The working principle of the RF circulator and the application of the RF circulator

RF or microwave circulators or isolators are devices that typically have three, sometimes four, ports and are used in RF system designs that require the transmission of power from one port to another while isolating the power supply from another.

RF circulators are used as duplexers in many RF applications to allow simultaneous transmit and receive functions, and they are widely used in RF design applications, including radar systems and various specialized radio communication systems.

RF circulators get their name because they transfer power from one port to another, for example, from port 1 to port 2 output, and from port 2 to port 3 for cycling.

The working principle of the RF circulator

The connection to the RF circulator is often called a port, and in addition to that, they are usually numbered 1, 2, 3, etc.

The RF cycler gets its name because it only loops the power that goes into one port to the next. Signals applied to port 1 are passed to port 2, and signals entered to port 2 are passed to port 3 but not returned to port 1. Input to port 3 is passed to port 1, but not backwards to port 2.

The ideal circulator transfers all power from one port to the desired port and not to any other. But in reality, there will always be some attenuation in the transmission path, and some signals will always leak onto ports that should be isolated. The key RF circuit design challenge for these devices is to ensure optimal transmission and isolation occurs.

Circulators may use stripline printed circuit board technology (but typically use a dielectric or PCB material with very low losses) and are contained in metal boxes with connectors or other connections to the outside world – and some even use surface mount technology. Other circulators may be waveguide-based, and these circulators can be used in RF system design applications incorporating waveguide technology. The type of interface and technology required for any given instance will depend on the RF circuit design of the application. In terms of their operation, most RF circulators are based on the use of ferromagnetic materials. There are two main types:

Three-port circulator: The three-port "Y-junction" circulator is based on eliminating waves propagating through two different paths near the magnetized material. Waveguide circulators can be of any type, while more compact devices based on striplines are the 3-port type.

Four-port circulator: Faraday-rotated four-port waveguide circulator based on waves propagating in magnetized materials. Using this technology, they were able to route RF signals to four ports.

RF circulator circuit symbol

In addition to other electronic components, RF circulators have their own circuit symbol that is used to represent it on an electronic circuit diagram or schematic.

The basic symbol consists of a circle and an arrow that indicates the direction of the power cycle. Usually ports are displayed in a circle in clockwise order: port 1, port 2, and finally port 3.

It is worth noting that each port, whether a coaxial feeder or a waveguide, appears as a single wire instead of a pair of conductors.

One of the commonly used forms of RF circulator is formed from the Y-shaped portion of a microstrip or stripline transmission line located on a printed circuit board or other dielectric. The ports are placed 120° apart, so they are distributed equidistant around a circle.

The printed circuit board assembly is then sandwiched between two ferrites, and on top of them, two powerful magnets are held in place.

This assembly establishes a strong magnetic field axially through the ferrite disk, which concentrates the magnetic field around the Y junction, called the bias.

When a signal is applied to one of the ports, an electromagnetic field is created in the stripline that interacts with the magnetic field from the magnet, and there is a complex interaction between them. This results in the signal only traveling around the circulator to the next port. The circulator assembly, consisting of a Y junction and a ferrite disk, has a pronounced resonant frequency - this component actually forms a resonator. For obvious reasons, the circulator does not operate at this frequency, but above or below that frequency because the insertion loss, that is, the attenuation is much lower.

RF circulator applications

circulators have a variety of applications in various RF circuit design applications. Usually they tend to be used at microwave frequencies, so they are often referred to as microwave circulators.

Duplexers: One of the most obvious and common applications of RF circulators is in radar systems or radio communication systems where transmitters and receivers use common antennas.