Circulators and Isolators are vital devices that guide the flow of microwave signals in RF systems and equipment.  They are passive electronic devices with three ports.  A port is a connection point for signals which can either be an input, output, or termination.  The standard schematic symbol for a circulator’s arrow demonstrates the unidirectional flow of signals from port to port.

How Do Circulators and Isolators Work?

In a circulator, any port can be an input or an output.  If a signal is applied to the first port, it will be passed on to the second port with minimum attenuation.  The same signal input will pass from the second to the third port.  It will continue back to the first port.

Since the flow of signal is unidirectional, it never moves in reverse.  Typically, there is an amount of insertion loss from 0.2- to the 0.75-dB range.

If one of the ports is terminated and the resistance is equal to the impedance of the port, the circulator becomes an isolator.  An input signal at the first port will pass to and exit the second port.  This typically occurs if the second port’s impedance is appropriately matched.  However, if there is a mismatch at the second port, the reflected signal will move on to the third port and be absorbed by the load.  This occurrence protects or isolates the two ports from signals that may move in the reverse direction.

How Are They Constructed?

Typically, a circulator is formed to be a Y-shaped section of a strip line or microstrip transmission line on a printed circuit board or any other dielectric.  The general impedance level for RF circulators is 50 ohms.  It consists of three ports spaced 120 degrees apart and is commonly terminated with SMA or N-type coaxial connectors.

Standard construction of a circulator may show a Y-strip line, magnets, and ferrite disks.  There may or may not be spacing between disk components.  

The heart of the circulator lies in the Y-shaped stripline circuit.  This assembly is situated between two layers of ferrite material.  Positioned on either side of these ferrite disks are two strong permanent magnets.  These send a strong magnetic field axially, called the bias.  The ferrite material supports and directs the magnetic field around the Y-junction assembly.

When a signal enters one of the ports, an electromagnetic field is in the strip line.  Then, this field interacts with the applied bias (axial magnetic field).  This, in turn, causes the signal to rotate in one direction to the adjacent port.

The whole assembly also forms a dielectric resonator which produces a resonant frequency.  However, the circulator does not operate at this frequency.  The circulator process occurs above or below the regions of the resonant frequency.  Here, the attenuation is minimal.

What Are the Common Applications?

As a Duplexer

A duplexer permits the transmitter and receiver in a radio or radar unit to share a similar antenna.  Undoubtedly, this is the most common use of a circulator.  The transmitter output is applied to the first port and will pass on to the second port, to which the antenna is connected. 

Because the receiver input is connected to the third port, the signal received by the antenna moves along here but not back to the first port.  The transmitter output does not pass on to the receiver input.  This process prevents damage to the receiver input circuits from high transmitter power.

As an Isolator

As mentioned above, when the circulator is connected as a duplexer, it allows a transmitter and receiver to share a standard antenna.  It could also function as an isolator.  When one port is terminated, the three-port circulator can be used as an isolator.  This is known as isolation if the power loss from the second to the first port is more significant.  The power loss from the first and second ports is less understood as insertion loss.  Dual port circulators can provide more isolation.

Another everyday use of an isolator is when it is connected to a signal generator and a DUT (device under test).  Given that all impedances are matched, the signal can pass freely to the DUT.  But if there is a mismatch at the DUT or the DUT is disconnected, the signal creates a high-voltage standing wave ratio (VSWR).  This will cause a significant reflected signal.  Acting as an isolator, the circulator absorbs this signal, protecting the signal generator.

As a Diplexer

The diplexer provides the same function as a duplexer.  It combines two or more ports into a single port.  Similar to the duplexer, the diplexer is typically broadband but frequency selective.  It allows two transceivers on two different bands to share a standard transmission line. 

A two-meter transceiver and a 70-cm transceiver can each be connected to a single diplexer that connects to a single coax.  On the other end of the coax, there might be a dual-band antenna or another diplexer separating the signals into two antennas.

In short, a diplexer separates two frequency bands from the input frequency band.  It uses a high pass filter at the second port to detach the desired frequency bands from the input frequency band at the first port.

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How Do You Choose the Circulator or Isolator to Purchase?

When specifying a circulator or isolator, pay attention to the following characteristics:

  • Frequency of Operation and Bandwidth:  Circulators and isolators typically operate over 700 MHz to 20 GHz.  Unique designs may be permitted if you need frequencies as low as 50 MHz and as high as 100 GHz.  Most available devices possess a narrow operating range but finite bandwidth.  Lower frequencies are better accommodated if your project operates above resonance than more limited bandwidths.  Otherwise, projects requiring below resonance operation allow for wider bandwidths.
  • Power:  The average maximum power the device can handle is essential.  Check the power rating range, which generally is from 1 to 1,000 W. Forward and reverse power levels are usually indicated, especially for the reflected signals.
  • Insertion Loss:  Pay attention to the possible attenuation from port to port in the forward direction.  The loss typically places between 0.1 and 0.75 dB with a three-port device.
  • Impedance:  The most common characteristic impedance of the ports is at 50 ohms.
  • Isolation:  This characteristic refers to the attenuation in the reverse flow direction.  Typically, you will find it in the 17-dB to the 35-dB range.
  • Temperature Range:  Since ferrite material characteristics and magnetic field strengths vary with temperature, this is a vital factor to look into.  The maximum range should be between –50 and +100°C.
  • VSWR: Also known as the voltage standing wave ratio, it is the value that indicates the maximum mismatch and reflected power as the signal moves from the output port to the input port.  The standard range is 1.1:1 to 1.4:1.

SEI offers affordable and high-quality circulators, isolators, adapters, connectors, antennas, and cable assemblies.  What is the configuration you need for your project?  Talk to us today to discuss your requirements.  Call us at 408-374-1031 or 408-622-4887.  You can also email us at [email protected] to request a quote.