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Circulator

BASIC PRODUCT DETAILS

RF & MICROWAVE CIRCULATOR:

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.

  • An RF Circulator is designed to circulate the input RF signal in one direction in either clockwise or counterclockwise direction.

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 ensures that it passes RF signal from the input port to the adjacent output port when it is used in the RF system.
  • An RF circulator connected at the output port of a sensitive Rx/Tx system is used to transmit the output signal to the antenna and transmit the received signal from the antenna to the Receiver circuit.
  • RF circulator is a 3-port passive unidirectional microwave device used to control the RF signal flow direction inside the RF circuit.
  • A port in RF circulator is defined as a connection point for either the input or output of the RF signal or the termination port.
  • RF circulator works as a unidirectional RF component, which helps to eliminate the reflected RF power from the Load to the source, by terminating the 3rd port.

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.

  • What is RF Circulator?
  • Types of RF Circulator?
  • Applications of RF Circulator?
  • What are the specifications that need to consider in the selection of RF Circulator?
  • RF and Microwave Circulator Manufactures.

What is RF Circulator?

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,

  • A signal entire to port 1 will come out only at port 2;
  • RF signal reaches at port 2 only comes out from the port 3;
  • RF and microwave signal applied to port 3 only comes out of port 1, and so on.

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.

  • RF circulator work by the principle of electromagnetic interference with ferrite material and magnetic field which created by the magnet surrounded the material.  Due to the created field, the RF signal enters the port cause to circulate in clockwise or anti-clockwise direction inside the RF circulator.

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.

Working Principle of RF isolator in an RF circuit:

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.

  • In the Circulator, the magnetic field is applied Y-axis of the assembly which results in a clockwise circulation of the RF energy from one port to the adjacent port.
  • The arrows on the package of the RF circulator represent the direction of the circulation of the magnetic fields and the and microwave signal when applied to the port.
  • RF signal experiences very little loss in the direction of circulation and high loss in the opposite direction( Reverse) while propagating through the Circulator.

Construction of RF Circulator:

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.

  • When a circulator is used as an isolator the level of isolation of the RF circulator is based on the impedance matches of the terminated port.

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Design and Application

Types of RF Circulator:

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.

Classification of RF circulator on the base of the package:

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.

  • SMD package RF.
  • Connectorized RF.
  • Waveguide RF circulator.   

Classification of RF circulator based on the direction of RF signal flow:

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.

  • RF circulators can be made to circulate the RF signal either clockwise (CW) or counterclockwise (CCW) direction on the base of the application. 

In addition, RF circulators are available with a circulation direction switching feature. 

Switchable RF circulator:

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.

  • The switchable circulator latches into a particular circulation orientation in the absence of the electrical control 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.

Classification based on the material used:

Depending on the materials used in the construction of the RF Circulator, classified into two categories: Ferrite RF circulator and Non-ferrite RF circulator.

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:

  • Differential phase shift.
  • Junction circulator.

In both types of RF circulators, when waves propagating in opposite paths will get cancelled inside the magnetized ferrite material.

  • Stripline circulators are of the junction type and waveguide circulators are of any of the above 2 types.

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:

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.

  • The major drawback of non-ferrite circulators is signal-to-noise degradation and power limitation.

Now integrated circuit-based non-ferrite circulators are available for full-duplex communication applications.

Applications of RF and Microwave Circulators:

 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 -

  • Radio communication and Television
  • Distributed antenna & amplifier system.
  • Aviation & Navigation industries.
  • Military equipment & Radar systems.
  • RF & Microwave measurement LAB test requirements.

1. Circulator can be used as Duplexer:

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.

2. Protection of sensitive RF components:

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.

3. Provide the Isolation between source and DUT in testing:

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.

4. Radio Link Combiner:

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.

5. Isolator used to offer matched impedance:

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.

6. Reflection amplifier:

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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Basics & Selection Feature

Important RF specifications for RF Circulator.

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.

Frequency of operation: 

The frequency range of operation of the RF circulator needs to cover the full range of frequency of operation, with flat RF response.

Isolation (dB): 

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: 

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.

RF Input Power:

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.

  • Forward power handling of the RF circulator is the maximum power handle at the input port ( port 1).
  • Reverse power is the level of power handling of the RF circulator at the output port. The reflected power handling indicates the level of RF reflected power absorption of the load.

In General, an RF Circulator with higher reverse and forward power handling is needed for the RF and microwave chain design.  

Impedance: 

The input impedance of the RF and Microwave circulator must match with the source impedance to minimize any internal reflection. 

Operating Temperature Range:

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.

Physical Size:

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.

Conclusion:

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.

Thanks & Regards

Team-RFMWC:- RF & Microwave Community

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