Different microwave components play an important role in the microwave and RF system design. At SEI, we will be sharing with you an article discussing everything you need to know about how RF circulators function. 

RF or microwave circulators and isolators are important devices used in radio communication systems. They are devices that usually have three or four ports used in the RF system designs. They require power to transfer the input from one port to the other ports while isolating power from the other ports. RF circulators are used in different RF applications by performing as a duplexer that allows both receiving and transmitting functions to take place at the same time. They are typically used in RF design applications such as radar systems and different types of professional radio communication systems. 

RF circulators are responsible for transferring power from one port to another; it allows the power to circulate. The power will enter at one port to output at port two and enter at port two to exit at port three. 

How Does An RF Circulator Work?

The RF circulators have connections called ports, and they are numbered as 1, 2, 3, and so on. The RF circulator gets its name because it helps the power to circulate by entering from one port to the next one. 

A signal that passes from port 1 will be passed correspondingly to port 2, then will be passed to port 3, but it will not go back to port 1. The signal passes through a clockwise direction, so when input passes from port 3, it will pass to port 1 and not in a reverse direction towards port 2. In theory, an ideal circulator must transfer all the power from one port to the required port and not to any other ports in the system. 

However, sometimes, there is weakness in the transfer path and there will be signals that can leak onto the other ports that should be isolated. The ideal RF circuit design for the system devices is to make sure that the optimal transfer and isolation of other ports must occur during the process. 

Circulators typically use stripline printed circuit board technology contained inside a metal casing with connectors or other connections to the outside world, while some may use surface mount technology depending on the application it is used for. Some circulators are based within waveguides which can be used in the RF system design applications that use waveguide technology. The type of technology and interface needed will be based on the RF circuit design used for the application. 

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There are three main types of RF circulators based on the uses of ferromagnetic material. 

  • Three port circulators – The 3-port circulator, also known as “Y-junction”, is based on the cancellation of waves generating over two distinct paths nearby a magnetized material. Waveguide circulators can be either 3 port or 4 port circulators, while compact devices that use a stripline are 3 port circulator types only. 
  • Four-port circulators – The 4 port waveguide circulators are created based on Faraday’s circulation of waves generated in a magnetized material. By using the said technology, they are capable of routing the RF signals to the four ports of the circulator. The RF circulator has a circuit symbol, which is used to indicate it on electronic circuit schematics and diagrams. 
  • 3 port stripline technology circulators – The most commonly used form of RF circulator is formed with a Y-shape section of a stripline transmission, which can be found on a printed circuit board. The ports are positioned 120 degrees apart so that they have equal space on the circle. 

The circuit board assembly will then be placed in between the two layers of ferrite and two strong magnets will be attached outside. This assembly produces a strong magnetic field through the layers of ferrite circulators, which is also known as the bias of the Y-junction. 

When RF signals are applied to either of the ports, it creates an electromagnetic field in the stripline which then interacts with the magnetic field caused by the magnets which results in a complex interaction between them. 

This causes the signal to only be allowed to move to the next port on the circulator. The circulator assembly consists of the ferrite disks and Y-junction which have a unique resonant frequency and forms a resonator. The circulator will then be operated at frequencies that are either lower or higher than the frequency of the circulator assembly because the insertion loss will be lesser. 

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What Are The Different RF Circulator Applications?

There are different applications for RF circulators. They are popularly used at microwave frequencies, so they are also known as microwave circulators. 

Duplexer

RF circulators are mostly used in radar systems and radio communication systems; their transmitter and receiver use a common antenna. When used as a duplexer, the RF circulator functions with a single antenna as its transmitter and receiver. 

The transmitter output can be connected to one port, while the antenna is connected to port 2, and the receiver is in port 3. When the transmitter power enters, it will circulate to the antenna, but it will not go to the receiver. On the other hand, when the antenna receives signals, they will circulate to the receiver and not the other way around. The receiver is isolated from the transmitter, while the antenna gets its power from the transmitter and allows the received signal to pass through to the receiver without the need for mechanical switching. 

RF Isolator

The RF circulator is also capable of working as an RF isolator. They function to protect a transmitter output amplifier that needs to run at high levels of Voltage Standing Wave Ratio. When high voltage current levels pass through the amplifier when it is connected to the antenna, there is a possibility the amplifier can become damaged because of it. 

Transmitters are known to work with a wide bandwidth. Maintaining good impedance can be difficult over a wide range of bandwidth and the high levels of VSWR can cause damage. To prevent this issue, a circulator can be used to protect the power amplifier from the reflected power’s effects. 

The transmitter can be connected to port 1 and the antenna to port 2. Port 3 will be connected to a 50 ohms load; the levels of isolation will depend on a good impedance match on the different ports. If an open circuit is presented, it can affect the isolation performance as it can become impaired. For example, the RF circulator is used as an isolator where a resistor is attached to port 3 to provide the same load for the port to preserve the level of isolation and disperse the unwanted reflected power during the transmission. 

The power will be transmitted and passed from port 1 to port 2 and it will travel to the antenna. If there will be reflected power, it will be restored on the feeder and it will pass through port 2, and it will be dissipated in the load in port 3. 

Through this process, the RF power amplifier can still function using a high-level VSWR but be protected from the effects of the reflected power through the RF circulator that acts as an RF isolator. The load in port 3 must provide matched impedance because isolators need to match impedance on the different ports to keep the isolation power from the specified port. 

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What Are The RF Circulator And Isolator Specifications?

The different specifications for an RF circulator will allow you to make the right decisions when making a specific RF circuit system or design. Here are some of them:

Frequency  

RF circulators are known to have a restricted frequency range where they normally operate. The frequency range is restricted due to the isolation between the ports so that insertion loss can occur. RF circulators function between 750 MHz up to 20 GHz, but some RF circuit designs are known to operate on frequencies between 100 MHz and 100 GHz. 

Insertion Loss

This is measured in decibels of the signal from one port to the other with a forward direction. It usually ranges between 0.1 to 0.75 decibels and may depend on the circulator, frequency band, and other factors. Keep in mind that the insertion loss of a circulator is affected by the frequency used. The higher the frequency, the higher will be the insertion loss. 

You must choose a circulator that has the least insertion loss as it can help preserve the transmitted power. The higher the powers that will be used in the system, the higher the loss in the circulator, which handles the required power for it to function correctly. 

If you need RF circulators, you need to choose a reliable company like SEI. We manufacture microwave and RF components like RF circulators. You can order Ferrite circulators here. If you want to know more information or if you have any questions regarding the product that we are currently offering, you can contact us by filling up this contact form here

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SEI is a supplier of different microwave and RF products and components; please let us know what you need. You can email us at [email protected]. If you have any inquiries or questions about the products that we are currently offering please let us know. Always remember to choose a reliable supplier of microwave and RF products like SEI.