Ferrite Circulators
Figure 1: Ferrite circulator: principle of operation
Ferrite circulators are often used as a duplexer. The operation of a circulator can be compared to a revolving door with three entrances and one mandatory rotating sense. This rotation is based on the interaction of the electromagnetic wave with magnetised ferrite. A microwave signal entering via one specific entrance follows the prescribed rotating sense and has to leave the circulator via the next exit. Energy from the transmitter rotates anticlockwise to the antenna port. Virtually all circulators used in radar applications contain ferrite.
Figure 1: Ferrite circulator: principle of operation
At first in Ferrite circulators the energy divides into two equal parts at the entrance (1) but these parts get a different propagation speed by the influence of the ferrite.
At the connection 3 both signal parts are on in opposition, they well extinguish himself.
Both signal parts are in-phase at the connection 2, well, they add themselves up to the complete signal again.
By the symetric construction of the ferrite circulator it is possible always to determine a defined way direction by the choice of the connection. If an antenna is in a connection, transmitting energy is always escorted to the antenna while the echo signals always find the way to the receiver!
Ferrite circulators can be used also as protection against reflections. In this case a terminator is attached to connection 3. This terminator converts the reflected energy of the connection 2 in heat while the desired signal of the connection 1 can pass to connection 2.
Figure 3 shows a disassembled ferrite circulator. The largest part of the total volume is occupied by two massive metal blocks, which have only a supporting function. The conductor in the middle is made of copper. The width of the conductor is chosen to form an impedance of 50 ohms (together with the surface of the metal blocks). The two ferrite platelets are shaped like a small tablet and are enclosed in a ceramic disc. The entire design is mechanically constrained only by the RF-sockets. All parts are fixed with glue in addition.
