Circulators help direct the flow of microwave or radio-frequency signals in RF systems. Usually 3 or 4-port device that transmits input signal from 1 port to the next, figure 1 shows a normal circulator symbol. The ports are either input signal, output signal or termination and typically have a line impedance of 50Ω. Common connectors are SMA or N-type coaxial. The signal will transfer from port 1 to 2, 2 to 3, and 3 to 1. The ports are connected to a symmetrical Y junction that is coupled with magnetically biased ferrite material. Ferrite along with a magnet creates a flow that helps push the signal in the circular path we see and ensure the port-to-port assignments are followed. Circulators are designed to have minimal loss when transmitting an input signal from one port to the next.
Common application for circulators are using them as a replacement for a duplexer. This allows bi-directional communication over a single path. Example being a radar and radio communication system where the receiver and transmitter are isolated, but can share a common antenna. These systems can be based on frequency, polarization, or timing. The difference between circulators and duplexers are circulators are usually less expensive. Figure 2 shows the application.
An isolator is a circulator but has only 2 ports that transfer a signal, while the third has a termination resistor that is connected to ground. Think of it as a dumpster for unwanted signal. It protects RF components from excessive signal reflection. Figure 3 shows the general isolator symbol. If signal from port A is well matched with port B, the signal will go into port B with minimal loss. If they are not matched the signal will pass to port C, which will dissipate the signal into heat. Since port A is the input and port B is output, you don’t want anything bouncing back from port B back into A, as this will distort the signal or damage the port.
Common application for an isolator is to put between signal source and a device under test. This prevents the reflections from the device under test from going back into the signal source. With this you must ensure the termination port can handle all of the source signal, just in case the device under test is disconnected. If it cannot handle the full incoming signal the termination could be damaged, passing the circular flow back to the input signal. Figure 4 shows the application.
Selecting RF Circulators and Isolators
Some parameters to consider for choosing a circulator or isolator:
Frequency- Range over which the device can operate and keep constant isolation along with minimal loss.
Insertion Loss- The signal loss from one port to the next. Difference in the incident power at primary port to the power received at the next port.
Power- The amount of power the device can handle while maintaining the it’s electrical characteristics.
Pack type- Various form factors: Drop-in, surface mount, modules etc.
Circulators and isolators are a lower-cost solution to control signal in a RF circuit. They help ensure minimal loss between ports and prevents damage to applications connected while protecting themselves.