RF isolators and circulators play an important role in many RF communication systems. Electrical behaviour of each individual unit contributes to overall system performances. Engineers often monitor and test each component during troubleshooting of an RF system.
RF Isolators: are two port passive devices used in RF and communication networks to allow transmission in one direction and give maximum signal isolation in opposite direction.
RF Circulators: are three port devices used in RF communication systems to control the signal flow within the system channels (with 120 degree symmetry). It transmits signal from port 1 to port 2 and port three will be isolated, similar way from port 2 to 3 and port 1 will be isolated, signal will flow from port 3 to port 1 and port 2 will be isolated.
Let’s see how to test an RF isolator and circulator for its electrical (response) performance.
Typical performance of any RF device can be found at manufacture’s data sheet. In real application we have to test and verify each characteristic to optimize with our system requirements.
The performance of these individual units tends to degrade its electrical behaviour over time. It may affect our systems like power loss, cross talk and intermodulation etc.
Network analyzers are one the best devices to test the electrical characteristics of RF isolators and circulators. It can measure and sweep across the operating frequency range and display the response of the unit.
Most of the network analyzers operate in low power. This is one limitation of network analyzer if we require testing high power devices.
S21 with forward response: must be minimum to allow maximum signal flow.
S12 with reverse response: must be maximum to protect signal reflection.
Power meter is another option to test response of RF isolator and circulator. It can’t sweep across the frequency and need to be tested individual points across the operating range.
Spectrum analyzer also can be used to test RF isolators and circulators. It can sweep across the operating frequency and analyse response behaviour. Spectrum analyzers are suitable to test at high power with the addition of sufficient attenuators.
In order find any damage of an isolator or circulator unit, compare the forward and reverse performances at different input power levels. Typically, it should give minimum loss in the forward direction and maximum attenuation in the reverse direction.
Unexpected response (non uniform behaviour or spikes) will be due to damage of the isolator / circulator unit and need to be replaced. It is advisable to compare the data with manufacturer’s specification to confirm the electrical performance of each unit.
Note: In RF Isolator, port 3 will be terminated with 50 Ohm load (internally). All three port combinations need to be tested for RF circulator for its functionality.