An RF and microwave circulator is a three-port RF device used in the RF and microwave circuit to regulate the RF signal flow in a particular direction and protects the RF system from excessive RF signal reflection from the load.
In the RF circulator, the RF signal that enters through port 1 is transmitted to the next near port. I.e port 2 and a signal incident at port 2 of the RF circulator is transmitted to port 3 and RF signal entered or reflected at port 3 is transmitted to port 1.
RF Circulators will have minimal loss in the direction of the signal flow I.e when transmitting RF signal from entering port to next port in the direction of circulation of RF signal and full RF signal cancellation in the reverse direction.
RF circulator is designed using the ferrite materials covered with a magnets material to determine the direction of RF signal flow in it.
There are many RF circulator manufacturers who offer the RF circulator in connectors modules with connectors like SMA, N, etc type, SMD package types, and also in the waveguide structure.
This article will cover RF and Microwave circulators following features.
An RF circulator is a passive, three or four-port non-reciprocal RF device that only allows radio-frequency and microwave signals to exit through the port directly immediately after the port on which it entered. RF and microwave ports of the circulator can be either connector, SMD, or the waveguide.
For a three-port circulator,
An RF circulator can be easily converted to an isolator by terminating the 3rd port and making it a two-port ferromagnetic passive RF component to protect RF systems from excessive reflected signals. By terminating 3rd port all the reflected signals from port 3 will get terminated in the terminator and no reflecting signal will reach port 1.
The rotary field inside the RF circulator, created due to electromagnetic interference should be very strong enough to circulate the RF and microwave signals enter into port 1 to port 2, and if any signal reflected from port 2 will circulate to port 3. Due to this strong circulation field, RF and microwave signals will not circulate in the opposite direction inside a circulator.
Isolation of the RF circulator is measured in the unit of dB value, It shows the level of separation of RF and microwave signal levels from the output port to the input port. The greater the RF isolation level, the less signal flow from the output port to the input port.
An RF Circulator utilizes a transversely magnetized ferrite junction to circulate incoming RF energy from port 1 to the immediate next port 2, from port 2 to port 3, and from port 3 to port 1.
RF circulator is manufactured using a stripline circuit sandwiched between magnetic ferrite discs, on both sides. The power handling and frequency of operation of the RF circulator are based on the ferrite materials and strength of magnets used for the manufacturing.
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RF or microwave circulators are very essential RF components for RF tests and measurement systems. RF circulators are classified on the base of the package, the material used, and the direction of the signal flow.
RF circulators use strip-line printed circuit boards and these boards and the magnet material is enclosed with a metal box with RF connectors or with a surface mount interface. For the high-frequency application, an RF circulator is constructed using the waveguides with a waveguide interface. The type of interface for the RF circulator is based on the RF circuit design and the application of the RF system.
Basically, RF Circulator is classified into 3 types.
RF circulators can be classified as counter-clockwise RF circulators (CCW) and clockwise ( CW) circulators on the base of the direction of RF signal flow inside the circulator.
In addition, RF circulators are available with a circulation direction switching feature.
In RF switchable circulator, the orientation of the circulator of the RF signal can be switched from CW to CCW and vice versa using an electrical signal.
For switching the direction of circulation, the customer can use a high-current DC pulse from the driver circuit. Switchable RF circulators are available from many manufacturers with low-loss and high power handling.
Depending on the materials used in the construction of the RF Circulator, classified into two categories: Ferrite RF circulator and Non-ferrite RF circulator.
In the Ferrite RF circulator, magnetized microwave ferrite materials are used to create the circulation of RF signals. They are classified into 2 classes:
In both types of RF circulators, when waves propagating in opposite paths will get cancelled inside the magnetized ferrite material.
For better isolation for isolators, two or more junction circulators can be combined to make a single isolator by terminating the reflected ports.
Even though ferrite circulators provides good forward circulation of RF and microwave signal, they are bulky at a low-frequency range and offer only a narrow bandwidth.
Non-Ferrite RF circulators are non-reciprocal in nature and designed by using transistors. As transistors require power to operate, non-ferrite circulators are considered active devices.
Now integrated circuit-based non-ferrite circulators are available for full-duplex communication applications.
RF and Microwave Circulators are used for many applications in the domain of RF and Microwave systems, test, and measurement applications.
The main industrial applications of RF and Microwave circulators are below -
The most obvious and regular application for an RF circulator is used as a duplexer in radar or radio communications systems where the RF transmitter and RF receiver use a common antenna path.
By connecting Port 1 to the RF transmitter and port 3 to the RF receiver and Port 2 to the antenna, a circulator can use as a duplexer. Whatever RF signal enters port 1 from the transmitter will pass to port 2 and the RF signal entering port 2 from the antenna will pass to port 3, which will receive to RF receiver chain.
RF circulator can be used as an RF isolator by terminating the third port. By connecting the RF circulator port 1 to the RF sensitive RF component and port to the load, it will act as a passive component to protect sensitive RF components from excessive power reflection. Whatever refection comes from the load will get terminated at the third port RF terminator, hence no reflected signal will reach port 1.
For RF testing applications in test laboratories, an RF circulator will place between a signal source and the device under test (DUT), with a third port terminated to avoid any reflections due to mismatches at DUT input and cause wrong measurements. By using a circulator with proper RF termination at the third port can offer 100 % isolation from reflected power.
An RF Circulator can be used as an RF signal combiner by connecting multiple transmitters in the circular port and the final combined signal will be available at the last port. This feature helps to combine different transmitters and receivers in communication systems.
The principle of the RF circulator working as an isolator will help the designers to use it as an impedance match circuit when connected to the device with unknown impedance, as all the reflections will get terminated at the termination port ( Third port).
This feature of the RF circulator will use in the RF power amplifier's output port to operate with different loads that have poor impedance matches.
A reflection amplifier is a type of microwave amplifier designed using tunnel diodes and Gunn diodes. As the diode is a one-port device, a circulator is used to separate the input and the outgoing amplified signal from the diodes by connecting the signal input port 1, and the biased diode connected to a second port, and in the third port load will connect.
This helps to uncouple the output and input RF signals.
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Below are the few electrical specifications needed to check in the selection of RF and Microwave circulators in the design of the RF and microwave systems.
The frequency range of operation of the RF circulator needs to cover the full range of frequency of operation, with flat RF response.
The level of Isolation offered between the ports is one of the important parameters that need to consider in the selection. The higher the isolation of the RF circulators the better the protection.
Insertion loss of the RF circulator indicates the level of signal loss from the input port of the RF signal to the output port. RF and Microwave circulator with lower Insertion loss, when inserted in a transmission and receiver path as a duplexer, is better for the application.
Lower insertion loss of the RF circulator will help to prevent the power loss in the RF circuit.
The RF power level of an RF circulator indicates the level of power handled by maintaining its electrical characteristics. Power handling needs to define for both forward and reverse power handling.
In General, an RF Circulator with higher reverse and forward power handling is needed for the RF and microwave chain design.
The input impedance of the RF and Microwave circulator must match with the source impedance to minimize any internal reflection.
The operating range of the RF circulator shows the temperature over which the RF circulator operates without any variation in its electrical characteristics.
For industrial RF circulators, -40ºC to +85ºC is acceptable.
RF circulators are available in any type of package like Drop-In, surface mount, connectorized modules, and also with waveguide connectors. The physical size of the RF circulator needs to select as per the RF design requirement of the system.
RF circulators are widely used for the applications like Radio communications, high-power Radar systems to protect the source from varying loads, and for RF and microwave LAB testing applications.
The fact that RF circulators and isolators transfer power from one port to another and offer suitable isolation in the reverse direction enables them to be used as a duplexer allowing a receiver and transmitter to operate simultaneously on nearby frequencies bands.
RF and microwave circulators are available from many manufacturers in different packages. As the RF circulator and isolators are available with different connector interfaces and gender options, designers can select a suitable component as per the customer’s design needs.
We hope this RF and Microwave circulator article help system designers to understand the Function, Application, and Selection specification in the communication system. If you would like to share any further details about RF circulators, we are happy to hear from you over the comment box or through the article submission button above.
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