Circulators and Isolators

The ferrite circulator is a practical component used to provide directional isolation at
the output of the transmitter. The isolation achieved has to be considered in conjunction
with the insertion loss and bandwidth, with the quality of the terminations
directly affecting the isolation performance. For very high isolation (>40 dB), a dual circulator
version may be fitted.
Circulators are electromagnetic components having three or more connections
(ports) in which RF energy circulates in one direction from one port to another,
whereas a relatively high attenuation occurs in the opposite direction. Additionally all
ports are matched.
The (low) attenuation in the circulation direction is called the insertion loss and is in
the order of tenths of a dB. The (high) attenuation in the opposite direction is called isolation
and is usually in excess of 20 dB. Other parameters of importance are VSWR at
the ports and the bandwidth of the circulator. The bandwidth is usually limited by
construction resonances but is always sufficient for mobile wireless applications. Isolators
have the following characteristics:
 They have only two ports.
 The insertion loss is very low in the forward direction.
 The isolation (attenuation) is very high in the reverse direction.

A magnetically biased ferrite core absorbs the electromagnetic waves in one direction
but does not influence them in the other direction. It is the most important part of
an isolator or circulator. Let’s consider a couple of examples.
Example 1
Acirculator can be used as a duplexer, where a receiver and transmitter are connected
to a common antenna. The circulator decouples the receiver from the transmitter to the
antenna and from the antenna to the receiver. Normally, the transmitter and receiver
operate at different frequencies within the bandwidth of the circulator so that additional
filters can be used to increase the isolation (see Figure 13.13).
Example 2
A terminated circulator may also be used to decouple two or more transmitters connected
to the same antenna. The object is to reduce intermodulation distortion by preventing
output energy from one transmitter passing to the second transmitter and
creating intermodulation products. The terminating resistor need only be dimensioned
for the reflected power. The transmitters can either operate at different frequencies
within the bandwidth of the circulator or on the same frequency (where power doubling
is required). 318