the higher the risk.
This is where the ferrite-based circulator comes into play as a non-reciprocal key component between the generator and applicator – a component that the microwave can only rotate in one direction. This function is made possible by a microwave ferrite that is magnetostatically biased.
A 3-port circulator passes the generator’s forward power unidirectionally and with low losses to the applicator, while the reverse power from the applicator is diverted into a low-reflection microwave absorber (matched load) on the third port of the circulator. In this case one also speaks of an isolator (circulator + load).
The generator is thus effectively and reliably protected against damaging reverse power and its life time is increased, but that’s not all. Circulators enable stable and continuous operation of generators in all forms of applicators, especially in strongly reflective resonant cavities of high Q-factor.
The circulator thus makes essential key contributions to high-performance microwave systems. It promotes the reliability and availability of microwave systems and also helps reduce long-term operating costs.
In addition to the isolator, the circulator is also used as a transmit/receive switch or duplexer in T/R modules of modern radar systems with actively phase-controlled antennas.
Moreover, a circulator can be used as a microwave switch by magnetically changing the sense of rotation.
All applications require high isolation and low insertion losses to effectively protect systems and significantly improve system performance and efficiency
