RF Isolators and RF Circulators

RF isolators and RF circulators are passive devices that are used to control the propagation of RF signals. RF isolators are two-port units that allow signals to pass in one direction while providing high isolation for reflected energy in the reverse direction. RF circulators have three or more ports and are used to control the direction of signal flow in a circuit. They allow the signal entering one port to pass to an adjacent port in either a clockwise or counter-clockwise direction, but not to any other port. Three-port or Y-junction circulators are commonly available. In these devices, the signal flow is normally expressed as 1 to 2, 2 to 3, and 3 to 1. For example, when the input is port 1, the RF signal exits port 2. Ideally, port 3 remains isolated and does not receive or pass the signal. Some RF isolators and RF circulators are designed for use with a coaxial cable. Others are designed for use with a waveguide. RF isolators and RF circulators that contain magnets are not designed for mounting near ferrous materials because the close proximity of metals such as iron can change the frequency response.

Important specifications for RF isolators and RF circulators include frequency range, isolation, insertion loss, voltage standing wave ratio, and operating temperature. Frequency range is measured in megahertz (MHz) and includes minimum and maximum frequencies. Isolation is measured in decibels (dB) and expresses the degree of attenuation between ports. For example, the isolation from port 1 to port 2 is the degree of attenuation at port 1 measured from port 2 with all other ports properly terminated. Insertion loss is the total RF power transmission loss resulting from the insertion of a device in a transmission line. It is also defined as the ratio of signal power at the output of the inserted device to the signal power at the input of the inserted device. Voltage standing wave ratio (VSWR) is a unit-less ratio ranging from 1 to infinity. A value of one indicates that all energy passes. Any other value indicates that a portion of the energy is reflected. Other performance specifications for RF isolators and RF circulators include reflected power, an amount usually expressed as a percentage. 

There are many applications for RF isolators and RF circulators. For example, many RF isolators are used in test bench applications where the placement of an isolator in the measurement path causes variations in the tested device to appear at the termination of the isolator and not at the signal source. RF circulators are often used as duplexers, devices in which a transmitter and receiver share one antenna.

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