RF isolators and circulators are passive components essential to modern communication systems. They act as traffic routers for RF signals, moving them to any desired place in the circuit. Isolators and circulators are manufactured using anisotropic ferrite materials, often biased by a permanent magnet, making them non-reciprocal devices.
Isotropic and Anisotropic Materials
In isotropic materials (from the Greek, iso meaning equal, and tropos meaning direction) the value of a particular property is the same in all directions (x, y and z). For instance, the density of gold—an isotropic material—is the same in all directions. Anisotropic materials, on the other hand, are materials with particular properties that may vary depending on direction. For example, wood is an anisotropic material because its grain orientation is not the same in all directions. This is the reason wood is easier to split along its grain. Anisotropic materials exhibit different electrical properties that depend on the direction an electrical signal moves. Ferrite, a common isolator and circulator material, is an anisotropic material.
Reciprocal and Non-reciprocal Devices
A reciprocal device is one in which signals travel in the same manner in forward and reverse directions. The transmission of a signal between two ports does not depend on the direction of propagation. From a network analysis point of view, a reciprocal device is a passive component manufactured using only isotropic materials. Resistors, attenuators and cables are examples of reciprocal components. In non-reciprocal devices or networks, on the other hand, signals behave differently depending on the direction of the movement. These are normally made with anisotropic materials. Examples of non-reciprocal components are circulators, isolators and RF amplifiers.
A circulator is a passive, non-reciprocal device, usually with three or four ports. Once a signal enters one port it is transmitted to the next port following a prescribed circulation path. Based on how signals “circulate” once inside the circulator, there are two types: clockwise (CW) and counterclockwise (CCW). Figure 1 shows the electronic symbol for a three-port circulator, for each circulation type.
Because circulators are non-reciprocal devices, a signal entering into port 1 exits port 2, a signal into port 2 exits port 3, and signals entering port 3 exit port 1.
When a magnet is applied to the ferrite material inside the circulator a gyrating (rotating) magnetic field is generated. This magnetic field is very strong, so any RF signal—in a particular frequency band—that enters the circulator at one port will be forced to move in the direction of rotation of the field toward the next port. Signals entering a port will not be able to move opposite to the direction of the magnetic field.
One common application of circulators is as a simple duplexer—a device that allows bi-directional communications using a single path. Figure 2 shows a transmitter and a receiver sharing one antenna while isolated from each other.
When the transmitter sends a signal to the circulator it goes directly to the antenna. The signal is transmitted and the receiver is isolated form this process. When the antenna receives a signal, it is directed to the receiver and not to the transmitter.
RF isolators are two-port devices that are built by terminating one port of a three-port circulator with a matched load (normally 50 Ohms). Because it has only two ports only one path is available for energy to flow. Figure 3 shows the schematics of both a circulator and an isolator. Notice that for the isolator, energy can only enter through port 1 and travel to port 2. Any signal that enters port 2 will be re-routed to the matched termination in port 3, and quickly dissipated as heat.
Isolators are most widely used to protect high-power RF sources. If a source is connected directly to the load (an actual impedance or equipment), and the load is not well matched with the source, some power reaching the load will be reflected back to the source that could destroy the source. An isolator between the source and the load will absorb most of the reflected energy. This situation is depicted in Figure 4.
If energy is reflected back from the load, as seen in the figure, it will enter port 2 where the isolator will deflect most of the energy to the matched resistance (R) and to ground.