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An important consideration when specifying an isolator or circulator is to ensure the
device has adequate isolation for your given application. Isolation is a unit of
measure (in dB) that states the separation of signal levels on adjacent ports of a
device. The greater the isolation value, the less interference from a signal on one
port is present at the other. The amount of isolation is directly affected by the VSWR
presented at port C of the isolator. If the match on port C is poor, you can expect
isolation below 10dB, but if the match is improved to 1.10:1 by using a good
termination device in the circuit, then the isolation would improve to over 20dB.

One common application of circulators is as a simple duplexer. A duplexer is a
device that allows bi-directional communications using a single path. Figure 32-14
shows a transmitter and a receiver sharing one antenna while isolated from each
other.

Figure 32-14. Isolation between transmitter and receiver using a ferrite isolator

When the transmitter sends a signal to the circulator, it goes in port A and out of port
B to the antenna with little loss, typically 0.4dB. The signal is transmitted and the
receiver is isolated from the transmitted signal by about 20dB (100 times). When the
antenna receives a signal, it enters port B and exits port C with little loss.

It may be strange for you to think about transmitters receiving signals from other
transmitters. Why not? Transmitters have antennas - those antennas receive signals
from other transmitters and funnel their energy into the output stage of the
transmitter. The transmitter, you will recall, often has a non-linear output stage. The
external signals from other transmitters will enter our transmitter and mix with our
transmitter signal in the power amplifier and reradiation will occur. The result is a
great mess of unwanted signals very likely to cause interference.

This interference is called transmitter-intermodulation.

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