The RF circulator has its own circuit symbol to facilitate its representation on electronic circuit diagrams or schematics. The basic symbol consists of a circle and an arrow, indicating the direction of the power cycle. Usually, the ports are shown in clockwise order around the circle: port 1, port 2, and finally port 3.
If the component is done by counter-clockwise, it is marked in counterclockwise order.
One of the commonly used forms of RF circulators is formed by a Y-shaped section of a microstrip or strip-line transmission line on a printed circuit board or other dielectric. The ports are placed 120° apart so they are equally spaced around a circle.
Y shape strip-line RF circulator PCB layout
The PCB assembly is then sandwiched between two pieces of ferrite, and on the outside of it, two strong magnets are held in place. This component creates a strong magnetic field axially through the ferrite disk, which concentrates the magnetic field around the Y-junction, called Biasing.
The cross section of the RF Circulator structure
When a signal is applied to one of the ports, an electromagnetic field is generated in the strip-line, which interacts with the magnetic field from the magnet, and there is a complex interaction between them. This causes the signal to only travel around the circulator to the next port. The circulator assembly consisting of a Y junction and a ferrite has a distinct resonant frequency, equivalent to forming a resonator.
In various radio frequency circuit design applications, the circulator has a wide range of applications benefit from its much lower insertion loss, that is, the attenuation is much lower. Usually, they tend to be used at microwave frequencies, so they are often called microwave circulators.